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I will add momentary push button to car infotainment panel (instead of blank). It's VW/Audi original button which I want to use to switch relay (ys-rt1c - Timer delay turn off) to supply 12 V for rear parking camera. Power supply for camera will be presented for 15 seconds.

I will use ys-rt1c to supply camera - Timer delay turn off on photo.

My question is.

Is momentary button designed for current load (12 V) for relay coil switching?

Is good practise to switch 12V impulse over push button (button current load) to switch relay coil?

Should I add some resistor to limit current over button to switch relay coil?

I read some articles about switching ground instead of 12 V over button.

I will dissasebmly car infotainment panel and I want to make it right.

enter image description here enter image description here

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    \$\begingroup\$ It is not good practice to add misc hobbyist boards to car electronics without type approval. It is in fact illegal, far as I know. \$\endgroup\$
    – Lundin
    Commented Jun 14 at 10:07
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    \$\begingroup\$ @Lundin maybe in Germany but it’s fine in most jurisdictions \$\endgroup\$
    – Frog
    Commented Jun 14 at 11:06
  • \$\begingroup\$ @Frog What's your definition of "most?". UNECE R10 applies all over the European Union and in many other countries as well. \$\endgroup\$
    – Lundin
    Commented Jun 14 at 11:29
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    \$\begingroup\$ @Lundin, It looks as if the only mods to the car's existing "electronics" would be the part where OP taps in to the 12V supply. Where do those regs draw the line (do they draw a line?) between adding a powered accessory and modifying the car's electronic systems? \$\endgroup\$
    – Solomon Slow
    Commented Jun 14 at 11:44
  • \$\begingroup\$ @SolomonSlow It's more a matter of the thing you put in there being EMC compliant. The regulation does draw the line by anything that may affect traffic is subject to 3rd party type approval. If you can guarantee that a particular add-on isn't live when the car is moving through traffic, then R10 doesn't apply. This isn't just cars btw, anything containing electronics put on market has to be EMC compliant. Cars just have tougher requirements and in some cases mandatory type approval, whereas regular CE marking can use "self-certification" procedures. \$\endgroup\$
    – Lundin
    Commented Jun 14 at 12:02

3 Answers 3

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That appears to be a timer relay, so the switch probably won’t carry the relay coil current at all, there’s no need for a resistor in any case. Switching in the 12Vvor ground side of the load can be done for a number of reasons, e.g. a ground connection is very easy to find in a vehicle, and low-side switching with an NPN bipolar or N-channel FET is a little easier but in this case it makes little difference.

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  • \$\begingroup\$ Can you please a go more into details? Can you recommend some modul for it? I'am kinda confused what is proper solution. Thank you. \$\endgroup\$
    – Jason
    Commented Jun 14 at 8:42
  • \$\begingroup\$ @Jason I assumed that you have the module in the photo, am I wrong? That appears to be a timer relay of some kind \$\endgroup\$
    – Frog
    Commented Jun 14 at 11:08
  • \$\begingroup\$ Yes I have this module. I wonder about current load of relay coil over the momentary button? \$\endgroup\$
    – Jason
    Commented Jun 14 at 14:48
  • \$\begingroup\$ Most likely the button feeds into some electronics and the current is very small. But I’m sure the button is designed to have a long life when operating with a small load like the relay coil \$\endgroup\$
    – Frog
    Commented Jun 15 at 4:45
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Here's the appropriate schematic.

enter image description here

The timer module is not designed to permit a momentary push button switch on the ground side and a current limiting resistor in series with the relay coil. In any case, its not advisable to use that resistor.

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  • \$\begingroup\$ Thank you for schematic. I worried about that if momentary button is designed for switching current load of relay coil (ACC signal)? \$\endgroup\$
    – Jason
    Commented Jun 14 at 14:46
  • \$\begingroup\$ Anytime, Jason! The relay coil current is not carried by the momentary button. The signal current through the button will be far less than the 68 mA working current of the module. \$\endgroup\$
    – vu2nan
    Commented Jun 15 at 2:37
  • \$\begingroup\$ I checked kinda timing relay module. Relay coil have UMW817C - NPN Phototransistor. So I assume if relay coil is switched by transistor (less trigger current). \$\endgroup\$
    – Jason
    Commented Jun 15 at 5:17
  • \$\begingroup\$ That's correct. \$\endgroup\$
    – vu2nan
    Commented Jun 15 at 10:04
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Your relay requires continuous power in addition to the trigger signal. As others have said, you will not be driving the coil directly. A current limiting resistor will not be required.

I don't know if your 12V is switched when the car is turned off, but this module will try to draw 10mA at all times from that rail. That might noticeably increase the parasitic draw on the battery when the car is off.

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  • \$\begingroup\$ Relay will be supplied 12V only when engine is on. \$\endgroup\$
    – Jason
    Commented Jun 15 at 5:11

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