Monday, March 30, 2009

Engineering Projects - V

  • POP FREE MICROPHONE ON/OFF SWITCH CIRCUIT
    This circuit uses a simple technique to eliminate the usual “pop” sound when an electret type microphone is switched on or off. The circuit is completely passive and will work on dynamic and electret type microphones
    .
  • 1 LINE TELEPHONE LINE STATUS INDICATOR
    This circuit is similar to the above circuit, but only monitors one phone line.
  • 1.5V LED FLASHER VERSION A
    Judging from the numerous references I have received, this electronic circuit, hobby circuit seems to be rather popular. Many published circuits which flash LEDs need 3 or more volts. This electronic 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 with the needed voltage. This electronic schematic will only work on red, green and yellow LEDs. It is not be able to flash white LEDs.
  • 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.
  • 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.
  • 2 LINE TELEPHONE LINE STATUS INDICATOR
    Many home businesses use multiple phone lines. This circuit gives you a visual indication when a line is in operation. The two AA battery cells should provide enough power for about one year of operation. The circuit is line polarity insensitive.
  • 3V CAPACITANCE PROXIMITY SWITCH
    This circuit was designed to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years. It is discussed in more detail in the section on Capacitance Proximity Switch Technology.
  • 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...
  • 9v Battery Voltage Monitor
    Nine voltage batteries are used in a wide variety of electronic devices. But, you often don’t know when it is time to change the battery until the device stops operating. The hobby circuit below provides a visual indication when the battery needs to be replaced. It uses a neat IC from Linear Technology, which contains a very low power voltage reference and a voltage comparator. The circuit is designed to turn on a LED indicator light when the battery voltage drops below 7.2 volts.
  • Audio Signal Detector Switch
    For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line. The hobby circuit below will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal. The switch could be used to turn on a relay, routing the signal to where it is needed. A single 3v lithium cell will power the electronic circuit for about 5 years.
  • 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.
  • 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.
  • Clear Fluid Monitor
    An optical technique seemed like the best choice for this application. The simple circuit below works well but only when the light detector can be shielded from bright ambient light. A visible red LED launches light pulses through the side of the clear tubing and emerges at the opposite side. A phototransistor forms a light detector. The circuit takes advantage of the lens effect from any clear fluid inside the tubing. When there is only air in the tubing, the light scatters and spreads out quickly before it reaches the phototransistor....
  • 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.
  • DOORKNOB ALARM
    I designed this electronic circuit, hobby circuit many years ago and it seems be a very popular construction project. 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....
  • 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. (this link is off-site) The system can detect human and animal motion by the electric fields they disturb.
  • FINGER TOUCH ACTIVATED SWITCH
    It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch is not desired.
  • 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. Not all 74HCT74 devices will work in this circuit. I suggest using a quality device from Texas Instruments.
  • FLASHING LED POWER INDICATOR DRAWS LOW CURRENT
    This circuit flashes a power indicator LED to keep the average current low.
  • Freezer Over-Temperature Alarm
    A typical freezer can contain hundreds of dollars with of food. If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled. The hobby circuit below is a battery powered alarm system, which is attached to the outside of the freezer using a couple strong magnets. Two small gage wires are fed from the alarm box to a thermostat temperature sensor, mounted inside the freezer. The alarm beeper will be activated, whenever the electronic circuit detects a freezer temperature warmer than 0 degrees C (32 F). The overall current is so low that the 9v battery powering the device will last for many years
  • Freezing 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....
  • Home Made Geophone Detects Foot Stomp
    A home made geophone is made from a strong magnet, a coil of wire and a rubber band. The circuit is sensitive enough to detect the vibrations of a nearby foot stomp. It could be used as an earthquake detector.
  • 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.
  • Infrared Safety Switch
    Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch.
  • LONG PERIOD COMPUTER WATCH DOG TIMER
    This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer. Should the computer fail to send a pulse, the output changes state. The time can easily be set from seconds to hours.
  • 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
  • Low Power Oscillators
    OK, you may not ever need an oscillator to function using a power supply voltage below one volt, but if you ever do need one, here is one way to do it. Getting any electronic circuit to operate below one volt is a real challenge. Typical silicon bipolar transistors don’t work below 0.7 volts. Some old germanium transistors do work down at low voltages but those are hard to find and are usually offered only in large packages....
