I am thinking of making a set up whereby the latching relay will switch between two 9V circuits from separate power sources. However the relay coils themselves must be powered by either of the 9v running through either of the switching circuits. Thus, this may prevent the latching relay from working, but I wanted to get an opinion if this might still work. This latching relay coils are operated with 9v. The schematic of this latching relay is shown here: enter image description here

The first 9v circuit will run between pin 8 and 5, and the other independent 9v circuit will run between pins 9 and 12. My question is if the coil power is powered by either of the two switching circuits, can the latching relay still operate and make the switch? To explain further, what I mean is that assuming the relay starts off in a 'set' condition, I will use the 9v power supply running from pin 8 to 5 to power both set and reset coils. When I trigger a reset condition across pins 16 and 1, most certainly the 9v power supply from the circuit across pin 8 and 5 going to the set coil (pins 16 and 1) will momentarily falter, however, my question is whether will the reset coil still be able (to be fast enough) to make the relay switch to allow the circuit across pin 9 and 12 to work, or will if not work because the reset coil does not have enough power to make the switch?


simulate this circuit – Schematic created using CircuitLab

Circuitlab doesnt have the relay model I am seeking to use, however, shown here is the schematic of the cicruit I intend to use. Note that unlike the relay shown in the schematic, the configuration of my latching relay would be such that one of the 9v circuits would be connected at any time (set or reset). I have excluded the flyback diodes (which would otherwise connect from the GND to the 9v) to make the diagram less cluttered.

  • \$\begingroup\$ Please draw a schematic of your planned circuit. (I suspect that the answer will be obvious, also for yourself) \$\endgroup\$
    – MrGerber
    Jan 18, 2018 at 10:31
  • \$\begingroup\$ As long as one of the transistors are on, you have an oscillator, AFAICS \$\endgroup\$
    – MrGerber
    Jan 18, 2018 at 11:26
  • \$\begingroup\$ Do you realise that your LEDs are wired in parallel? \$\endgroup\$
    – Finbarr
    Jan 18, 2018 at 11:36
  • \$\begingroup\$ @MrGerber: Does that mean it may work? \$\endgroup\$
    – Craver2000
    Jan 18, 2018 at 11:40

2 Answers 2


I would combine the two supplies with diodes to provide power to the relay coils. And your LED seems to be in the wrong place so I've moved that too:

enter image description here

  • \$\begingroup\$ Hey, nice idea! \$\endgroup\$ Jan 18, 2018 at 11:57
  • \$\begingroup\$ But that is not an answer to OP's question. One of the relay contacts are not NO as in the attached diagram, but in fact NC. \$\endgroup\$
    – MrGerber
    Jan 18, 2018 at 12:07
  • \$\begingroup\$ @Finbarr: Thanks for the suggestion. The reason for why I am thinking of this set up is so that I can apply it potentially to a 9v solar power circuit with backup from another 9v DC supply. If I were to connect as shown with the diodes, the backup supply would be conitnously be supplying current. I would prefer if the solar power supply for instance would be the sole provider of power, and only when needed, the raspberry pi would trigger the relay to switch to the other (backup) 9V power supply. Hence, ideally, one 9V power supply circuit would be active at any time. \$\endgroup\$
    – Craver2000
    Jan 18, 2018 at 12:10
  • \$\begingroup\$ @Craver2000 No, it wouldn't be draining the backup supply all the time because the diodes only provide power to the relay coils - and that will only happen when the coils are turned on by the transistors, which will only be a short pulse. \$\endgroup\$
    – Finbarr
    Jan 18, 2018 at 12:42
  • \$\begingroup\$ @MrGerber That's right, one contact is NO and one is NC. That's how the relay switches power to the LED (and presumably whatever else the circuit is powering) from either the 9V power source on the left or the other 9V power source on the right. \$\endgroup\$
    – Finbarr
    Jan 18, 2018 at 12:45

This is an unusual use case for the relay and it probably won't be covered by the datasheet. Indeed there is a risk that the set/reset coil (or both) will not have enough time to toggle the relay before the contacts powering it get disconnected.

Putting flyback diodes on both coils should improve the situation, since those diodes will allow the current to flow in the coils even when the power supply is disconnected, increasing the chances to toggle the relay properly.

If flyback diodes alone are not sufficient, installing a big cap between the coil supply and ground (which will act as a back-up power source) should allow the relay to toggle properly.


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