I'm looking for what circuit I'd need using basic components to do this task.

Essentially I have a switch that turns on two relays, one of which needs to be on as long as the switch is depressed, but the second I would like to only activate for half a second or so, possibly longer, it's something I would have to test out and fine tune.

What's the best way of going about this.

Edit: I didn't feel like firing up eagle cad so here is a quick and dirty schematic of how I have it currently set up.

Using the example from the link BeB00 provided I added the 555 timer, sans high pass filter into my circuit.

The end goal is to have relay 2 stay on continuously while SW1 is depressed, but relay 1 needs to turn on for ~500ms, and then turn off, until the next time SW1 is depressed.

At the current state, regardless of whether a positive, negative or no connection is made to the trigger of the 555 timer, it does seem to trigger ever half second or so, but does this continuously when power is applied.

enter image description here

I would be able to use a positive or negative signal to the 555 trigger, positive is just the most convenient in it's current state, but the linked page by BeB00 does state it needs a negative signal.

I did have to substitute the suggested 10uf ceramic capacitor for a 20uf electrolytic capacitor, if that makes any difference, however I don't think that it should since it's not interacting with the trigger.

Edit 2: I did find this page which has the 555 timer connected differently, and appears like it may perform the function that I need. https://www.utm.edu/staff/leeb/3b3.htm

I'm not quite sure if the 555 timer example also fires for half a second though or if that is only the 74121

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    \$\begingroup\$ This is probably an X Y problem, but use a 555 in monostable, with a high pass filter on the input (electrosome.com/monostable-multivibrator-555-timer) \$\endgroup\$ – BeB00 Apr 6 '19 at 3:44
  • \$\begingroup\$ I'll look into that, I was figuring a 555 might be able to do it but I've never played around with them. I should have one sitting around somewhere. \$\endgroup\$ – XOIIO Apr 6 '19 at 4:22
  • \$\begingroup\$ I'd use an Arduino over a 555, it's simpler to set up and more versatile. Note that you'll need some kind of buffer on the output, just like with the 555. \$\endgroup\$ – Drew Apr 6 '19 at 5:34
  • \$\begingroup\$ Yeah, I have several pro mini's kicking around, but I want to keep this thing as analog as possible, and while the boot time for an arduino is short, it's still slightly annoying and less robust than the 555 solution would be. I found my 555 timer so I can give this a try and always change my mind later. \$\endgroup\$ – XOIIO Apr 6 '19 at 5:39
  • \$\begingroup\$ Alright, so I don't think that I have what I need laying around to make a high pass filter, I gave this a shot with what I had, I also had to substitute C with a 20uf electrolytic instead of a 10uf ceramic capacitor, it does seem to be triggering for about half second intervals, but I realized after that this isn't single shot which is what I need, unless the high pass filter would achieve that. I did try supplying the trigger pin with positive and negative connections but it triggered on both and with pin 2 free, not sure if this is expected or due to the electrolytic cap I used somehow. \$\endgroup\$ – XOIIO Apr 6 '19 at 6:32

You're soooo close. But, there are a couple problems with your schematic. First off, when the trigger pushbutton is not depressed the 555 trigger pin is OPEN. That means it could/will pick up noise and cause false triggering. Never leave inputs open unless expicitly allowed in the data sheet. Second, the 555 datasheet says that the circuit will trigger when the trigger pin is brought below 1/3 VCC so you need to use a negative trigger, not a positive one like you have. I urge you to read the datasheet carefully. The applications section usually provides an example circuit which is a good starting point.

I also note that there are no protection diodes on the coil drivers. These prevent the inductive kick, when the coil turns off, from blowing up the 555 output stage.

Here are my recommendations for cleaning things up. I'll call the 555 as U1:

1) Use a 10K pullup resistor from VCC to U1 pin 2 to keep the 555 stable (untriggered).

2) Connect the switch from pin 2 to GND, not VCC.

3) Connect the 2nd relay coil from VCC to the switch. Closing the switch will energize the coil but allows you to have the switch connected to GND.

4) Add a 1N4148 or similar diode across each relay coil. Anode to lowest voltage, cathode to highest voltage.

5) Consider using a 0.1uf bypass cap at U1 (the 555) from VCC to GND. It's always good design practice to filter the power supply to any IC.


You need a high pass filter on the input of the 555. The resistor/capacitor values here are fairly arbitrary, but they should work. You should also implement Randy's points 4 and 5 to generally improve your circuit.

If you don't implement a high pass filter, the output of your 555 will remain activated if you hold down the button longer that 500ms, until you let go of the button.


simulate this circuit – Schematic created using CircuitLab

Edit: I just realised that you want to control the second relay as well. Doing this is sort of possible by rearranging the components, but i would probably just use an arduino. The values here are a bit more critical.


simulate this circuit

  • \$\begingroup\$ An excellent point about the retrigger problem. My design did not deal with the situation where the user continues pressing button. This post does deal with that (UPVOTE) though I would probably call it an AC-coupled trigger as for me the term filter conjures up noise suppression. \$\endgroup\$ – Randy Nuss Apr 7 '19 at 14:24
  • \$\begingroup\$ I'll give the mix of these solutions a try when I can get the parts. Noise suppression/filtering on the input would be good anyhow as there's a high voltage transformer triggered by one of the relays and it probably sends some noise back along the input. \$\endgroup\$ – XOIIO Apr 8 '19 at 0:35
  • \$\begingroup\$ If you're finding issues with this set up, you can try to add a more complicated passive network, but at some point, a microcontroller becomes far easier. The issue is that you're trying to control a digital input and a high power, noisy device with the same switch. If you used a transistor (instead of directly driving the relay with the switch), that would probably simplify things. \$\endgroup\$ – BeB00 Apr 8 '19 at 1:45

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