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I'm currently trying to design a circuit for my car which will switch itself OFF or ON depending on +12V signal from ACC(accessory) cable.

Car is providing me a +12V battery power, +12V accessory power and ground, i would like to make a circuit which detects +12V on ACC and then uses +12V from battery power.

Obvious solution is to use relay switch, but I'm curious are there any other options how it can be done ?

I'd be grateful for example circuits :)

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  • \$\begingroup\$ Why? Only difference between Accessory and Battery is that Accessory is switched on by the key... essentially a manual relay. \$\endgroup\$ – Passerby Feb 15 '15 at 21:52
  • \$\begingroup\$ @Passerby: On quite some systems accessory is regulated to 12V while battery is 12.xV or 13.xV when the motor is running. Additionally this regulation current limits accessory to a few amps max. \$\endgroup\$ – PlasmaHH Feb 15 '15 at 21:56
  • \$\begingroup\$ because on this particular acc cable only 100-200mA load is allowed/suggested by manufacturer - and my device will draw ~500mA-1A \$\endgroup\$ – NoderBlade Feb 15 '15 at 21:56
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    \$\begingroup\$ That would be important in the question. And @plasma I have yet to see a car where that is the situation. \$\endgroup\$ – Passerby Feb 15 '15 at 22:12
  • \$\begingroup\$ @Passerby: Try any of those that me and my family had in the last 10 years \$\endgroup\$ – PlasmaHH Feb 15 '15 at 22:26
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Here are options:

  1. Transistor with a heatsink if need be
  2. Infrared LED along with phototransistor with heatsink if need be

I would only suggest you use these options if you have the components laying around, otherwise you would just be reinventing the wheel as a solid state relay(SSR) is typically made out of one of the options above.

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  • \$\begingroup\$ doesn't a mechanical relay switch also use power constantly because of electromagnet is working constantly ? (in case of monostable switch - because that's the only one I'm interested in) \$\endgroup\$ – NoderBlade Feb 15 '15 at 23:37
  • \$\begingroup\$ @NoderBlade Good point, I'm curious which one uses less energy. \$\endgroup\$ – Addison Feb 16 '15 at 0:45
  • \$\begingroup\$ I found this one - it looks promising i think docs-europe.electrocomponents.com/webdocs/130e/… \$\endgroup\$ – NoderBlade Feb 16 '15 at 0:49
  • \$\begingroup\$ @NoderBlade That does seem nice. Only 300mW of power for the input, which at 12V input is 25mA. \$\endgroup\$ – Addison Feb 16 '15 at 1:09
  • \$\begingroup\$ Mechanical relays use power while the coil is energised (activated) but not very much, and your car generates a LOT of electricity (typically 50-100 Amps @12v) when running so ~100mA to keep a relay on is a flea fart in a hurricane. \$\endgroup\$ – John U Jun 10 '15 at 14:58

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