A timer in electronics is essentially a device which is used for producing time delay intervals for switching a connected load. The time delay is set externally by the user as per the requirement.
A simple timer can be built through many countless options. You can transistors, CMOS gates like NAND gates, NOT gates, linear ICs like 555, 741, 324, or simply more specific type of CMOS IC like the 4060.All these devices besically generate oscillations which are adjustable right from a few Hz a second to a atomic fraction of a Hz.
Further more these ICs can be integrated together to form more complex timer circuits that may be used for generating very long time intervals.
In this post we see how two different modes of ICs are coupled together to form a long duration timer circuit.
Referring to the circuit diagram.
- IC1 is an oscillator counter IC consisting a built in oscillator stage and generates clock pulses with varying periods across its pins 1,2,3,4,5,6,7,9,13,14,15.
- The output from pin 3 produces the longest time interval and therefore we select this output for feeding the next stage.
- The pot P1 and the capacitor C1 of IC1 can be used for adjusting the time span at it pin 3.
- The higher the setting of the above components the longer the period at pin #3.
- The next stage consists of decade counter IC 4017 which does nothing but increase the time interval obtained from IC1 to ten folds. It means if the the time interval generated by IC1s pin #3 is 10 hours, the time generated at pin #11 of IC2 would be 10*10 = 100 hours.
- Similarly if the time generated at pin #3 of IC1 is 6 minutes, would mean a high output from pin#11 of IC1 after 60 minutes or 1 hour.
- When power is switched ON, capacitor C2 makes sure that the reset pins of both the ICs are appropriately reset, so that the ICs begin counting from zero rather than from some irrelevant intermediate figure.
- As long as the counting progresses, pin #11 of IC2 remains at logic low, such that the relay driver is held switched OFF.
- After the set timing lapses, pin#11 of IC2 goes high activating the transistor/relay stage and the subsequent load connected with the relay contacts.
- The diode D1 ensures that the output from pin#11 of IC2 locks the counting of IC1 by providing a feed back latch signal at its pin #11.
Thus the whole timer latches until the timer is switched OFF and restarted again for repeating the entire process.
R1, R3 = 1M
R2, R4 = 12K,
C1, C2 = 1uF/25V,
D1, D2 = 1N4007,
IC1 = 4060,
IC2 = 4017,
T1 = BC547,
POT = 1M linear
RELAY = 12V SPDT