Multipurpose Flip-Flop Timer

This particular timing circuit can be used
to time one-shot events from a few seconds to a few hours. And in
standby mode (ie, with RLY1 and LED1 off), its power consumption is very
low. The heart of this circuit is a low-cost CMOS 4011 quad NAND
gate, with IC1a & IC1b configured as a standard Set/Reset
flip-flop. Briefly pressing switch S1 to start the timing sequence pulls
pin 1 of IC1a low and, as a result, pin 3 switches high. Two things
happen while pin 3 is high: capacitor Cx begins charging via
potentiometer Rx; and (2) pin 11 of IC1d will be low, which means that
transistors Q3 and Q1 are both on.

As a result, both LED 1 and relay RLY1 are also on. RLY1 and LED
1 remain on until Cx has been charged up to about 70% of Vcc (ie, the
supply rail). At this point, pins 8 & 9 of IC1c are pulled high and
so its pin 10 output goes low and resets the flip-flop by applying a low
to pin 6 of IC1b. This causes pin 3 of IC1a to go low and so LED1 and
RLY1 switch off and the timing period ends. At the same time, pin 4 of
the flip-flop goes high and this turns on transistor Q2 while ever the
flip-flop is held reset. This ensures that Cx is discharged, so that the
circuit is ready the next time S1 is pressed.

Multipurpose Flip Flop Timer Circuit

Multipurpose Flip-Flop Timer Circuit Diagram

Diode D1 and its associated 10µF capacitor reset the flip-flop when
power is first applied, so that LED1 and RLY1 remain off until S1 is
pressed. D4 is included to protect Q1 against the back-EMF
that’s generated when the relay switches off. Choosing appropriate
values for Cx & Rx for a given time delay is straightforward. The
formula is T = 1.24 x Rx x Cx, where T is the delay time in seconds. As
an example, let’s assume that we require a time delay of 10s using a
value of 100µF for Cx. Now we just need to calculate the value of Rx as
Rx = 10s/(1.24 x Cx) = 80,645O
In this case, an 82kO resistor would be the closest value. You can use
either a fixed resistor for Rx or you can use a potentiometer (or
trimpot) which can be adjusted to give the required time delay. Note
that the value of Rx should not be any more than a few megohms. Power
for the circuit can be derived from any 12V DC source. This is then fed
to 3-terminal regulator REG1 to derive a 9V rail to power the circuitry.
The exception here is the relay circuit, which is powered from the 12V
rail. Diode D3 protects the circuit against incorrect supply polarity.

Author: Trent Jackson – Copyright: Silicon Chip Electronics

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