Monday, March 30, 2009

Engineering Projects - VIII

Automotive

  • AC Line Powered LED Strings
    This shows 4 different ways to string white LEDs that are powered by a 120vac or 240vac power line. The circuit was designed by Ken Schultz.
  • CHARGE COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
    The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line into the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed to turn on and off 400 watts of AC or DC power to a load.
    Published in Electronic Design, Nov. 8, 1990.
  • SOLID STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
    This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays. It is ideal for battery-powered systems. Using a combination of a high current TRIAC and a very sensitive low current SCR, the circuit can control about 600 watts of power to load while providing full isolation and transient protection.
  • ISOLATED AC CURRENT MONITOR
    This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 amps of AC current and provides at least 500vac of isolation. With the components shown, the output AC voltage is scaled so 1 amp of current produces 100mv of AC voltage.
  • Run Switching Type AC Power Adapters on DC NEW
    I have received a couple emails from people looking for inexpensive DC to DC converters, which can convert 40v to 60v DC into say +5 or +12v. Such input voltages are often found in new automotive and industrial applications with a typical DC voltage of 48 volts. It turns out that many, not all, but many, standard AC line operated power adapters, which use switch mode techniques, will indeed work great when supplied with DC instead of AC. The unit below works down to about 30v DC and delivers 500ma at 5v. If you draw less current, it will operate at even lower DC input voltages....
  • VARIABLE ISOLATED AC VOLTAGE SPANS 0VAC TO 280VAC
    I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel for the140vac range and in series for the 280vac range. The selector switch also diverts power to the appropriate output socket to avoid mishaps in sending the wrong voltage to the load. My home-built unit also included an AC volt and amp meter to monitor the output. However, this circuit only indicates the AC power connections.
  • ISOLATED AC CURRENT MONITOR
    This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 amps of AC current and provides at least 500vac of isolation. With the components shown, the output AC voltage is scaled so 1 amp of current produces 100mv of AC voltage.
  • Run Switching Type AC Power Adapters on DC NEW
    I have received a couple emails from people looking for inexpensive DC to DC converters, which can convert 40v to 60v DC into say +5 or +12v. Such input voltages are often found in new automotive and industrial applications with a typical DC voltage of 48 volts. It turns out that many, not all, but many, standard AC line operated power adapters, which use switch mode techniques, will indeed work great when supplied with DC instead of AC. The unit below works down to about 30v DC and delivers 500ma at 5v. If you draw less current, it will operate at even lower DC input voltages....
  • VARIABLE ISOLATED AC VOLTAGE SPANS 0VAC TO 280VAC
    I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel for the140vac range and in series for the 280vac range. The selector switch also diverts power to the appropriate output socket to avoid mishaps in sending the wrong voltage to the load. My home-built unit also included an AC volt and amp meter to monitor the output. However, this circuit only indicates the AC power connections.


  • 240VAC TO 5VDC POWER SUPPLY
    This is simple way to power some 5v logic from a 240vac source. If a 120vac power adapter is used, the circuit will also work for 120vac power lines.
  • CAPS PROVIDE VOLTAGE BOOST TO SERIES REGULATOR
    This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply.
  • LOW POWER 12,000 VOLT POWER SUPPLY
    If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line. Although the circuit can only deliver about 5uA of current it can produce dangerous shocks, so be careful.
  • MINIATURE ISOLATED AC/DC POWER SUPPLY
    This circuit uses a novel approach to produce a fully isolated and regulated 5 volts @30ma from the 120vac power line. It uses two tiny SCRs that alternately discharge two capacitors through a miniature high frequency transformer. The voltage spikes produced through the transformer are rectified, filtered and regulated. A very common 8 ohm audio impedance matching transformer can be used for the transformer. Published in EDN, Feb. 17, 1992
  • AC Line Powered LED Strings
    This shows 4 different ways to string white LEDs that are powered by a 120vac or 240vac power line. The circuit was designed by Ken Schultz. (added 12/04)

  • CHARGE COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
    The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line into the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed to turn on and off 400 watts of AC or DC power to a load.

  • SOLID STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
    This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays. It is ideal for battery-powered systems. Using a combination of a high current TRIAC and a very sensitive low current SCR, the circuit can control about 600 watts of power to load while providing full isolation and transient protection.

  • ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
    For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%.

  • HIGH PASS ACTIVE FILTER COLLECTION
    This is a collection of inverting and non-inverting active high pass filter circuits. I included one, two three, four and six pole filter circuits. You can change the component value ratios shown to achive any frequency cut-off you may need. The circuit does not specify an operational amplifier. The circuits should be used to select the needed resistor and capacitor components for a particular frequency knee and slope.

  • 3KHz LOW PASS FILTER PLUS AUDIO AMP
    This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio. The schematic includes an audio amplifier that is designed to drive a standard audio head phone. The circuit is described in more detail in the receiver section of Dave Johnson's
    Handbook of Optical Through the Air Communications. (this link is off-site)

  • LOW PASS ACTIVE FILTER DESIGN COLLECTION
    This is a collection of inverting and non-inverting active low pass filter circuits. I included one, two, three, four and six pole filter circuits. You can change the component value ratios shown, to achieve any frequency cut-off you may need. The circuit does not specify an operational amplifier. The circuits should be used to select the needed resistor and capacitor components for a particular frequency knee and roll-off slope.

  • CIRCUIT FORMS DIVIDE BY 1.5 COUNTER
    Two inexpensive ICs divide a TTL clock signal by 1.5. By following the circuit with another flip/flop, you could also generate a divide by three function.
  • DVD Recorder + TV Camera Make Cheap Data Logger
    Sometimes recording the results of a test requires as much ingenuity as designing the product. Consider a situation which requires the collection of many different variables over a long period of time. Perhaps you are conducting temperature cycle tests on an electronic circuit and you want to know what changes occur in circuit performance over a 24 hour period. You could stay up all night and collect the data yourself....

· CMOS Logic Inverter Amplifier Characteristics
This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

· FLASHING LED ADVERTISING BADGE #1
This circuit is similar to Flashing LED Advertising Badge #1. It uses a CD4013 dual D Flip/Flop IC. The 74HCT74 IC in Flashing LED Advertising Badge #1 does not always work. As in #81, a single lithium battery will provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing.

  • 20MHz VCSEL 3mW LASER TEST CIRCUIT
    This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that don’t require light output control circuits. The circuit shows how to drive the device from a single high speed CMOS IC. The circuit can easily be modified to transmit signals from kilohertz to about 50MHz.

  • 10MHz TO 20MHz LASER LIGHT DETECTOR
    This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.

  • BROAD BAND 2MHz OPTICAL FIBER RECEIVER
    If you need more sensitivity than the above circuit this circuit provides about ten times more gain. It too is designed around an inexpensive plastic optical fiber detector.
  • BROAD BAND 5MHz OPTICAL FIBER RECEIVER
    This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers. It has a bandwidth from 1KHz to over 5MHz. It is great for experimenting with various modulated light sources.

· CMOS Logic Inverter Amplifier Characteristics
This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

  • C-MOS INVERTER MAKES LOW POWER AMP
    With the addition of one resistor and a capacitor, some common logic ICs can be transformed from digital to analog duties. This circuit outlines some the features to expect from different inverter ICs.
  • FET INPUT HIGH SPEED LIGHT DETECTOR
    This circuit is yet another design that converts current from a PIN photo diode to a voltage. It has a bandwidth that extends beyond 50MHz.
  • N-CH AND P-CH TRANSISTORS FORM PUSH-PULL DRIVER
    This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of the transistors. I have used this circuit beyond 2MHz.

· WIDE BAND ZERO CROSS DETECTOR
This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal. It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak. The input frequency can range from a few kilohertz to about 150KHz.

· ELECTRIC FIELD DISTURBANCE MONITOR
This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at
Electric Field Disturbance Monitor. The system can detect human and animal motion by the electric fields they disturb.

  • 10MHz TO 20MHz LASER LIGHT DETECTOR
    This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.
  • 1uS LIGHT PULSE RECEIVER PLUS POST AMP
    This circuit is designed to detect very weak light pulses lasting 1uS. It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity. A post voltage amplifier is included with a gain of about X20. The circuit is described in more detail in the receiver section of my
    Handbook of Optical Through the Air Communications. Note: The LF357 op amp is no longer available, this circuit is for reference only.
  • 40KHz LASER BURST DETECTOR
    This circuit was originally designed to detect weak flashed of laser light bounced off of a fabric video projection screen. It was used as part of a firearm training system. It generates a 100mS output pulse whenever it detects a 3ms to 5ms-laser burst, modulated at 40KHz. It is very sensitive and could be modified for long-range laser communications.
  • 40KHz LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT
    If you want even more sensitivity than the above circuit, try this design. When used with a one-centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
  • 40KHz MODULATED LIGHT DETECTOR
    This circuit uses a unique cascode amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20.
  • 10MHz TO 20MHz LASER LIGHT DETECTOR
    This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.


