I'm trying to build a circuit with multiple timers, each with an SPST output relay. I want to connect up my circuit so that if any of the timers reaches their set point, they turn on a beacon. It doesn't matter how many timers reach their set point, as long as one has I want the beacon to be on. Additionally each timer will turn on its own light when it reaches the set point.

I've drawn up a circuit, but have a question, is it possible to control two lights with one SPST switch? The simplest form of the circuit I'm visualizing is shown below:

(All the Lights and Beacon and Power Supplies are 12V)

enter image description here

The outsides (including the switches) are the timer circuit section, while the middle part is the beacon and its power supply.

I'm particularly confused about what happens in the red path. Would that be bad for the lights? Or turn them on before the timer switches close?

  • \$\begingroup\$ Seeing that there is 2 switches in the circuit shown here, I think the title of the question is misleading \$\endgroup\$
    – Tofandel
    Oct 15, 2023 at 21:43

2 Answers 2


No real need for 3 power supplies here, so I simplified. Specific parts are circuit-lab defaults for the part type, not suggested part numbers.


simulate this circuit – Schematic created using CircuitLab

Add 2 diodes and I think you could make it work. As it stands either switch will turn on all three lamps. In the "red path" the voltage sources (as drawn) cancel, so no current should flow. The "beacon" needs a diode pointing at each switch which will permit it's current to flow through the switch, but prevent the current from the "individual timer" light on the opposite side from flowing to the switch beyond.

  • \$\begingroup\$ If you don't like the defaults, you can take them off. It makes things less confusing that way. :) \$\endgroup\$
    – AaronD
    Feb 4, 2016 at 2:43
  • \$\begingroup\$ Great thanks!! That helps a lot. Any tips on what type of Diodes to use? Just anything that can take the current that might be flowing through? \$\endgroup\$
    – sam
    Feb 4, 2016 at 2:48
  • \$\begingroup\$ @AaronD Just spent 5 minutes fighting the stupid thing and it did not appear to want to accept the concept of not labeling them. Must be "user friendly" and "obvious." Pretty much any run of the mill diode should do, unless you really are using incandescent lamps rather than LEDs (which don't burn out, unlike incandescent lamps.) If you need big power, they make big diodes. \$\endgroup\$
    – Ecnerwal
    Feb 4, 2016 at 3:00
  • \$\begingroup\$ @Ecnerwal Hmm. I've never had a problem with blank entries. If it insists on having "something" there, then maybe a space or a dot? \$\endgroup\$
    – AaronD
    Feb 4, 2016 at 3:51
  • \$\begingroup\$ Dunno how you did it, but you had two of each component so when you deleted the labels of one, the one right behind would make it look like you didn't change anything. \$\endgroup\$
    – horta
    Feb 4, 2016 at 4:05

Here's a circuit that I think will do everything you're wanting. Each timer controls its own LED (D1-D3), and the 4th LED (D4) is in place of your beacon (if beacon takes too much power for a low-cost transistor, you can always run a relay or power MOSFET from the transistor to handle your load) and turns onn whenever any timer turns on its LED.

Logical OR Circuit


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