  • MACHINE VIBRATION TRIGGERS HOUR METER
    This circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine. Machine vibrations are detected by an off-the-shelf piezoelectric wafer. The wafer is normally used as a sound generator. The 3v power could be tapped from the hour meter's own lithium battery or from a separate 3v lithium battery. The circuit only draws about 1.5uA, so a 250mA-hr battery would power the circuit for about 10 years. Published in EDN, Jan 16, 1997
  • Motion Shadow Alarm
    The hobby circuit below works quite well in typical indoor room lighting. I would not recommend its use in direct sunlight. Two small PIN photodiodes positioned about one inch apart form a shadow detector. With no shadow cast on the devices, both devices produce nearly identical current levels. The current is converted to a voltage with a load resistor in parallel with the photodiode. One resistor is adjustable so the two voltages can be carefully balanced under uniform lighting. When a hand or an arm is moved over the sensors, casting a shadow,
  • 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.
  • OPTICAL INTERRUPTER DRAWS MICROAMPS
    This circuit is great for battery-powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery.
  • Oscillator Draws 2 Nanoamps
    This page shows two low frequency oscillators that draw ultra low current.
  • 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.
  • Piezoelectric Vibration Sensor Activates Switch
    An inexpensive piezoelectric wafer is used to detect vibration and when the vibration is sufficient a switch is activated.
  • MICRO POWER 40KHz BURST LASER DIODE DRIVER
    Some laser tag or simulated combat games can use this circuit to send short bursts of modulated laser light at the opponent's vest, equipped with a matching light receiver. The circuit operates from three 1.5v cells (4.5v) that should provide enough energy for about 200,000 shots.
  • MICRO POWER OVER-TEMPERATURE ALARM
    The circuit is powered for years by a single 3-volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a cheap but accurate thermostat as the temperature sensor.
  • 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.
  • MICROPOWER PULSE GENERATOR
    it is hard to design a pulse generator that produces clean logic pulses while drawing very low current. This circuit is designed to produce short 2mS pulses at a rate of one per second while drawing only 1 micro amp from a 9-volt battery.
  • Motion Alarm
    Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm.
  • Motion Shadow Alarm
    The hobby circuit below works quite well in typical indoor room lighting. I would not recommend its use in direct sunlight. Two small PIN photodiodes positioned about one inch apart form a shadow detector. With no shadow cast on the devices, both devices produce nearly identical current levels. The current is converted to a voltage with a load resistor in parallel with the photodiode. One resistor is adjustable so the two voltages can be carefully balanced under uniform lighting. When a hand or an arm is moved over the sensors, casting a shadow,
  • 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.
  • OPTICAL INTERRUPTER DRAWS MICROAMPS
    This circuit is great for battery-powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery.
  • Oscillator Draws 2 Nanoamps
    This page shows two low frequency oscillators that draw ultra low current.
  • 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.
  • PRECISION ULTRA LOWER POWER OSCILLATOR
    This circuit works much like the classic 555 timer, but draws only about 1.5 micro amps from a 3 volt battery. It is highly stable under varying temperature and supply voltages.
  • Pump Motor Monitor
    The traditional method for determining if a motor is running or not is to measure the current flowing through one of the wires supplying power to the motor. When the motor is off, the current should be near zero and will jump to the running current when the motor turns on. The classic sensor for measuring AC current is a current transformer. In a typical AC current transformer configuration, one of the wires carrying current to the motor would be routed through the transformer. The one wire forming a single turn primary. The magnetic field produced by the current flowing through the wire induces a voltage in the secondary winding of the transformer,...
  • Reflected Infrared Light Switch
    This hobby circuit has been used in some museum displays and exhibits when the electronics needed to trigger an audio/visual presentation is behind a plastic or glass panel. A finger pressed against the glass, over a designated area triggers the switch....
  • 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.
  • Solid State Relay Requires Ultra Low Control Current
    Most solid state relays require at least 5ma and often more input control current, to fully turn on the device. But such current levels often force battery powered circuits to use excessively large batteries. The relay hobby circuit shown below demands only 50uA of input current. This about 100 times lower than that needed by a typical optically isolated solid state relays. The circuit uses a combination of a high current triac and a very sensitive low current SCR, to control up to 600 watts of power to a load, while providing full isolation and transient protection.
  • Thermocouple Temperature Activates Relay
    Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F.
  • THREE 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
  • 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....

