I have a 12VDC automotive relay controlled by an open-collector NPN output on a remote device. I would like to be able to have an indication of when the relay is activated. I tried (and failed) the following:

  • 12V buzzer in series with the relay - sound indication OK but relay is not being activated.
  • 12V buzzer in parallel with the relay - open-collector does cut-off, sound and relay are activated continuously (this is weird). Once the buzzer is disconnected, the open-collector will cut-off in a couple of seconds.
  • 12V LED in series with the relay - visual indication weak (compared to regular led light), relay is not being activated.
  • 12V LED in parallel with the relay - visual indication OK, relay activated/deactivated OK, LED dead after one run (I suspect due to relay not having a suppression diode).

How can I have an indication of the relay's state? I would prefer to use the control information, not the relay output.


2 Answers 2


Both the LED (with a series resistor!) and a buzzer are viable options.

For the LED put a resistor in series with the LED. For 12V and 20 mA the value would eb 600 Ohm, I suggest you start with a 1k resistor. Do add a suppressor diode across the relais coil, any 1N400x will do.

For a 12V buzzer no series resistor is needed, but I would still add the suppressor diode. The weird effect you observed might be due to wiring the buzzer between the output and ground (instead of between output an +12V).

  • \$\begingroup\$ The buzzer is wired between +12V and output. I will test with a suppressor diode. \$\endgroup\$
    – Vlad
    Commented Mar 25, 2013 at 15:35
  • \$\begingroup\$ Thanks, the series resistor saved the LED (doh!). Also, it seems the weird behavior when using a buzzer was due to EMI between piezo buzzer and trigger input for the open-collector (an iButton reader). \$\endgroup\$
    – Vlad
    Commented Mar 26, 2013 at 15:27

It sounds like, from your text, that the output is an open collector of the PNP type and this means that the relay is connected between the open collector and ground. This means you simply wire your buzzer across the relay. Be careful in case the buzzer is polarity concious or it won't work. I think (again inferring your connection) you had the buzzer between open collector and positive supply. Or perhaps the buzzer has an built diode protection device and you wired the buzzer in reverse.

The buzzer in series won't work because you'll never attain the voltage on the relay to drive it. The LED in series is problematic too because of the way you have used it.

The parallel LED you have figured out.

Another option is to use a spare contact on the relay if you have one but you don't prefer that.

  • \$\begingroup\$ The relay is connected between +12V and open collector output. I was careful about polarization of buzzer and LED. \$\endgroup\$
    – Vlad
    Commented Mar 25, 2013 at 15:37
  • \$\begingroup\$ @Vlad it "lloks" like a wiring mistake Vlad. When you said "the open collector will pull up" this made me believe it was a PNP open collector up to 12V \$\endgroup\$
    – Andy aka
    Commented Mar 25, 2013 at 19:13
  • \$\begingroup\$ Sorry, I think I don't quite master the English terminology. I think what I meant is best put as "the open collector will not cut-off". Please let me know if this is better so I will update my question! Thanks. \$\endgroup\$
    – Vlad
    Commented Mar 26, 2013 at 8:28
  • \$\begingroup\$ @Vlad just say that the open collector transistor is NPN type \$\endgroup\$
    – Andy aka
    Commented Mar 26, 2013 at 9:01

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