Good morning/afternoon/evening everyone

This is my first time posting, so please excuse if the format is not 100%

I'm trying to build a circuit, that has logic pulses from a gap sensor as input. The circuit has to output a logic pulse once the count has reached 20.

So far I have successfully created a circuit that displays the count of logic pulses by using two seven segment displays. The displays also successfully reset once the counter has reached twenty, however there are two issues I'm struggling with:

  1. Instead of using a gap sensor, I have used an Ac voltage source, coupled with a diode, where I have adjusted the frequency, to essentially act as a source of logic pulses. Is this correct, or should I directly try to implement the gap sensor.

  2. I'm struggling to implement an output that outputs a logic pulse once the counters reached twenty.

I have included a simulation of the circuit. The top seven segment display counts the "ones" and the bottom one counts the "tens".

Thanks for your time.

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1 Answer 1


I don't see enough information to offer any advice on the sensor.

As far as detecting a count of 20, if you are sure that the counter will only count "up" you don't need to use a full comparator (such as two 74LS85s) but only need to 'AND' all the '1's in the desired count. In this case there is only a single 1, and that is bit 1 (QB) of the second counter. It will go high when a count of 20 is reached (and not before if the count starts at 00).

![enter image description here


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