Tapping the PPM signal
The so called “PPM” signal is actually a series of PWM channels which are time division
multiplexed. This signal is normally received generated by the receiver after
demodulating the FM signal from the transmitter. Under normal RC operation, it is then
demultiplexed and the channels sent to specific pins assigned to different servos.
When used with a Paparazzi autopilot, the RC receiver is only used for FM signal
reception and demodulation. The multiplexed “PPM” signal is tapped before it is
demultiplexed and sent to the autopilot for demultiplexing and assigning to servo
channels. Incidentally, this means an inexpensive receiver which is designed to output to
only a few servos can be used with the eight channel tiny autopilot board as the full PPM
signal is still sent to the autopilot.
When we tap into the PPM signal, we are reverse engineering the work of the receiver
designers. