rising and falling edge detection problem..

[dokuyucu]

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
od@postaci.com

 

[BrianG]

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.