Discussions

PPM input logic level?

Posted by Boris Matuschka on June 1, 2012 at 10:39am

Hi folks,

I recently tried to connect a ezUHF receiver to the ppm input of the APM2. I soldered the jumper on the back and connected the cable to ch5. This setup was working fine with a Simprop 2G4 receiver in ppm mode. When I now connect the ezUHF receiver two things are happening (on two different APM2 hardware). The first APM does not boot correctly. the A B C LEDs are blinking in a row A B C fast and then remain blinking slow in the same order - what does this mean)? The second APM2 starts and I can connect via Mission Planner but in the config tab there is no input from my transmitter visible.

The support of ezuhf told me that it outputs a 3V logic level. Maybe this is not working for the APM so anyone any hint where the problem could be?

I tested it with two differnt ezUHF recievers - same game.

Best regards,

Boris

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Replies

Ravi June 13, 2012 at 6:10pm

hi john, thanx for the guide for giving PPM IN. i would be using frsky FASST receiver with PPM out on PIN 1. i am using Futaba 7C. by default the mode channel is on channel 8 of Rx. but 7C has only 7 channels. earlier i used to input channel 7 of Rx to CH8 IN of APM. how do i assign mode to channel 7 in APM. since i am using PPM IN so i cannot change channels just by switching the servo connectors form Rx.
Ravi June 12, 2012 at 9:58pm

can someone please help me how to input PPM in APM2 board. which jumper has to be enabled on APM2. thanx
Boris Matuschka June 1, 2012 at 1:09pm

ok. I watched the links nick gave me; can you tell me which of the connectors would i have to connect? I get from the reciver the gnd, + volatage and signal line; where shoud i connect it eg to the levl shifter from diydrones? sorry for askin, but i have to get into this first...

Thanks in advance

Boris
Boris Matuschka June 1, 2012 at 12:38pm

ok thanks a lot for your answers this helps me a lot. ill try it with the given scematic.

thanks again to you jake ad nick :-)

Boris
Nick Arsov June 1, 2012 at 11:46am

Hi Boris,

Just buy a simple level translator like this one - https://store.diydrones.com/I2C_SMBus_Voltage_Traslator_I2C_Level_S... or http://www.sparkfun.com/products/8745

Note it has 2 supply voltages - 3.3V and 5V and also 3.3V datalines and 5V datalines.

Also you can make your own by using 2N7002( BSS138 ) +2x10k resistors for each channel....see the attached schematics.

Best regards

Nick
Jake Stew June 1, 2012 at 11:16am

3v logic probably isn't going to work with a 5v system. Trying to use 3v logic with a 5v processor is notoriously unreliable and I wouldn't suggest it.

Your ezUHF probably isn't putting out true 3.3v logic. It might have a pulldown resistor that is eating some of the voltage. You could try running the RX at 3.6v if you can change that. Otherwise you'll have to get a proper level converter or search the internet for alternatives. It can be done with a diode and a pullup resistor supposedly.

I wouldn't take chances with this sort of thing though. Receivers are cheap compared to a crash.