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I need to make a circuit that has a momentary switch and a light (30mW). When the switch is pressed, nothing should happen, but when the switch is released, I want it to pulse the light for a short duration (around 20 - 30ms)

I think I might be able to accomplish this with a capacitor and some 555 timers, but I'm honestly at a loss as to how to assemble it, or if it can be accomplished with just these components.

I feel bad asking for help with something that sounds so simple. But honestly, I'm pretty ignorant in developing even simple circuits. The affordability of Arduinos has really made me lazy and it's a problem. In the past, if I needed to make a light blink, I used $2 nano clone from China. Now I need to make a real circuit and I'm at a loss.

Any help would be much appreciated!

Thanks.

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  • \$\begingroup\$ Sounds like you want something to debounce the switch (finicky, otherwise) and use the trailing edge of that to trigger a one-shot arranged for 30 ms, or so? \$\endgroup\$
    – jonk
    May 25, 2019 at 4:20
  • \$\begingroup\$ A 555 with a clever capacitor switch circuit will do but it will rake some tuning. Instead, consider a 74LS123 or 74123, which unlike a 555 are strictly rising or falling edge triggered. \$\endgroup\$
    – Heath Raftery
    May 25, 2019 at 12:10

2 Answers 2

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You can do this using a 555 in a monostable configuration and, as you said, a pushbutton switch and an extra capacitor and resistor to trigger the pulse. See the attached image.

R2 is a pullup resistor to keep the trigger high. The pushbutton switch will normally be connecting the capacitor to the trigger pin which will also keep the capacitor charged through R2. When the button is pressed the capacitor will be shorted to ground and simultaneously disconnected from the trigger pin, so the trigger pin will still be pulled up by R2. However, when the button is released, it will connect the now discharged capacitor to the trigger pin which will pull it below 1/3 Vdd momentarily, before it is recharged through R2. Per the datasheet, pulling the trigger pin below 1/3 Vdd will trigger the pulse set by R1 and C2, in this case to give you a ~30ms pulse.enter image description here

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  • \$\begingroup\$ Wow! Thank you very much for drawing this circuit up for me! I've got some 555 timers arriving today and will test the circuit out on my breadboard today or tomorrow. What program do you use for designing your circuits? \$\endgroup\$
    – sly2.0
    May 25, 2019 at 19:31
  • \$\begingroup\$ @sly2.0 I actually just downloaded the app, Quick Copper to sketch out the circuit for you. It seems like a very useful app, let's you export as multiple file types and has many built in component symbols. Let me know if theres anything else I can help you with! Many of the component values are approximate, so it would be good to do a once over with the 555 datasheet as a reference. Hope it works! \$\endgroup\$
    – Ben Watson
    May 26, 2019 at 6:45
  • \$\begingroup\$ Thanks again for the help. I was able to put the circuit together today and test it out. it does exactly what I was looking for! \$\endgroup\$
    – sly2.0
    May 27, 2019 at 7:40
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A duel switch, NPN transistor with a LED connected to the collector could work.

Collector of the transistor is connected to LED

Base of the transistor is connected to a capacitor. The base is also pulled low by a 100 kOhm resistor.

The other end of the capacitor is connected to VDD.

Emitter is connected to GND.

First power on case:

The LED will be on until the capacitor charges. Then it blocks the VDD thereby turning off NPN. So, with proper cap value you can choose the blink duration.

Switch press case

Where is the switch connected? This switch I assume it to be two switch in one package. Then by pressing, you effectively cut the VDD to the circuit with one switch and the other half part of the switch discharges the capacitor.

Releasing the switch is same as giving the power back to the circuit. The capacitor will again charge and the LED will be on for that duration.

You can also use another NPN to make use of normal switch types, but I cannot draw circuit now. Will share when I get to the computer.

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