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I have a car wireless door lock system I want to modify. The basic operation is that when the key fob's button is pressed, a light relay is triggered, to flash some of the car's lights, to indicate the doors are locking. It does this by grounding a pin attached to the relay. This same relay is used on multiple systems.

I want to add a second relay, that only triggers, when the wireless door lock system triggers the light relay, not when anything else does. General schematic attached. (Red wires and New relay is what I know I'll be adding).

schematic

simulate this circuit – Schematic created using CircuitLab

Any of the nodes labeled "Switch" "Alarm" or "Misc" can ground the relay, turning on the lights. If I simply tap into the circuit as shown, that would make the new relay trigger as well, and I don't want a 24/7 horn blasting.

Obviously, I only want the New relay to trigger, when the WDI grounds the Existing relay. How can I do this?

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  • \$\begingroup\$ I'm assuming a diode would work, but where and how? \$\endgroup\$ – Passerby Sep 13 '14 at 21:19
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You will need to cut the grounding path between your light relay and WDI and insert a diode pointing towards the WDI, and finally connect your new relay minus terminal directly to the WDI pin. This will work assuming that your light relay can work with some 0.7V less coil voltage, use a schottky if you want to be extra sure, but I think that relay can reliably turn on off of 9V so that should not be a problem.

See this schematic for a little clarification.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ That should work. I used a Schottky diode (1N5819) in my old Acura for a similar purpose (400mV drop at 0.1A) but a 1N400x should be good enough too. \$\endgroup\$ – Spehro Pefhany Sep 13 '14 at 22:21
  • \$\begingroup\$ Ah, so the current from the 12v through the New Relay's coil, to the "other ground paths" is blocked by D1 being reversed biased, while allowing current from the Light Relay to sink through when WDI's In pin is grounded. Do I have that right? \$\endgroup\$ – Passerby Sep 13 '14 at 22:25
  • \$\begingroup\$ @SpehroPefhany Place the schottky in the same direction? And what was your project (just curious)? \$\endgroup\$ – Passerby Sep 13 '14 at 22:27
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    \$\begingroup\$ @Passerby you have it right. that's also assuming that your wdi can sink enough current for two relays, if you are not sure you can use a transistor. and yes, the schottky would go in the same direction \$\endgroup\$ – Vladimir Cravero Sep 13 '14 at 22:37
  • \$\begingroup\$ Yes, the same direction. My project was to adapt a generic power antenna. \$\endgroup\$ – Spehro Pefhany Sep 13 '14 at 23:00

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