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I have a 13V supply powering a brake light in my car. I'd like to take 5V from the line but I still want my bulb to get the power it's supposed to get. How can I do this?

toolic
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Jack Tyu
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    You should take the 12 V (that's what it's normally called in a car even if it isn't) from elsewhere and use the voltage on the brake wires as a signal, IMO. Taking current from the brake light may interfere with things like brake light failure detection. – Andrew Morton Apr 25 '23 at 18:20
  • I know it's a shopping question ultimately but whatever https://www.amazon.com/WMYCONGCONG-LM2596-Converter-Adjustable-Supply/dp/B01NCRYXEW – vicatcu Apr 25 '23 at 18:21
  • I actually want the brake wires to be a signal to an Arduino. I want the Arduino to detect the signal from the brake line when I press the brake. In short. I'm making a DRS-type spoiler/wing for my car that turns respective to the brake applied or not. – Jack Tyu Apr 25 '23 at 18:25
  • And yes, I do have the step down module too – Jack Tyu Apr 25 '23 at 18:25
  • I would put in some robust protection on that line; automotive power is notoriously dirty. – vir Apr 25 '23 at 19:02
  • @JackTyu In that case, wanting just a signal, Spehro Pefhany has given you [the answer](https://electronics.stackexchange.com/a/664292/36731) for your air brake actuation signal. I assume you have an off-road track to try it out on, what with the possible sudden change in balance of the vehicle? – Andrew Morton Apr 25 '23 at 19:40

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You can do that with a simple DC-DC converter if the current is over about 80mA or so. There are many such small modules available, they are called step-down or buck converters.
If the current required is fairly small (below 100mA), you can use an IC regulator with only 3 pins, like LM7805 (max. 1A current), LM78M05 (max. 0.5A) or LM78L05 (max. 0.1A or 100mA).

Edin Fifić
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As a signal you can simply use a voltage divider from the high side of the bulb. It's possible the resistors will interfere with the lamp failure detection however, so maybe test with resistors half the value.

Eg.

schematic

simulate this circuit – Schematic created using CircuitLab

Spehro Pefhany
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  • Would it be prudent to stick a Zener diode across R2 to limit the voltage of the signal to the GPIO pin in an automotive environment, or will the current be limited sufficiently by R1 for the Arduino's built-in protection to keep it safe? – Andrew Morton Apr 27 '23 at 21:12
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    @AndrewMorton I think the 100kΩ (1mA for 100V) is enough. – Spehro Pefhany Apr 27 '23 at 21:52