I need to create a simple circuit that toggles when one pulses a push button and then stays on no matter what you do with the push button again. The toggle will reset when the system is powered off (starts off)

Any clues where to start from ? I've playing with relays,push buttons and those but .. cannot really get it. I'm currently working with logic gates to see where it leads..

Any idea ?

  • 2
    \$\begingroup\$ You can use an SR latch with a power-on reset circuit. \$\endgroup\$
    – kva
    Commented May 29, 2017 at 7:10
  • \$\begingroup\$ A logic gate is not a passive device – so, I'm not sure what you mean with "passive circuit", can you elaborate? \$\endgroup\$ Commented May 29, 2017 at 7:21
  • \$\begingroup\$ Sure, What I meant is not micro-controllers, nothing that needs to be programmed. I edited my question. Thanks for the detail. \$\endgroup\$
    – javirs
    Commented May 29, 2017 at 7:59
  • \$\begingroup\$ A confusingly-written question, I'm afraid. "no matter what you do with the toggle again" - but I couldn't anything with the toggle, on a push-switch. The toggle is the output. What's a toggle here, anyway? Please describe it like you're talking to someone trying to draw it on a blank sheet, giving voltages, output types, everything - skimp nothing. Diagram too. \$\endgroup\$
    – TonyM
    Commented May 29, 2017 at 8:51

2 Answers 2


This is actually quite simple to do with a single SPST relay:


simulate this circuit – Schematic created using CircuitLab

This is completely generic. The power source can be AC or DC. The only requirements are:

  • Both the switch and the relay contacts must be rated to handle the load current.
  • The relay coil must accept the same power (AC or DC, same voltage) as the load.

When you push the button, power is applied to both the load and the relay coil. After that, the relay contacts keep the current flowing until the power source is removed.

I once built a high-power version of this to run all of my computer systems. I was living in an area that was subject to bursts of short-term power outages. Since computers and other electronic gear don't particularly like to be "short cycled" like this, I used the circuit to make sure that once the power went off, it stayed off. I would restart everything manually by pushing the button, once the power had settled down again.

  • \$\begingroup\$ This is plain genius \$\endgroup\$
    – javirs
    Commented May 29, 2017 at 18:42
  • 1
    \$\begingroup\$ What is good about a solution such as this is that in addition to practically working, it illustrates in a very intuitive, mechanical way the concept of having something that once turned on will then continue to hold itself on - pretty much all other solutions will use a similar idea, but often leveraging some particular effect which is harder to initially understand. \$\endgroup\$ Commented May 29, 2017 at 19:47
  • \$\begingroup\$ hi, why relay is not shorting the power source for DC power? \$\endgroup\$
    – muyustan
    Commented Jun 4, 2020 at 16:37
  • \$\begingroup\$ @muyustan: Why would it? The relay contacts are in series between the power source and the load. The relay coil is just a small additional load in parallel with that. \$\endgroup\$
    – Dave Tweed
    Commented Jun 4, 2020 at 18:36
  • 1
    \$\begingroup\$ @muyustan: A relay coil designed for DC power has enough resistance to limit the steady-state current to a reasonable value. \$\endgroup\$
    – Dave Tweed
    Commented Jun 4, 2020 at 22:04

The most simple circuit can be made using a sensitive gate thyristor (SCR), like this:


simulate this circuit – Schematic created using CircuitLab

You can also make a one shot trigger with NPN and PNP couple immitating the SCR:


simulate this circuit Depending on load (pull up) current and transistors used R2 can be shorted.

Both circuits act in a similar way.


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