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.