Electronic Circuits
Digitally Adjustable Timer electronic circuit
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This is the circuit for the digitally adjusted timer.
In this project, the timing can be adjusted digitally with the help of the 2 digital IC’S.
The relay gets operated as soon as the circuit is triggered and becomes off after a selected time period.
This kind of the ckt. Can be the heart of many industrial operation cycles, where the definite timings are to be selected manually very often.
The ckt description is as follows.
IC1 (THE FIRST 555) supplies the clock pulses of 0.5 sec. To ic2( 7490). A decad counter counts the number of clock pulses and gives the corresponding binary output at pins 12,9,8 and 11.
The decoder chip (7442) converts the binary output to a decimal output. Means, pin 5 is at 0v where as all the other output pins are at vcc(+5v).
For any count, the corresponding output pin is grounded and all the other pins are high.
Resistors RC and R1 to R4 are connected in such a way that the voltage at point A is different for different counts and the corresponding LED glows for the required count.
IC4 ( THE LAST 555) is connected as the mono-stable multi vibrator and its control pin5 is connected at point A such that for each value of voltage at A we get different –gate widths at IC4.
When leds 1 to 4 glow, the circuit operates approximately in 60 sec, 75 sec, 90 sec, and 105 sec.
Timer operating method: suppose one wants to set the timer at 105 sec, first press S3 once to ensure that the out put pin of IC4 goes low thtn press S1.
and select the led that is in front of the 105 seconds time.( led 4 glows). After releasing S1 press S2 to trigger the mono-stable.
At this instant the output of ic3 pin4 goes high. After approximately 105 seconds, the output of the timer ic IC4 ( LAST 555) will become low.
The put put drives a relay with a transistor bc548.
Any instrument connected to the relay can be made ON for a specified time with this ckt. And as the timings are selected by the digital IC’S its called the digitally adjustable timer.
Note:
If you have any confusions or questions, or if you need pcb design of this circuit, please contact on learnpcbdesign.online(at)gmail.com
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