I have a 12 V relay module, as pictured below (not this exact image, the relay has 12 V logic, not 5 V as in the picture.)

I am getting a voltage reading from the signal, it's floating around 12.2 V all by itself when there is nothing connected to the signal.

The relay works as expected: when signal is connected to a battery the relay switches; disconnect the battery and it goes back to the previous position.

After doing this it will show 0 V on the signal, but leave it for 10 minutes and signal will start to give off a voltage reading again.

The above signal is a 12 V high signal, when a vehicle ignition is turned on it provides a 12 V signal to this relay and a GPS tracker signal input.

The problem is when the ignition is turned off (12 V signal is turned off) the relay goes back to its default position and the GPS tracker turns off as expected.

Give it 10 minutes and the INPUT on the relay module starts to give off a voltage which triggers the input on the GPS module to think the ignition is turned on.

I have tracked everything back to this relay module, everything works fine if this relay module isn't connected, and I have put it on the bench and replicated it using a bench power supply.

My current idea is maybe the input is floating and needs a pull-down circuit added to it, but I would have thought a module like this would have covered this scenario already.

enter image description here

  • 1
    \$\begingroup\$ (1) I am referring to Part D of my answer to this Q&A: electronics.stackexchange.com/questions/505318/… \$\endgroup\$
    – tlfong01
    Jul 23, 2022 at 7:17
  • \$\begingroup\$ (2) Please confirm your wiring (a) Jumper select = High or Low trigger? (b) Vcc = 3V or 5V?. (c) Input signal logic = 3V o4 5V? (3) Wiring reference: imgur.com/gallery/kYOxQme \$\endgroup\$
    – tlfong01
    Jul 23, 2022 at 7:24
  • \$\begingroup\$ We don't know how each and every relay module works and how to use them. You need to tell us which exact product it is and where to find a manual or schematic for it. \$\endgroup\$
    – Justme
    Jul 23, 2022 at 10:02
  • \$\begingroup\$ And, the relay module has a "cnetral output terminal" with 3 terminals COM, NC, NO. We don;'t need to mess with these three terminals for troubleshooting the normal operation of the relay module. Now you seem to input the GPS output signal to either NC or NO. This makes things too complicated. It would be nice if you can draw us a complete schematic of your GPS control gear. \$\endgroup\$
    – tlfong01
    Jul 23, 2022 at 11:44
  • \$\begingroup\$ Or are you playing with SIM8/9/7600, or Neo7/8M, with Vcc = 5V? \$\endgroup\$
    – tlfong01
    Jul 23, 2022 at 13:33

1 Answer 1


It sounds like there is a pull-up on the signal input on the relay module. You need to bleed away any current coming from that input. Add a 10 kΩ resistor between signal and GND and see if that cures the problem. Report back!

My current idea is maybe the input is floating and needs a pull-down circuit added to it, however I would have thought a module like this would have covered this scenario already?

The opposite is more likely. Using a pull-up allows switching to be done by connecting the input to GND. Using an open-collector NPN transistor as the switch, for example, facilitates a very simple interface between two systems working on different supply voltages.

  • \$\begingroup\$ If it takes a number of minutes for the input to float high, it is almost certainly not an intentionally installed pull-up. This seems more like leakage or capacitive coupling along with a floating ground on the relay module. \$\endgroup\$
    – PStechPaul
    Jul 23, 2022 at 22:03

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