RPM measurement weirdness

Hello everyone!

I’m working on a DLA 32cc gas engine, trying to feed the RPM output from its ignition to a Pixhawk 1.

Since I’m using a kill switch which keeps the chassis on Vbat when disconnected (low switched), I need to use an solution such as an optocoupler to prevent the ignition from powering up when I connect the RPM wire ground to Pixhawk.

I did a quick build with a 4N25 optocoupler but for some weird reason, when I connect the RPM servo cable output of the ignition to the 4N25 input side, the ignition RPM signal gets overloaded and the ignition ceases to generate a spark. Essentially, it stops working.
It does work however with a run-of-the-mill 4 7segment RPM meters.

I tried to measure the loading currents of the RPM signal line to compare the two consumer devices.
I went to as low as 1mA with my optocoupler with a series resistor and the ignition always keeps shutting down. 0 mA drawn when the magnet is triggered, 1mA anywhere else.

On the other side, the RPM display draws about 0.6mA on one case and -0.35 on the other (can’t remember which is which).

What’s going on here? What am I missing? Clearly the ignition output can’t be overloaded by 1mA load.
Still, if I connect the RPM signal to ignition ground through a mere 5k resistor, the ignition stops working.
I’m using the same system ground for Pixhawk and the ignition.


Hi, I’ve got a similar engine and just want to monitor the RPM via the “RPM sensor output and tachometer” servo cable coming out of the Ignition module. Where did you plug that into on the Pixhawk and what did you enable in Mission Planner? I’ve got a Pixhawk 2.1. Thanks

Hello and sorry for the late reply. I hadn’t noticed your post until now.

As I wrote in the first post, I couldn’t connect the RPM signal directly to the Pixhawk. It would overload the ECU output and seize the motor.

Here’s the schematic of the amplification circuit I built:

P1:1 is the RPM output from the engine ECU.
P2:1 is the output RPM signal towards Pixhawk, which should be connected according to this