· Ultra Low Power 32KHz Crystal Oscillator
I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 micro amps.

  • 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 micro amps. With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal. (added 12/04)
  • Ultra Low Power LED Flasher
    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 King bright.....
  • Universal Flasher using a Latching Relay
    Latching relays are nifty devices. Most contain two separate coils. When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil. Since the latching and unlatching pulses only need to last about 25 milliseconds, it is possible to control a sizeable amount of power using little energy.
  • UNPLUGGED POWER CORD ALARM
    Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries. However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries. So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time. The device was designed to snap onto the outside of a power cord, where it senses the AC electric field emitted. No direct connection to the internal wires of the power cord is needed. I also included a low battery voltage monitor.
  • VERY LOW POWER ASTABLE MULTIVIBRATOR
    This classic circuit draws only 200 nanoamps from a 1.5v supply.
  • VERY LOW POWER GATED CRYSTAL OSCILLATOR
    The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input of a C-MOS buffer when clock pulses are needed. The technique gets around the problem of a slow starting crystal oscillator by keeping the oscillator going and switching on a transistor power stage only as needed. The method keeps the standby power consumption to a very low 1uA when used with a 3v supply.
  • Very Low Voltage Oscillator
    OK, you may not ever need an oscillator to function using a power supply voltage below one volt, but if you ever do need one, here is one way to do it. Getting any electronic circuit to operate below one volt is a real challenge. Typical silicon bipolar transistors don’t work below 0.7 volts. Some old germanium transistors do work down at low voltages but those are hard to find and are usually offered only in large packages....
  • WATER SEEPAGE ALARM
    This simple circuit sounds a beeper when its electrodes detect water. It is powered by a single 1.5v N cell. A small 1.5v button battery will also work.
  • XENON LAMP FLASH DETECTOR
    This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp. The coil makes the circuit immune to normal room lights. Its 10mv sensitivity can detect light flashes from a range of over 100 feet. Reflections from a room’s walls and ceiling is usually enough to trigger the circuit. The entire circuit draws only 3 Micro amps from a 6 to 9 volt battery.
  • Electrical Device Harvests Earthworms
    A 120vac or 240vac isolation transformer is used to force earthworms from the ground. (added 7/06)
  • Visible Light Blocking Filter
    This isn’t an electronic circuit but the drawing below shows the curve for an optical filter made from exposed color film. The material blocks most visible light but is very transparent to 880nM infrared light. Kodak 100ASA is exposed to "cool white" fluorescent light for about 5 seconds.
  • Wave Power Generator Demo System
    An inexpensive stepper motor is used as a voltage generator in this wave generator demo. (added 7/06)
  • Worm Harvester
    When I was a kid my engineer uncle built a circuit similar to the one below. My cousin and I used it to force earth worms out of the ground to be used for fishing. It worked like a champ. Please be careful since there is high voltage at the probe tips.
  • HO Train Model Lighthouse Flasher
    HO train sets often have authentic looking scale model homes and buildings. The hobby circuit below can be mounted inside a model lighthouse. The electronic circuit drives a single LED lamp in such a way that it produces light which simulates the rotating light from a lighthouse beacon. It uses a dual low power op amp IC.
  • Miniature Lighthouse Light Simulator
    The circuit I came up with is shown below. It uses two dual low power op amps. The first two devices form a classic oscillator circuit whose output is a triangle waveform signal. That signal is routed to a current regulator circuit, which converts the triangle voltage signal to a triangle current signal through the lighthouse LED. A third op amp generates pulses timed so they occur at the peak of the triangle waveform. The result is a LED, which gradually grows brighter, then flashes even brighter before dimming again. This should produce a light, which simulates a rotating lighthouse light.

· Electrode less Water Level Monitor
This system operates much like a classic float switch but without any moving parts. The illustration shows how a system using two copper coils sealed inside a plastic pipe, can detect the level of water outside the pipe. Whenever the water level is lower than the upper coil, no signal is coupled between the coils. Once calibrated, this technique might also work on the outside of a plastic water tank. (added 7/06)

· WATER SEEPAGE ALARM
This simple circuit sounds a beeper when its electrodes detect water. A single 1.5v N cell powers it. A small 1.5v button battery will also work.

· Delayed Pulse Generator
This circuit generates a short 10ms pulse 15 minutes after a “start” pushbutton switch is activated.

