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I found many disussions on the internet regarding this issue, but as an engineering noob I cannot get a proper circuit working :(

What I want to achieve: I want to control the ignition line of a car by small chip. The ignition therefore has to be connected to the 12V car battery. The chip's output pins only provide 3.3V @ ~10mA.

I thought of a 3V relay (e.g. G5V1-3), but this requires more than 10mA of power. What's the best/easiest/cheapest solution to this? Power consumption should be <0.1mA when the circuit is off. I would like to keep the car's and chip's circuits as independent as possible (that's why I thought of a relay). Toggling frequency is <10Hz.

Thank you!!

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  • \$\begingroup\$ There are optoisolated relay modules that need only a logic signal and a drive voltage. \$\endgroup\$
    – ratchet freak
    Commented Jun 14, 2018 at 11:39
  • \$\begingroup\$ Do you want the best solution or the easiest solution or the cheapest solution? The best solution is probably not the cheapest, depending on your definition of "best". \$\endgroup\$
    – Elliot Alderson
    Commented Jun 14, 2018 at 11:44
  • \$\begingroup\$ "10mA of power" and "Power consumption should be <0.1mA" are not promising. \$\endgroup\$
    – user133493
    Commented Jun 14, 2018 at 11:44
  • \$\begingroup\$ @ratchetfreak Do you have an example? Most modules require 5v I think and are quite big. I would prefer a small one. \$\endgroup\$
    – Tobias Mai
    Commented Jun 14, 2018 at 11:44
  • \$\begingroup\$ @ElliotAlderson the easiest, so I don't have to buy 100 different things. And it should be small (space is limited). \$\endgroup\$
    – Tobias Mai
    Commented Jun 14, 2018 at 11:47

1 Answer 1

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I thought of a 3V relay (e.g. G5V1-3), but this requires more than 10mA of power.

enter image description here

Most folk would use a 12 volt relay coil controlled via a BJT like the above. Power is measured in watts or mW; Current is measured in amps or mA etc..

The diode is to protect the transistor when it deactivates the relay and magnetic stored energy is pushed back into the circuit.

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    \$\begingroup\$ T1 could be a power MOSFET for lower dissipation. However, you have to make sure the threshold is under about 2V. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Jun 14, 2018 at 12:24
  • \$\begingroup\$ @CristobolPolychronopolis it certainly could but I'm guessing the op is comfortable with relays and it would work with relays by the sounds of it. Going straight to a MOSFET would be my personal choice but may be a step too far for the OP. \$\endgroup\$
    – Andy aka
    Commented Jun 14, 2018 at 12:27
  • \$\begingroup\$ Thanks! And how do I know which transistor (or MOSFET?) and diode fits? Never worked with anything like that before. \$\endgroup\$
    – Tobias Mai
    Commented Jun 14, 2018 at 12:49
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    \$\begingroup\$ Virtually any regular transistor will do and ditto for the diode. What do you have available? What distributors do you have dealings with? \$\endgroup\$
    – Andy aka
    Commented Jun 14, 2018 at 14:21
  • \$\begingroup\$ @Andyaka Currently I don't have any transistors at home, so I'll have to be some. I saw many circuits using the 2n2222, so what about them? And i only have 1N4007 diodes, are they ok? Could the second circuit in the first answer of this question solve my problem? Thank you so much for your help! \$\endgroup\$
    – Tobias Mai
    Commented Jun 14, 2018 at 19:07

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