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Using a push button as start for countdown timer with 74190 ICs

SandierAlex

New Member
I´ve been thinking for hours and just can't seem to find a way to do this. As a final project i need to make a presetteable countdown timer using 74190 ICs, i know how to preset numbers using the DATA inputs and how to connect between the 74190s, the problem is that after presetting a number i need to use a push button or a momentary action toggle to actually start the countdown. I did a diagram using a latched action instead of a push button, i would really appreciate some ideas to change the latch into a momentary action. Thank you
 

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SandierAlex

New Member
Im sorry, the 74190 is the one on the right but i hid the names bc they took up space. Do you have any suggestions for latching the pushbutton? Do you think i'll need to add a debounce circuit?
 

AnalogKid

Well-Known Member
Most Helpful Member
There is no clock source in the schematic. I'm assuming that by "countdown timer", you mean something that decrements at 1-second intervals. If so, then are you asking about an alternate-action circuit that enables and disables a 1 Hz clock into the timer with every other press?

If not, then please expand on your question.

ak
 

SandierAlex

New Member
Thanks for your help, i used a 7474 to latch the momentary action. But now i have another problem and after running out of ideas and things to try i need help. The thing is that, as a down minute counter i need to modify the tens and minute counters, to block them from displaying numbers above 5, to do this i connected the Q0 and Q3 outputs to an AND gate so that when the 74190 outputs a 9 (1001) the AND gate gives a 1 and inverting that i activate the PL input from the 74190, thus letting me load a 5 every time the counter reaches 9. The problem is that for some reason, the timer doesn´t work anymore, in the picture i put a logic probe to see the logic state of the XOR output, it should work but it doesn´t, it only works when the PL input doesn´t come from the outputs of the 74190.
1 is for increments of 1.
2 is for timer start.
3 is for timer reset.
 

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