  • 20MHz VCSEL 3mW LASER TEST CIRCUIT
    This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that don’t require light output control circuits. The circuit shows how to drive the device from a single high speed CMOS IC. The circuit can easily be modified to transmit signals from kilohertz to about 50MHz.
  • BROAD BAND 2MHz OPTICAL FIBER RECEIVER
    If you need more sensitivity than the above circuit this circuit provides about ten times more gain. It too is designed around an inexpensive plastic optical fiber detector.
  • BROAD BAND 5MHz OPTICAL FIBER RECEIVER
    This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers. It has a bandwidth from 1KHz to over 5MHz. It is great for experimenting with various modulated light sources.

· CMOS Logic Inverter Amplifier Characteristics
This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

  • C-MOS INVERTER MAKES LOW POWER AMP
    With the addition of one resistor and a capacitor, some common logic ICs can be transformed from digital to analog duties. This circuit outlines some the features to expect from different inverter ICs.
  • FET INPUT HIGH SPEED LIGHT DETECTOR
    This circuit is yet another design that converts current from a PIN photo diode to a voltage. It has a bandwidth that extends beyond 50MHz.
  • N-CH AND P-CH TRANSISTORS FORM PUSH-PULL DRIVER
    This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of the transistors. I have used this circuit beyond 2MHz.

· WIDE BAND ZERO CROSS DETECTOR
This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal. It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak. The input frequency can range from a few kilohertz to about 150KHz.

  • DOORKNOB ALARM
    Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected by the electrical capacitance change that occurs from the human hand contact between the knob and the box. Almost all of the commercial devices sold use a more expensive and power consuming radio frequency circuit approach to detect the capacitance change. But, a very inexpensive and micro power technique can also work. This circuit schematic should dramatically reduce the cost of the device and allows it to operate for many years from one set of batteries.

    Also see
    Completed Project by Matthew C. Ruschmann

  • Earthquake Alarm Circuit
    Geophones are dandy motion sensors. They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire. The magnet functions as a floating mass. The slightest vertical motion of the device causes the coil to move relative to the magnet. This induces a small voltage in the wire coil...
  • Fake Car Alarm Light
    Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)
  • Freezer Over-Temperature Alarm
    This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F). A 9v battery powers the hobby circuit. The average 9ua current is so low that the battery should last for many years....
  • Freezing Temperature Alarm
    This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade. The circuit draws only a few microamps from a 9 volt battery. It uses a glass bead thermistor accurate to 1 degree C.
  • Human Traffic Footstep Alarm
    This circuit processes the amplified signals from the footstep detector circuit and activates a microprocessor when the circuit detects footsteps. At has an adjustable sensitivity.
  • Simple Security Wire Loop Alarm
    A wire loop is used to protect valuable objects in this simple alarm circuit. The electronic hobby circuit is powered by a 9v battery. The alarm beeper is activated if the wire loop is severed. The standby current is so low that the 9v battery should last for many years.
  • Wire Security Loop Alarm
    A wire loop is used to protect valuable objects in this alarm circuit. The circuit is powered by a 9v battery. (added 7/06)

· 24V DC Powered Beeper with 4 Separate Inputs
24v DC is a very popular voltage used in industrial settings. The circuit below was designed to accept four different 24v DC alarm input signals, which are then used to drive a single low power beeper. The beeper is a magnetic type with its own oscillator/driver. The four diodes form an “OR” gate so any one of the four inputs will cause the beeper to make noise. A CMOS version of the popular 555 timer is used to strobe the beeper on and off at about 1Hz.

· 3V Sweeping Siren Alarm
The circuit uses a LTC1799 precision frequency generator from Linear Technology. A 74HC14 hex Schmitt trigger from Texas Instruments is also used to perform several other functions. One section is wired as a simple 7Hz square wave oscillator....

