how can i make the count down stop at zero and led illuminate when it reach zert


Joined Jan 29, 2010
hi mm.
Do you want it to count Up to 99, then Down to Zero.?

How do you expect to display the count on the 7 segment LED's using the LS192 outputs.?
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Joined Jun 17, 2014
I can give you a hint.

When dealing with binary counters like the 7490 or 7493 or the CMOS versions or any other binary counter, when you want something to happen at a particular count you use logic gate(s) to detect the count at the output of the counter(s), then have the output of those gate(s) do something that will achieve what you want it to do.

A typical use for this is when counting up to some count like 9, then reset it to zero when it reaches 10 (binary 1010).
For this i would use an AND or NAND gate to detect the binary state 1010 and have the output go to the RESET pin on the 7493 or whatever counter im using at the time.
For another example, when creating a 12 hour AM/PM digital clock one way to get the hours count to reset to zero after 12 hours is to detect the hour 13 with logic gates and have the output connect to the RESET pin on the counter. So the count goes like this:
0,1,2,3,4,5,6,7,8,9,10,11,12, and when it reaches 13 it quickly resets back to 0 and starts over again, and if you have an AM/PM indicator flip flop it toggles that flip flop too so that if it is AM it toggles to PM and if it was PM it toggles to AM.

In the past i worked for hours and hours with TTL logic because it was prevalent at the time. I built clocks and counters. One such device was a frequency counter. Having a gating time period of 1 second, the counters had to be reset every second. Detecting the states of a secondary counter driven by an accurate crystal, it was able to achieve this 1 second constant reset.
It's the same with most logic circuits like this although sometimes the count is detected in other ways when it is simpler.

So try that. Figure out what count you would need to detect, what kind of gate you would need, and what the output should do when it detects that count. If you need to detect two or more counts, then just repeat this procedure for each and every count.
Note some counts are easy to detect if the counter is known to count in only one direction. For example, if a counter is always counting UP and you need to detect '8' (binary 1000) then you just have to use the output of the last flip flop in the counter without any gates.

Please let us know what you end up doing, and good luck with it.