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Background
I'm doing some automotive wiring to add a light to a cargo area with the following requirements:

  1. The light turns on/off when the cargo door is opened/closed
  2. The cargo light can be turned off with a switch on the dash
  3. If the vehicle is off but the cargo light is on, an audible buzzer should sound

I don't know much about electronics, but from what I understand after some research I've developed two options. The first option uses a relay, and the second option uses diodes.

My Question
Can I use either of the two options below? My preference is the diode circuit because it saves space by not needing a second relay.

enter image description here

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  • 1
    \$\begingroup\$ Neither of those are likely to work reliably. Option-1 relies on the cargo light drawing enough current to activate the relay (and the relay to not drop the voltage enough to affect the light), while Option-2 similarly requires the light to draw enough current to activate the buzzer. You need to be able to tap into the wire between the cargo door switch and the light to make this work properly. \$\endgroup\$
    – brhans
    Commented Nov 28, 2022 at 1:37
  • \$\begingroup\$ @brhans can you explain why? I don't know enough to understand why tapping in between the cargo door switch and the light is different. \$\endgroup\$
    – ak_
    Commented Nov 28, 2022 at 1:43
  • \$\begingroup\$ @brhans I must be missing something - the option 1 circuit works as expected when I have it all wired up on the benchtop. \$\endgroup\$
    – ak_
    Commented Nov 28, 2022 at 4:05
  • \$\begingroup\$ One thing that @brhans is pointing out is if the cargo area light burns out the buzzer will not sound when the door is left open. \$\endgroup\$
    – ErikR
    Commented Nov 28, 2022 at 5:14
  • \$\begingroup\$ @ErikR thats fine. The door can be left open any time, I just don't want the light to drain the battery. I didn't make that very clear earlier, so I've edited the question to reflect that. \$\endgroup\$
    – ak_
    Commented Nov 28, 2022 at 13:15

1 Answer 1

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Option 1:

  • The cargo light will not turn on with the relay coil wired in series with it.

  • Even without the series relay coil, the light will turn on only when the door is opened and the dash switch is actuated.

Option 2:

  • the cargo light will turn on only when the door is opened and the dash switch is actuated.

  • When the door is opened without the dash switch being actuated, the buzzer will come on without the cargo light.

Here's my schematic.

enter image description here

Here's my revised schematic considering the dash switch as an override to turn the light off.

enter image description here

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  • \$\begingroup\$ Thanks for this. I'm pretty ignorant about electronics, can you explain the function of the diode in the upper left? Also, the dash switch must be before the cargo light because they are 6m apart. Having the dash switch after the light means a total 12m run of wire. What is the reason for having it on the negative side of the light? \$\endgroup\$
    – ak_
    Commented Nov 29, 2022 at 14:21
  • 1
    \$\begingroup\$ Hi AK_, You are most welcome. It's a flyback / freewheeling diode, connected across the relay coil, to suppress the voltage spike that occurs when the relay is de-energised. Having the dash & door switches on the negative side simplifies the buzzer wiring. I need to figure out how to reduce the wire length. I have edited my answer to point out the drawbacks of your schematics. \$\endgroup\$
    – vu2nan
    Commented Nov 30, 2022 at 6:10
  • \$\begingroup\$ Thank you, that explanation is very helpful. If I purchase a relay that has a diode or resistor internally, do I need that extra diode? Also, your bullet of "the cargo light will turn on only when the door is opened and the dash switch is actuated." This is actually the desired behavior. The dash switch serves as an override to turn the light off. \$\endgroup\$
    – ak_
    Commented Nov 30, 2022 at 14:03
  • \$\begingroup\$ One more question, you said that "The cargo light will not turn on with the relay coil wired in series with it." Why won't this work? I'm pretty new so I don't yet understand. \$\endgroup\$
    – ak_
    Commented Nov 30, 2022 at 16:25
  • \$\begingroup\$ An extra diode would not be required should the relay have one inbuilt. I have included a revised schematic considering the dash switch as an override to turn the light off. The cargo light will remain unlit with the relay coil resistance in series considerably limiting its current. \$\endgroup\$
    – vu2nan
    Commented Dec 1, 2022 at 4:10

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