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I'm trying to drive a relay off a dome light in a car, where the dome light fades in smoothly. That is, say, I have a constant ground and the other wire goes from 0 to 12V (up to 14V) over a couple seconds. The dimmer is being run by the car's computer.

While the relay does come on, it does so while making a sound of entering a hyperspace. I figured the sound is due to the relay not getting enough voltage during the initial startup. In so far, I've tried a zener diode in series. 5v zener reduces the noise, but not completely. 10v zener is not enough to activate the relay (datasheet claims coil activation is around 8v) due to zener voltage drop. I also tried a capacitor in parallel to the coil 560mu 36V I had laying around seems to work the best. 300mu 24V and smaller did not seem to work. Still, being an automotive application, I'd rather not have a large electrolytic capacitor near the hot roof. Is there a better yet simple solution? A comparator using zener as a reference seems like an overkill. I can also try a much smaller PCB-type relay with a hope of it being more tolerant to the startup conditions. Any suggestions? Bonus points for not frying the car's computer that drives the dimmer!

EDIT: I have simplified a bit. In reality, I have 3 wires: 12+, ground, and 'door'. With the door closed, the 'door' wire is at 12+, when the door opens - the 'door' wire goes to close to 0. My goal is to convert the 'door' wire into 'interrupted 12+' (no signal when door closed, positive when door open) required by a dimmable mirror.

EDIT2: Would simply comparing ground to 'door' signal make more sense?

EDIT3: My ultimate goals are 1) to provide a positive 'door' signal to a Gentex 221 mirror that came from a Ford using a 'door' signal from Toyota that, upon opening the door, goes from +12V to 0V; 2) trigger a Hella timer relay that feeds +12V to this mirror. My original plan was to use the Toyota door signal to trigger a second relay that would send +12V to the mirror's door pin.

Upon much struggle with the chatter on the relay, I realized I don't actually need the second relay. All it takes is a PNP transistor like 2N3906. The base goes to the Toyota door signal, Emitter goes to +12V, and Collector goes to Gentex door pin. This setup essentially reverses the +12->0 signal from Toyota into a 0->+12 signal sufficient to drive the Timer relay and the mirror's door function.

Now, I'm not 100% positive why the timer relay does not exhibit a chatter; most likely because I'm switching on the full +12V rather than the PWM'd ground.

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  • \$\begingroup\$ Is one side of the bulb grounded or is one side connected to +12? \$\endgroup\$
    – Spehro Pefhany
    Apr 21, 2020 at 3:58
  • \$\begingroup\$ Your trying to capture an pwm or otherwise modulated signal. Use a microcontroller and level translation to trigger off it and control the relay. \$\endgroup\$
    – Passerby
    Apr 21, 2020 at 4:14
  • \$\begingroup\$ entering hyperspace makes a sound? ... in vacuum? ... how? \$\endgroup\$
    – jsotola
    Apr 21, 2020 at 5:01
  • \$\begingroup\$ @Spehro Pefhany I have updated the description, hope it clarifies things a bit. \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 11:29
  • \$\begingroup\$ Turns out everything was way simpler than I thought. Live and learn! \$\endgroup\$
    – Maksym
    May 10, 2020 at 3:27

3 Answers 3

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Here is one approach (this assumes the bulb has one side grounded, if not then just flip everything over and use a P-channel MOSFET).

The idea is that the PWM signal to the bulb discharges the capacitor C1 (through Lamp1 and D1) during the 'off' part of the PWM cycle until the lamp is almost completely on.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The OP requires the dimmer output to switch on the relay. I presume there is no 12V line nearby. \$\endgroup\$
    – vu2nan
    Apr 21, 2020 at 10:53
  • \$\begingroup\$ Sorry, I should have clarified. I do have 3 wires - 12V, ground, and PWM. My goal is to convert PWM back to discrete 12V/no 12V. \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 11:30
  • \$\begingroup\$ Does the bulb play an active role in the circuit? Can we take it out or replace with a smaller resistance and still get the same behavior? \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 11:41
  • \$\begingroup\$ Yes, the bulb discharges C1 when the high-side switch turns off. The resistance is not critical, provided it's a resistor or tungsten bulb and not an LED. \$\endgroup\$
    – Spehro Pefhany
    Apr 21, 2020 at 12:51
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    \$\begingroup\$ I made this work with the following changes. Since there is no lamp1, I have connected Lamp1 wire to the collector of the mosfet. I also changed the capacitor to 10uf, which is still electorolytic, but a lot smaller one. 1uf was still turning on too soon. It seems 10uf turns on a little after the PWM wire (ground in my case) reaches the full 12V potential. Thanks for your help! \$\endgroup\$
    – Maksym
    May 9, 2020 at 2:50
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I realized I don't actually need the second relay. All it takes is a PNP transistor like 2N3906. The base goes to the Toyota door signal, Emitter goes to +12V, and Collector goes to Gentex door pin. This setup essentially reverses the +12->0 signal from Toyota into a 0->+12 signal sufficient to drive the Timer relay and the mirror's door function.

Now, I'm not 100% positive why the timer relay does not exhibit a chatter. One possible reason is because I'm switching on the full +12V rather than the PWM'd ground. Second reason is the timer relay measures 100 kOhms while the normal relay is 10 ohms, perhaps because of a built-in resistor.

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Here's one using a transistor to drive the relay.

enter image description here

The schematic has been altered to show the relay being fed by +12V instead of the dimmer output.

enter image description here

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  • \$\begingroup\$ Would I get any benefits from using a comparator as opposed to the transistor? Also, can I substitute with a different zener (say 10V) without changing other components of the circuit? \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 11:43
  • \$\begingroup\$ Maksym, It's a 10V Zener only. The error has been corrected thanks to you! I suppose the transistor circuit is good enough for the requirement. \$\endgroup\$
    – vu2nan
    Apr 21, 2020 at 12:15
  • \$\begingroup\$ Thanks! 8V zener should work as well, but 10V is closer to where I'de like to turn it on. Since, in addition to the signal door wire, I have a ground and a 12V (see the updated description) - would it make more sense to throw away zener and just use the ground as a reference? \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 14:34
  • \$\begingroup\$ Hi Maksym, After going through your edits, it appears that your original scheme is the right one. Your objective is to simulate the door switch using a relay which is energised by the voltage across the dome light. Using the relay contacts, the voltage applied to the dimmable mirror is to be 0V with the door closed and +12V with the door open. This scheme effectively avoids any interfacing with the 'Door Switch' input of the ECU. My schematic is to be altered to show how the supply to the dimmable mirror is switched. \$\endgroup\$
    – vu2nan
    Apr 21, 2020 at 16:15
  • \$\begingroup\$ Nah, your schematics is correct. My only 'door' signal is the dimmer wire; I'm trying to convert this dimmer signal into a more binary door switch signal. I will try to assemble your schema and accept the answer if I see if it works good enough. I don't see why it wouldn't though; l though of this approach myself, but was curious if I can get away without using transistors and other logic elements. \$\endgroup\$
    – Maksym
    Apr 21, 2020 at 16:48

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