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rising and falling edge detection problem..

rising and falling edge detection problem..

rising and falling edge detection problem..

Dear Mr./Mrs.,

I need your help about a problem on my circuit. I am using 74hc123 in order to detect rising and falling edges of square wave signal. I can detect with using 74hc123. there is another function of  74hc123 is to adjust duty length. Square wave's period is 20 ms. i can adjust the duty cycle with RC components. with using RC components i can adjust the duty cycle's period 4ms, 2ms, 1ms, 0.5ms, 0.25ms. but when I analyse in the scope from 1ms to down the 74hc123 don't work correctly. the output signal is not match with the theorical result. i tried both TTL and CMOS ic. after that i find another ic 74hc4538 and CMOS 4538. but i cant solve the problem. I am waiting for your ideas and solutions about problem.

Best Regards,

Oguz Dokuyucu

RE: rising and falling edge detection problem..

More information is required to answer your question fully as you don't say whether you are trying to trigger the monostable on both edges or just one edge.

If I understand correctly you are trying to produce a shorter pulse from the 20ms square wave (as suggested by your list of timing component results)? If so, then I would suggest that the 74HC123, or any family "123" device is not the best to use for the task. This is because it is a re-triggerable monostable and thus any noise spikes present on the 20ms signal could re-trigger the timing generator before it has reached the full timing period. This will result in a longer pulse than expected for the theoretical R-C values chosen.

A better device may be the non-retriggerable type 74HC121. This will give a fixed pulse length for each trigger event, however, do note that a noisy input signal can still trigger this type repeatedly as well. If the signal is very bad a large R-C filter followed by a Schmitt-input gate may be needed to clean up the 20ms signal, before it is applied to the monostable input.

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