  • ANOTHER VERY LOUD PIEZO ALARM BEEPER
    This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezoelectric device to more than 200 volts peak to peak. It cranks out an ear splitting 120db when measured at 12 inches.
  • Basement Doorbell Beeper
    If you can't hear your doorbell when you are in your basement try this circuit. This circuit takes advantage of the 24vac power source located near the furnace. Using a simple current transformer technique, the circuit sounds a beeper whenever the main door chime is activated.
  • Basement Doorbell
    This circuit will activate a beeper in the basement, whenever the front doorbell is pressed.
  • Check Engine Beeper
    When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on. (added 7/06)
  • Front Desk Service Beeper
    This device functions like a lobby desk service request bell. A short beep is sounded whenever the large pushbutton on top of the device is pressed. Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed. This prevents impatient customers from ringing the bell over and over.
  • HIGH SOUND OUTPUT BEEPER CIRCUIT
    I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries. Published in Electronic Design, Aug 5, 1993
  • MACHINE POWER LOSS BEEPER
    For some medical equipment it is important for an operator when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts.
  • Piezoelectric Triggered Switch
    Two different switch circuits are shown. One sources current and the second sinks current. Both switches are connected to a piezoelectric wafer. When the wafer is tapped, the switches are activated.
  • Power Reminder Beeper
    This circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box. It might be ideal for computers, printers or some test equipment that shouldn't be left on all night.
  • Service Beeper: This device functions like a lobby desk service request bell. A short beep is sounded whenever the large pushbutton on top of the device is pressed. Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed. This prevents impatient customers from ringing the bell over and over.
  • Very Loud Piezoelectric Beeper Circuit
    This circuit is similar to circuit number 7, but generates a continuous tone instead of a pulsing one. The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency.

· CHEAP LOW POWER 3V 2Hz OSCILLATOR
This is yet another method to generate low frequency output pulses. The circuit uses an inexpensive under-voltage monitor IC and a couple of cheap MOSFETs. It could easily be modified to produce pulse frequencies from about one cycle per minute to a maximum of about 10KHz. It draws only about 1uA from a 3v battery. This circuit should not be used if frequency stability is important, since it will change as a function of supply voltage.

· CMOS SCHMITT TRIGGER IC MAKES VCO
By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal.

  • Low Power Oscillators
    This page has two unusual two-transistor oscillators. I set the component values for a low frequency application. Both circuits draw only about 1 microamp of current.
  • MICROPOWER C-MOS OSCILLATOR, DRAWS ONLY 0.5uA
    If truly low power oscillators interest you, this circuit draws a mere 2 microwatts (500nA) from a 6v battery. It uses a very inexpensive C-MOS IC to produce a frequency of 2Hz. However, by changing the component values you can push it to 300Hz. The circuit draws much less current than traditional C-MOS oscillators using a Schmitt trigger inverter. It also produces fast leading edges. However, if you need more drive current, a C-MOS buffer should be included.
    Published in EDN, September 2, 1993.
  • PRECISION ULTRA LOWER POWER OSCILLATOR
    This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery. It is highly stable under varying temperature and supply voltages.
  • Ultra Low Power Astable Multivibrator
    Taking advantage of some new voltage comparators, this circuit can produce a nice square wave signal while drawing only 1.6 microamps. With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal.

  • 120VAC Touch Switch
    This is a capacitance based touch on/touch off switch, which can control power to a 200 watt 120vac load.

  • AUDIO AMP + 3KHz FILTER
    This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electret microphone. The circuit is described in more detail in the receiver section of Dave Johnson's

  • CMOS SCHMITT TRIGGER IC MAKES VCO
    By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal.

  • ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
    For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%.



  • Check Engine Beeper
    When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on. (added 7/06)
  • Fake Car Alarm Light
    Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)
  • Voltage Indicator
    When troubleshooting the 12v electrical system of an automobile, it is nice to have a simple voltage indicator tool instead of a voltmeter. This electronic circuit, hobby circuit is powered by two AA or even two N cells, which will provide enough energy for years of service. The circuit has only two leads...

