noob Help with Monostable 555 Circuit

Thread Starter

DoctorWkt

Joined Aug 10, 2017
17
Hi all, I'm building a mainly logic circuit, and I'm using a 556 to provide a slow (1kHz) clock signal and a one-shot clock signal for debugging. The astable side is fine, but I'm stuck on the monostable side. This is so noob that I thought I'd ask here.

I've started with this 555 circuit which gives roughly a 0.11s period with R25 and C7.


I've redrawn the schematic for one side of a 556:



Here is a photo of the breadboard with the circuit:



The problem is, when I hit the pushbutton, the output goes to Vcc (5V) and stays high. I've checked the resistance of the 1M and 1K resistors. I've checked that both caps are marked 104. I've checked continuity on the discharge/threshold link. I've looked at the waveform on the discharge/threshold: it shows a square wave instead of the expected ramp. The astable side is showing a nice ramp waveform.

Can anybody spot what I've done wrong, and/or suggest other things for me to check? It's going to be so blindingly obvious, I'm sure.

Many thanks in advance for any ideas. Warren
 
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