I wanted to ask for some help regarding the setup of a 555 timer. I'm using it in the standard monostable mode, running at 5 V, basically as a 1 min timer, using a simple momentary pushbutton to trigger.
Now what I want to accomplish is that when triggered (meaning the output is now high for 1 min), if I actuate the momentary switch again during this 1 min high period, I reset the timer for an additional 1 min. So let's say I trigger the 555, and 30s later I hit the switch again, I'd like the output to now stay high for a cumulative 1:30 min. Is there a simple way to achieve this, perhaps with some additional transistors/MOSFETs and capacitors? Surely there already exists a neat solution for this "retrigger when already triggered to extend timer" mode of action? Ideally it would stay HIGH during the whole time, but it is OK if it goes HIGH-LOW-HIGH during the retrigger, as long as it is 'automatic', with no additional switch inputs.
For what it's worth, I tried (simulating at falstad.com) a simple P channel mosfet across the timing capacitor, with it's gate connected to the trigger pin, so every time the trigger is actuated, it sees a ground pulse, but it doesn't fully discharge the timing capacitor (only takes it down to ~2V, I guess due to GS threshold voltage)?
EDIT Well I retried the same setup as above (bridging the timing capacitor) with a PNP transistor this time, and it works perfectly, well almost, now pulling the timing cap all the way down to ~0.6V. Can someone explain why the transistor is pulling lower than the mosfet, I guess it's to do with the difference in 'threshold voltage', the mosfet requiring a higher difference between gate and source than the transistor between base and emitter?
I guess this can also be called a "Missing Pulse Detector" circuit for the 555.
Thanks for any help!