Band Pass Active Filters

Battery Chargers

Battery Monitor

Battery Tester

Beepers & Buzzers

Bistable Multivibrator

Blinkers

Buffer Amplifiers

Burglar

  • ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
    For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%.
  • Candle Power: A single lit candle in conjunction with an inexpensive thermopile voltage generator provides enough power to charge a small battery when used with a small low voltage DC to DC converter.
  • Solar Powered Lithium Ion Battery Charger uses a LTC1734
    The circuit below feeds a controlled current and voltage to a 3.6v lithium ion battery. The current is limited to 300ma and the voltage is limited to 4.2 volts. The circuit uses a LTC1734 IC from Linear Technology. No diode is needed between the circuit and a 6 volt solar panel....
  • Two 12v Battery Isolator with a LTC4412: Linear Technology has just announced a neat little chip (LTC4412). It has been designed to be used in conjunction with an external P-channel power FET, to form an ideal diode function with a very low 0.05v voltage drop. The chip monitors the voltage on either side of the FET. As long as the voltage on the drain side is greater than the source side, the FET is turned on....
  • 3 VOLT LOW BATTERY VOLTAGE FLASHER
    This circuit is designed to monitor two alkaline cells (3v) that form the battery often used in portable electronic equipment. It use an inexpensive IC from Panasonic that is connected to an efficient LED flashing circuit. When the battery voltage drops below a certain point the circuit flashes the LED. In the off state the circuit draws only 1uA, while in the active flashing state it draws 20uA. Published in EDN, Jan 2, 1997
  • 9v Battery Voltage Monitor uses a LTC1440 comparator
    This circuit turns on a LED whenever the voltage of a standard 9v battery connected to the circuit drops below 7.2 volts. It uses a LTC1440 comparator, which also contains a 1.18v reference diode. In standby mode, the circuit draws only 4uA.
  • 6v NMH/NiCd Battery Tester
    I designed this circuit to test rechargeable six volt battery packs under constant current conditions. As designed, the circuit applies a 10 amp load to the battery pack. A heat sink must be used on the main power transistor.
  • Button Battery Cell Internal Resistance Measurement Circuit
    The open cell voltage of a small silver oxide button cell battery remains very close to 1.5 volts over most of the life of the cell. However, there is a relationship between battery health and the internal resistance of the cell. This circuit when used in conjunction with an oscilloscope will measure the internal resistance. You will have to use a known good cell as a reference.
  • Small Battery Capacity Tester
    The only truly accurate way to measure the current capacity of a battery is by connecting the battery to a constant current load and measure how long the battery can sustain that current before its voltage drops below a recommended “cutoff” point. The circuit below is designed to do this type of test.....

· 24V DC Powered Beeper with 4 Separate Inputs
24v DC is a very popular voltage used in industrial settings. The circuit below was designed to accept four different 24v DC alarm input signals, which are then used to drive a single low power beeper. The beeper is a magnetic type with its own oscillator/driver. The four diodes form an “OR” gate so any one of the four inputs will cause the beeper to make noise. A CMOS version of the popular 555 timer is used to strobe the beeper on and off at about 1Hz.



  • 3V Sweeping Siren Alarm
    The circuit uses a LTC1799 precision frequency generator from Linear Technology. A 74HC14 hex Schmitt trigger from Texas Instruments is also used to perform several other functions. One section is wired as a simple 7Hz square wave oscillator....
  • ANOTHER VERY LOUD PIEZO ALARM BEEPER
    This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezoelectric device to more than 200 volts peak to peak. It cranks out an ear splitting 120db when measured at 12 inches.
  • Basement Doorbell Beeper
    If you can't hear your doorbell when you are in your basement try this circuit. This circuit takes advantage of the 24vac power source located near the furnace. Using a simple current transformer technique, the circuit sounds a beeper whenever the main door chime is activated.
  • Basement Doorbell
    This circuit will activate a beeper in the basement, whenever the front doorbell is pressed.
  • Check Engine Beeper
    When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on. (added 7/06)
  • Front Desk Service Beeper
    This device functions like a lobby desk service request bell. A short beep is sounded whenever the large pushbutton on top of the device is pressed. Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed. This prevents impatient customers from ringing the bell over and over.
  • HIGH SOUND OUTPUT BEEPER CIRCUIT
    I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries. Published in Electronic Design, Aug 5, 1993
  • MACHINE POWER LOSS BEEPER
    For some medical equipment it is important for an operator when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts.
  • Piezoelectric Triggered Switch
    Two different switch circuits are shown. One sources current and the second sinks current. Both switches are connected to a piezoelectric wafer. When the wafer is tapped, the switches are activated.
  • Power Reminder Beeper
    This circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box. It might be ideal for computers, printers or some test equipment that shouldn't be left on all night.
  • Service Beeper: This device functions like a lobby desk service request bell. A short beep is sounded whenever the large pushbutton on top of the device is pressed. Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed. This prevents impatient customers from ringing the bell over and over.
  • Very Loud Piezoelectric Beeper Circuit
    This circuit is similar to circuit number 7, but generates a continuous tone instead of a pulsing one. The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency.
  • 1.5V TOUCH ACTIVATED SWITCH
    A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years. It features both a normally open and a normally closed set of solid-state switch thermals. It also has an adjustable sensitivity, which can be set for a touch capacitance change as small as 1 picofarad.


