Capacitor for momentary switch?

Question

Dear all,I'm a complete novice at electronics and need help selecting acapacitor (I guess this is what I need).I have a momentary switch that is wired to a controller board.  Thecontroller has a resolution of one second, in that it scans it'sinputs once a second only.  Sometimes when I push the button too fast,the controller misses it and doesn't register the input.I'm hoping there is a way to connect a capacitor into the switchcircuit so that it's held closed for longer than a second no matterhow fast I push the button, and therefore registering the input on thecontroller without fail.  I've measure 5VDC across the switchingconnection, incase that information helps.Can anyone share a solution for this problem I'm having?  ...andperhaps even suggest the right capacitor to use, if in fact acapacitor will solve this?Many thanks,MrQuan

John Barrett · Accepted Answer

Capacitor wont do it -- you need a pulse stretcher, which may be more trouble than its worth.. you can make one with a 555 timer easy enough.. look for "one-shot" circuits where, when triggered, the circuit turns on for a specific period of time, then resets and waits for the next trigger.

EdV · Answer

Using a toggle switch might work.  Of course this may make operation alittle goofy but it is better than it not working at all.

John Fields · Answer

---Since you know the controller's limitations, why not just hold theswitch down for longer than one second?-- JF

MrQuan · Answer

Well, that's fine for me in the workshop, but this solution was really aimed more at the people whos offices and buildings these are installed in. The controller is used for HVAC and access control. I have seen some of my colleagues label push buttons with "push and hold for one second", I was just hoping for a more elegant approach!

MrQuan · Answer

I was actually looking at different switch alternatives, but I thinkfor aesthetic reasons a push button style is my only option as it willbe mounted to a small panel on a wall.

MrQuan · Answer

I see how this solution would work, however as you say it looks like more trouble than it's worth... I'll have to look at alternative solutions I guess. I have been mounting neon indicators next to the push buttons, so the user gets feed back whether they have actually switched anything - and this seems to be working. I was just hoping, if it were easy enough, to make it totally robust. Thanks

John Barrett · Answer

OK -- solution number 2 -- get a dual flip-flop chip (74LS74)... button hooks up to the Set line, which will "remember" the button press until the processor reads the data and deliberatly resets the flip-flop by pulsing the Reset line.. each chip will handle 2 buttons and no messy R/C timing needed.

Rich Grise · Answer

Then that is definitely the way you want to leave it - it protects thesystem from being turned on/off accidentally when somebody just bumpsthe button.Cheers!Rich

Don Lancaster · Answer

Have the pushbutton set a flip flop and the sampler reset it.-- Many thanks,Don Lancaster                          voice phone: (928)428-4073Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552rss:    email: Please visit my GURU's LAIR web site at

amdx · Answer

There are ways to use an SCR to stretch your pulse.Might take 1-cap 2-resistors and an SCR. Off the top of my head.                                         Mike

amdx · Answer

I posted a schematic on   of a circuit I used to debounce a switch, but I think you canmodify it to stretch your pulse.                             Mike

Capacitor for momentary switch?

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