  • 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.
  • 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.
  • Charge Coupled MOSFET Relay
    This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current. The differential oscillator supplies voltage to the gates of the two FETs with good isolation will draw only 1.5ma of current.
  • 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.
  • Circuit Protects from Battery Polarity Reversal
    This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.
  • FINGER TOUCH ACTIVATED SWITCH
    It does not get any easier if you want a solid-state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch is not desired.
  • 300V PEAK TO PEAK SIGNAL GENERATOR
    This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times.
  • VERY LOW POWER ASTABLE MULTIVIBRATOR
    This classic circuit draws only 200 nanoamps from a 1.5v supply.
  • 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 mill volts to hundreds of volts. The output current is only limited by the rating of the transistors. I have used this circuit beyond 2MHz.
  • CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
    This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.
  • CHARGE PUMP EFFICIENTLY DIVIDES SUPPLY VOLTAGE
    This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value.
  • 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.
  • Home Made Geophone Detects Foot Stomp
    A home made geophone is made from a strong magnet, a coil of wire and a rubber band. The circuit is sensitive enough to detect the vibrations of a nearby foot stomp. It could be used as an earthquake detector. (added 7/06)
  • Motion Alarm Using Piezoelectric Device
    An inexpensive piezoelectric device is used as a motion sensing device for this motion alarm. (added 7/06)
  • Pigeon Scarecrow
    I have an area on my roof where a large flock of pigeons like to roost. Their nesting materials and droppings really make a mess. Often there is enough junk from them that it clogs my roof gutters. Perhaps one way to scare them away is with a device mounted on the roof near their favorite roosting spot. The device....
  • Shadow Activate Motion Detector
    This circuit can sound an alarm when the shadow of a hand or arm moves over two small photo diodes.
  • 555 Timer Forms Simple PWM Motor Controller
    Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor.
  • Automatic DC Motor Brake
    In many DC motor powered systems you would like the motor to quickly come to a full stop. This circuit provides an automatic electronic braking action to any DC motor ranging from 6v to 24v up to 1 amp of current. Some of the component values could be changed for larger motors....
  • DC Motor Brake
    When the power switch is closed, the gate voltage of Q1 turns Q1 on. The drain of Q1 swings low, turning off Q2. Current flows freely into the DC motor with only one diode drop from D2. When the power switch is open. The gate voltage of Q1 swings low, causing the drain of Q1 to swing high. This turns on the transistor Q2. With Q2 on, the resistor R3 is switched in parallel with the motor, producing a braking action. The value of R3 can be selected for the desired braking action. A zero ohm resistance will produce the fastest slow down of the motor. The circuit below is good for about 500ma of motor current. A Q2 transistor with a lower on resistance would be needed for bigger motors. Also, D2 would need to increase in size

· Low Voltage H-bridge
TTL type Q and inverted Q inputs control a classic H-bridge circuit, rated at 50 volts and about 10 amps. The circuit can control power and direction of a DC motor.

  • Medium Power 12v Brush Motor Speed Controller
    The circuit is a speed control for a medium power 12v DC operated single phase brush motor. It uses the brush current pulse method to monitor the motor speed. It includes a method to limit the average motor current. I have also included a means to turn on and off the motor from a simple TTL logic signal. The circuit uses some rather old op amp ICs I had laying around. Other op amps and voltage comparators could also be used. The circuit uses a pulse width modulation (PWM) scheme to control both the motor speed and the maximum average current. The current control is handy when you want to limit the motor torque, since motor current and torque are directly related. The circuit uses the current pulses from the motor brushes to extract the motor speed. For the motor I used, there were exactly 12 pulses per shaft rotation. The typical average motor current was about 1.2 amps with a speed of about 1000RPM. At 1000 RPM, the pulse frequency works out to be 200Hz.
  • Motor Brake
    This circuit will bring a fast moving DC motor to a stop in seconds. The braking action is automatically applied whenever DC power is removed.
  • PWM Circuit for Motor Speed Control
    Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about 3 volts to 15 volts. The frequency is fixed at about 2KHz but the pulse width can be varied from nearly 100% to 0%. ...
  • Two Pushbutton Motor Controller
    Two small pushbutton switches, a few diodes and two relays form a method to control on/off power to a brush motor as well as the motor direction. The circuit was originally designed for a motorized lifting platform.

· 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.

· 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.

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