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Circuits designed by David Johnson,
P.E.
Last Updated on:
Wednesday, November 30, 2011 01:24 PM
Master Category List -
Dave's Circuits
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Times &
Counter Circuits |
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36 Hour Watchdog Timer
A watchdog timer is often used in computer systems to detect when a
computer is hung up and is no longer running properly. If it does become
locked up and is not able to supply a reset pulse to the circuit within a
defined time frame the timer times out and activates some kind of alarm.
Watchdog timers can also be used for humans....
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- 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.
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Closet
Light with Automatic 3 Min Timeout
The circuit below is powered by three 1.5v alkaline AA cells. With a finger
tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned
on. The lights remain on for about 3 minutes, then will turn off. The
circuit’s standby current is only a few microamps. A set of fresh batteries
should last at least 200 light applications. The circuit uses a Schmitt
trigger inverter and two transistors. When the pushbutton switch S1 is
pressed, the 10uF capacitor C1 is discharged. When the switch is released, the
capacitor is charged through R1. With the capacitor C1 discharged, the Schmitt
trigger output swings to 4.5v, which turns on transistor Q2. The combination of R4
and Q1 form a constant current control circuit. The current is set about 15ma
per LED. When the capacitor charges up to about 2.3v, the Schmitt trigger
changes state, turning off the LEDs.
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- DARKROOM CAMERA SHUTTER
TIMER
This circuit was designed to control a film exposure shutter for a darkroom. It has
8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot
switch and draws power from an external 12 volt DC supply.
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- 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.
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Line Powered 60hz
Clock Generator
This circuit is connected to the 120vac power line and transfers 60Hz clock pulses
to a logic circuit. The optoisolator used provides 5000 volts of isolation between
the power line and the logic side of the circuit....
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- LONG PERIOD COMPUTER
WATCH DOG TIMER
This circuit uses a simple 4060 IC oscillator/timer which 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.
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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 microamp from a 9 volt battery....
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- 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.
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Poor Man’s Digital Counter Using
Pedometer
There are many
occasions when you may want to count something electronically. Perhaps it is
car traffic on a street or items moving down an assembly line. It might be the
number of times a machine is activated or maybe you want to count the number of
people entering doorway. Commercial counting modules do exist but if you want
to use something cheaper, you can modify a pedometer to do much the same thing....
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Poor Man's Timer
Often during testing of certain equipment and components, you would like to keep
track of the elapsed time in hours, minutes and seconds. There are some nice
commercial digital timers and counters available for just this sort of application.
However, although it may not be as accurate, there is a cheaper way to go. I’m
sure many of you have some old wrist watches or battery powered dial faced alarm
clocks collecting dust in some drawer.
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