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Circuits of the Week
Last Updated on:
04/27/2008 06:17 AM
(Dec 2007 - July 2007)
(June 2007 - Jan 2007)
(Dec 2006 - July
2006)
(June 2006 - Jan 2006)
(Dec 2005 - May 2005) |
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| June 25, 2006 |
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. |
| June 18, 2006 |
ACCEPTABLE VOLTAGE INDICTOR
I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed. The circuit can also be used to adjust a particular voltage be within specific high
or low limits. The three LEDs will indicate if the voltage is high, low or OK. When connected to other converters, such as a frequency to voltage converter, a current to voltage
converter or a power to voltage meter, it could provide a quick indication of a proper level. |
| June 10, 2006 |
Car Window Closer
This circuit automatically rolls up a car’s electric windows when it rains. The windows are rolled up one at a time while the system monitors the window motor current. When the motor stalls, the
system moves to the next window and shuts off it’s the last window. Designed by
Andrew R. Morris
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| June 4, 2006 |
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. |
| May 28, 2006 |
THROUGH GLASS PUSHBUTTON SWITCH
This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass. Pushing the button moves a magnet near the reed relay,
closing the relay contacts. |
| May 21, 2006 |
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. |
| May 14, 2006 |
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. |
| May 7, 2006 |
Pump Motor Monitor
This circuit solves this problem in a novel way. It clips on the outside of the pump motor’s power cable and provides a transistor switch closure whenever the circuit detects current flowing
to the motor. The circuit draws so little current that a small lithium cell will power the circuit for years |
| April 30, 2006 |
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 |
| April 23, 2006 |
Freezer Over-Temperature Alarm 2
This circuit turns on a beeper whenever the inside temperature of a freezer is greater then 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. |
| April 16, 2006 |
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. |
| April 9, 2006 |
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. |
| April 2, 2006 |
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. |
| Mar 26, 2006 |
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. |
| Mar 19, 2006 |
CASCODE LIGHT RECEIVER CIRCUIT
This page provides a detailed explanation of how the modified cascode light receiver circuit operates. The cascode technique in conjunction with an inductive load provides very high current
to voltage conversion as well as very high speed. |
| Mar 12, 2006 |
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 Flashing LED Advertising Badge #1, 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. |
| Mar 5, 2006 |
Brownout Voltage Tester
I used this circuit years ago to test AC line powered devices under 95vac line voltage conditions. It has a rating of 250 watts. |
| Feb 26, 2006 |
VERY LOUD 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. |
| Feb 19, 2006 |
CAPACITANCE TYPE LIQUID LEVEL MONITOR
This circuit was originally designed to monitor the level of liquid natural gas in a tank but it can be used to also measure almost any liquid. Two custom insulated metal tubes form a capacitor
plate. The capacitance between the two tubes increases as the level of the liquid rises. The circuit converts an increase of capacitance into a positive voltage change. The circuit could be
changed to handle almost any scale factor needed. |
| Feb 12, 2006 |
Xenon Lamp Flash Detector
This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp. When connected to a flip/flop it can serve as an on on/off
Controller. |
| Feb 5, 2006 |
Low Value Capacitance Meter
This circuit was originally designed to measure the volume of fluid inside a medical syringe. As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of
about 10 picofarads. With a digital voltmeter, at its output, it can resolve a capacitance change of 0.002 picofarads or 2 femtofarads. |
| Jan 29, 2006 |
MODULATED LASER DIODE TESTER
This circuit can either be used to test laser diodes or as a general purpose modulated light source. The circuit has an adjustable diode threshold current and an adjustable modulation frequency.
The frequency can be adjusted from below 30KHz to above 50MHz. It is powered from a 9v battery. |
| Jan 22, 2006 |
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. |
| Jan 15, 2006 |
Human Traffic Footstep Detector
This circuit uses a surplus geophone to detect human footsteps from 100 feet away. It was designed for monitoring the Mexico/USA border. This preamp part of the system draws about 50
microamps from a 12 volt supply. |
| Jan 8, 2006 |
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. |
| Jan 1, 2006 |
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. |
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(Dec 2007 - July 2007)
(June 2007 - Jan 2007)
(Dec 2006 - July
2006)
(June 2006 - Jan 2006)
(Dec 2005 - May 2005) |
