Hello,

Can someone explain the purpose of the capacitor connected to the output pin of the first 555 monostable? Why
is it there and what does it do?

TIA

PJ

 

purpose of the capacitor connected to the output pin of the first 555 monostable?

It AC couples the trigger to the second 555 timer. Otherwise, the output of the second timer would have to be HIGH for the duration of the time the first timer output is LOW.

I'd assume that's explained in the text.

BTW, that's a poor symbol for the timers if you haven't memorized pin function.

 

A series capacitor followed by a load resistor constitutes an RC high-pass filter.
It is also a differentiator, in other words, it outputs the derivative of the input signal.



If the load resistor is tied to ground, then the output is a positive pulse and a negative pulse about a 0 V baseline.

If the load resistor is tied to Vcc, then the output is a positive going pulse greater than Vcc and a negative going pulse going below Vcc. Hence with regards to the specific question, the second 555-timer circuit is triggered only on the negative going edge of the output of the first 555-timer, and not the positive going edge. Hence the purpose of the first 555-timer is to produce a delayed trigger while the second 555-timer sets the output pulse-width.

 

In summary, the TRIG input of the 555 is level triggered and the output will stay high as long as the TRIG input is low, so the capacitor coupling generates a short negative-going pulse to trigger the second 555 when the first 555 times out and the output goes low, to avoid that problem if the TRIG input stayed low.

 

Here's what Signetics had to say:


EDIT: Add schematic.