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Question about triggering cd14538

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martin_t

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Hi all.
I've been doing some tests with a circuit based around a cd14538.
In the datasheet it says it can be triggered by either positive going edge or negative going edge.
In this situation I need it to be trigered by the second option (negative).
During breadboarding I noticed that the IC triggered with positive edges despite the fact that it was configured the other way. Could it be because of the wires' inductance causing some sort of oscillation?
I decided to put a low pass at the trigger input (circled in red) to smooth the signal a little and it worked, but I wanted to ask if anyone had some experience with this IC and faced the same situation.

temp_sch.jpg

Any comment is welcome.
Thanks very much.

Martín
 

martin_t

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These triggers inputs have ~30% hysteresis inputs, so your noise must be extreme.

I didn't see any noise in the scope. Just the usual.
Also, the trigger signal is DC. I'm not switching at any frequency. Just opening and closing from a XOR gate's output.
 

Audioguru

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I betcha the messy long wires all over your breadboard and the many strips of contacts on it are picking up mains hum, radio stations and more interference (crosstalk?) that causes false triggering. get rid of the breadboard and use a compact pcb layout instead.
 

crutschow

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At the time of breadboarding, just connecting a cable to +V to the XOR input (CD4030).
Eventually, it'll be a switch (push button).
That's the worst possible way to make a pulse signal. :wink:
Touching the wire to the +V will almost certainly give multiple bounce signals, which the circuit interprets as multiple pulses.
You need a good debounce circuit.
 

Audioguru

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A pushbutton switch will also create many bounce signals each time it is pressed. In "the good old days" I fixed it with a Motorola Cmos debounce IC called MC14490. I am surprised to see that they are still available, but surface-mount only.
 

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