  • D-FLIP/FLOP ONE SHOT CIRCUITS
    Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators. This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds.
  • FLASHING LED ADVERTISING BADGE #1
    I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted. My circuit is simple but efficient enough to provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing, extending battery power.
  • FLASHING LED ADVERTISING BADGE #2
    This circuit is similar to Flashing LED Advertising Badge #1. It uses a CD4013 dual D Flip/Flop IC. The 74HCT74 IC in Flashing LED Advertising Badge #1 does not always work. As in #81, a single lithium battery will provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing.
  • HONEYBEE COUNTER
    I designed a circuit similar to this one a long time ago to help a beekeeper count the number of bees going into or out of a hive. The low power circuit uses a slotted opto-sensor to detect the passing bees. The circuit advances an electronic counting module whenever a honeybee passes through the sensor. The device only counts the number of bees going through the sensor. A different circuit would be needed to count the number of bees only going out or only coming into the hive.
  • Miniature Pushbutton On/Off Switch NEW
    In many battery powered devices, you would like to turn the circuit on and off by pressing a small pushbutton switch. One press turns the device on and second press turns it back off. The flip/flop circuit below uses a minimum number of parts to perform this function. It operates from DC supply voltage which can range from 2v to 6v. It draws very little current and takes up little space. Yet, it can control up to one amp of current. The circuit uses a single flip/flop device made by Texas Instruments. ...
  • On/Off Flip/flop Circuit with Automatic Timeout
    This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs to turn itself off after some period of time. With the components shown, the output will stay on for only about 20 seconds. (added 12/04)
  • PUSHBUTTON ONESHOT AND LATCH
    This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v.
  • TIME TO DUST INDICATOR
    I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough. I have not yet built and tested this circuit completely but in concept it should work. The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a phototransistor, mounted at a 90-degree angle from the LED. When the dust reaches a particular level, sufficient light is reflected into the phototransistor to change the logic state of the circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED or to one of the LED flashing circuits in this hobby circuit section.
  • 1.5V LED FLASHER VERSION A
    Many published circuits that flash LEDs need 3 volts or more. This circuit uses only a single inexpensive C-MOS IC and flashes the LED for a full year on a single 1.5 volt AA alkaline battery cell. The circuit uses a charge pump technique to provide the LED the needed voltage.
  • 1.5V LED FLASHER VERSION B
    To squeeze even more energy from a alkaline battery cell, this circuit adds two transistors to a circuit similar to the above design to boost the efficiency. A small 1.5 volt alkaline N cell should flash the LED for a full year. It too uses a "charge pump" technique to provide a LED the needed voltage.
  • 3v Low Battery Voltage Flasher
    Many battery powered devices use two AA alkaline cells. Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating. The hobby circuit below can be connected to a 3v battery, to give you some warning when the battery is nearing its end of life. It will flash a LED when the battery voltage drops to about 2.4 volts. The electronic circuit draws only 1ua of current in standby mode and jumps to only 20ua when flashing, so it can safely...
  • 9v POWERED XENON PHOTOFLASH Controller
    This 9v battery powered circuit is designed for remote control flash needs. A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged. A neon lamp is included to indicate when the system is ready to flash.
  • Fake Car Alarm Light
    Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)
  • FLASHING LED ADVERTISING BADGE #1
    I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted. My circuit is simple but efficient enough to provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing, extending battery power.
  • High Intensity Line Powered LED Flasher
    There are times when you would like to generate an intensely bright flash of light using electrical power from the AC line. The circuit below can drive a one to three watt power LED with about 350ma of peak current. Since the circuit drives the LED with a constant current, any LED color can be used. I have also shown how you could use an array of 7 LEDs, wired in parallel. This array may not produce as much light as the single power LED but the array would be cheaper to build. ....
  • High Power LED Flasher August 3, 2008
    Power LEDs are becoming more popular these days. The DC input power to these devices ranges from 1 watt to 5 watts. Normal LEDs only receive about 0.05 watts. At this higher power, these devices can emit a lot of light. You can buy them in just about any color in the rainbow but white seems to be the most popular. One application for these devices is a flashing light. If the flash duration and flash rate is kept low, the average current can be low enough that even a small battery can last quite a while.....


