Tuesday, March 24, 2009

Analog Time Delay Circuit

Time Delay Circuits
Short timing functions such as a pulsed outputs or time delays
can also be created with one or two comparator sections.

Low Voltage, High Current Time Delay Circuit
In this circuit a LM339 quad voltage comparator is used to
generate a time delay and control a high current output at low
voltage. Approximatey 5 amps of current can be obtained
using a couple fresh alkaline D batteries. Three of the
comparators are wired in parallel to drive a medium power
PNP transistor (2N2905 or similar) which in turn drives a high
current NPN transistor (TIP35 or similar).

Turn on delay circuit
As you can see there is a RC timing network. Timing resistor is
variable type. Also there is a fast discharge network R2+Q1,
which provides fast discharge of capacitor when input signal
drops and fast turn off the relay . Q1-BD140 , D6-1N5819 or
similar, R2 -510R.

Turn off delay circuit
This circuit differs from upper circuit by momentary charging
timing capacitor. When input signal drops capacitor discharges through timing resistors

Alarm stays active until you turn it off.
A hidden touch point of two pinheads is all that you need.
Use a small capacitor at C1 (.1 uF) and it will appear to turn
off instantly (about a half second).
Or use a Push Button N.O. to instantly drain C1.

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