I had some heated seats retrofitted to my car. I have wired them so that a 12 V ignition signal activates a relay and powers a bus bar which then powers the seats.

The switches are round ON-OFF-ON switches. So if someone leaves the passenger switch ON and I just get in and don't notice, the seat will always be on.

My query is: can a relay, latching relay or something else be put inline somewhere that will, in effect, reset the power to each seat?

This means that if you left it on and cycled the ignition, you would then need to go to OFF, then back to ON to get the seat to work on the rocker switch. This is essentially overriding the switches' physical state.

  • \$\begingroup\$ Why are they ON-OFF-ON rather than ON-OFF? What's the difference between the two ON settings? \$\endgroup\$
    – Transistor
    Nov 17, 2020 at 10:43
  • \$\begingroup\$ @Transistor High and low temperature \$\endgroup\$ Nov 17, 2020 at 14:39

1 Answer 1


It's possible, using one standard relay and 2 push buttons per seat.

enter image description here

Four relays K1 - K4 are to be wired as shown for K1.

Each relay could be switched on or off with it's corresponding 'on' and 'off' push buttons. All the relays would go off together when the ignition switch is off.

Should 'single-button' operation be required for 'on' and 'off', the following ratchet relay based schematic would apply.

enter image description here

  • \$\begingroup\$ I am not sure this will work as it seems similar to what I have already. Key in the ignition currently controls the power to a relay to give/take away power to the seats What I want really is a momentary switch that pressing once goes to low, then again for high, then again for off, then cycling the ignition resets the switch so all is off \$\endgroup\$ Nov 17, 2020 at 9:42
  • 2
    \$\begingroup\$ @BenPettman This is a well known circuit often used in industry to prevent machines from starting back up by themselves after a power failure. When you press the on button, the relay is activated and a NO contact from that relay bridges the on button, causing the relay to keep itself activated. However, the relay needs power to do that. If you interrupt power in any way (ignition switch) the relay will open and will require another press of the on button when power returns. \$\endgroup\$ Nov 17, 2020 at 10:49
  • \$\begingroup\$ @Unimportant, Many thanks for your clarification. \$\endgroup\$
    – vu2nan
    Nov 17, 2020 at 11:02
  • \$\begingroup\$ Hi Ben Pettman, It'll most certainly work! However, it appears that you require 'single-button' operation for 'on' and'off'. Hence another circuit, which could do that, has been included in the answer. \$\endgroup\$
    – vu2nan
    Nov 17, 2020 at 11:57
  • \$\begingroup\$ The switch to operate the seats is a on-off-on switch, there are 2 states of on and one that is off. Essentially high, low and off. Does this make a difference? \$\endgroup\$ Nov 17, 2020 at 14:38

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