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Problem getting mutliplexer to switch


I’m using a Radiomaster TX16S transmitter with a LemonRx DSM receiver to drive a Pololu multiplexer and triggering each side of the multiplexer based on the output of one of the receiver channels. This is for use in an RC plane, so I don’t have 100% controller of the PWM signals coming from the receiver.

I’m going through the learning mode to try and set the trigger pulse width at around 900us. According TX16S, the output when I go through the learning mode is 834us. Normal range when operating for the channel says 998us - 2012us, with a mode switch dropping it down to the 834us level again to trigger the channel swap of the multiplexer. Note there may be some remapping happening to push the actual receiver output into the 1100us - 1900us range. I don’t have anything currently to test the actual pulse width coming out of the receiver channel.

The problem is, the learning mode seems to complete OK, and when I first power all the gear including the multiplexer up, the LED is showing the regular short blink indicating it’s running OK and the master channel is engaged (i.e. multiplexor inactive). I change modes so my receiver channel operates within normal limits, and the LED goes to the long blink, indicating it is active and swapped to the slave channels. However when I swap back to the initial mode again, the multiplexer stays active and I can’t get it to turn off again, unless I re-power the unit.

If the multiplexer was using the default setting of around 1700us for the trigger, I should see it flipping during the normal operation which goes above and below this value, but it doesn’t change.

I’m not sure what else to try. Any suggestions on why it’s fine when first booted, but then won’t reset to inactive once the active has been triggered? Maybe there’s something about the way the threshold is being set that I’m missing, but I’ve been through these instructions Pololu - 9.1. General Configuration Procedure many times and can’t see what I’m doing wrong. Only thing I can think is the pulse reading is not getting the values my receiver should be outputting. I’m not getting the error LEDs about a low value setting, so I’m not sure what value it’s setting the threshold at, but it seems to be outside the range my receiver can supply.


Well I think I’ve solved my own problem, after having to build an Arduino PWM measurer to check what’s coming out. I think a slightly floating stable setting meant the threshold average was pulling below my minimum value I could set on the transmitter, which is why it wouldn’t return to inactive, and the 64us hysteresis range for the detection threshold also means I need quit a wide gap between my trigger for the inactive state, and the allowable values on the channel in the active state. Once I re-calibrated everything and set appropriate output limits in each mode, it seems to be working properly now. Just means I’ve lost a little resolution on the servo travel for the control channel so I don’t accidentally de-activate the switch.


It sounds like you got your servo multiplexer working; thanks for letting us know! For future reference, an oscilloscope would be a useful tool for troubleshooting something like this since you could probably set one up to look at something like your RC hobby servo signals a lot faster and with much better resolution than programming an Arduino to do that, so if you don’t have access to one, you might look into getting one or finding one you could occasionally borrow.

By the way, please note our company name is Pololu, not “Polulu”. (I edited that in your post.)

- Patrick

Thanks for that. Yes an Oscilloscope has been on my shopping list for quite a while, but a decent one is not cheap and my uses are limited, so hard to justify the cost. My Arduino hack got eh job done, but I agree an Oscilloscope would have been much quicker and easier.

Also a suggestion? The multiplexer is a great product for a great price, but I do have trouble getting the learning jumping to connect as it seemed to require quick a heavy press to get the connection to work, so I’m worried about scraping the trace away. A microswitch there would be ideal.

As for the company name, sorry about that, my brain can’t process all the repeating letters :slight_smile:

Thank you for the suggestion; I’ll pass that on to other product development engineers here!

It sounds like the tool you are using might not be very conductive, so you might try experimenting with other tools to see if you can get a better result. You might also consider soldering small wires to the learning mode jumper pads to add your own mircoswitch or button.

- Patrick