  • Lighthouse LED Flasher
    This was originally designed for a model in a HO train set. It simulates the behavior of the light from a lighthouse. The LED intensity gradually increases, then flashes with a bright light and finally decreases slowly in intensity.
  • LINE POWERED XENON FLASH TRANSMITTER
    This line powered xenon flash circuit drives a small camera type flash tube. It has an optical isolator to allow the flash to be safely triggered from some remote device. A flash rate of 2Hz is possible with the circuit.
  • LOW BATTERY VOLTAGE FLASHER
    This circuit is designed to monitor two alkaline cells (3v) that from the battery often used in portable electronic equipment. It use an inexpensive IC from Panasonic that is connected to an efficient LED flashing circuit. When the battery voltage drops below a certain point the circuit flashes the LED. In the off state the circuit draws only 1uA, while in the active flashing state it draws 20uA.
    Published in EDN, Jan 2, 1997
  • Reduced Power Sequential LED Flasher
    The 74HC4017 is a neat decade counter which has 10 decoded outputs. When supplied with a low frequency clock and with a LED attached to each of the outputs, an interesting ten step sequential flasher effect can be produced. The LEDs can be configured in a wide variety of patterns of loops, lines or arrows. But, these circuits can often be power hogs. To reduce the average current needed to produce the same effect, you can strobe the individual LEDs with a short 10mS pulse.
  • SIMPLE NITROGEN SPARK GENERATOR
    Nitrogen or air sparks are very powerful light sources that produce flashes that last only a few nanoseconds. This line-powered circuit generates a continuous series of very small sparks across electrodes with a 0.05-inch gap.
  • Universal Flasher using Latching Relay: This circuit is powered by a 9v battery and controls any AC to DC load through a set of relay contacts, rated at 10 amps. To conserve power, the circuit uses a latching relay. A variable frequency oscillator controls the flashing speed from 0.2Hz to 2Hz. A pair of pulse generators first latch the contacts closed then unlatches them open. The power consumption is so low that a 9v battery will last for several months.

Ultra Low Power LED Flasher NEW
The efficiency of some newer LEDs is amazing. Some of the latest green LEDs can launch blinding light with just one milliamp of current. I take advantage of one of these newer devices in the circuit below. The flashing circuit uses a classic multivibrator oscillator, made from a tiny National Semiconductor’s LMC7215 low power voltage comparator. The circuit produces a short 10ms pulse every two seconds, drawing power from a 3v supply. I suggest using a surface mounted green LED from Kingbright.....



CMOS Logic Inverter Amplifier Characteristics
This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

  • FLASHING LED ADVERTISING BADGE #1
    This circuit is similar to Flashing LED Advertising Badge #1. It uses a CD4013 dual D Flip/Flop IC. The 74HCT74 IC in Flashing LED Advertising Badge #1 does not always work. As in #81, a single lithium battery will provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing.

ELECTRIC FIELD DISTURBANCE MONITOR-1
This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at
Electric Field Disturbance Monitor. The system can detect human and animal motion by the electric fields they disturb.

ELECTRIC FIELD DISTURBANCE MONITOR-2
This schematic is the motion discriminator alarm and battery monitor sections of the field monitor that is discussed in more detail a
Electric Field Disturbance Monitor. (link is off-site)

  • Simple Security Wire Loop Alarm
    A wire is looped through various objects which are to be monitored. The loop wire is typically an aircraft cable but could also be any insulated stranded wire. The loop is attached to a table or desk. Should the loop be cut, the alarm is sounded.

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