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Using a spst as a momentary...

S

synaps3

Gawd
Joined
Mar 15, 2006
Messages
957
I am working on a case mod, and I have an interesting question.

I have an LCD that I need to turn on, which uses a momentary push-switch. I will be making a custom control panel thing, and want to use a nice spst switch I have instead of the momentary. Is there some way I can do that, so that it appears to be an on / off switch?

Also, I have another device that I want to power in the same way, but instead of being a momentary, it needs to complete the circuit for about 2 seconds. How would I go about making something to do that?

Thanks for any / all help in advanced.
 
In both cases you can use a 555 monostable (see Alternative monostable circuit), running from the 5VSB line, which will output a single pulse when the switch is turned on. Pulse time depends on a simple resistor-capacitor combination.

In the case of the power switch, the pulse will be used to turn on an open-collector NPN transistor, shorting the appropriate wires from the usual mom switch connections.

You'll need to turn the switch back to the 'off' position before re-operating.
 
It might also be possible (maybe) to modify the switch so that it will return to its "off" position by spring action. This may be more difficult than it's worth, though. Depending on how long the button has to be pressed to turn it on, you might even be able to get away with nothing more complicated than two resistors and a capacitor--even simpler than cpemma's design!
 
There is such a thing as a momentary on toggle switch. Their spring-loaded so they only stay "on" when you hold it in the on position. As soon as you let go, it springs back to "off".
 
SarverSystems said:
There is such a thing as a momentary on toggle switch. Their spring-loaded so they only stay "on" when you hold it in the on position. As soon as you let go, it springs back to "off".
Click to expand...

Yep, we have a bin full of 'em here at work. MOM toggles, and the more usual ON/OFF/ON toggles or two-position toggles. They are spring loaded indeed.
 
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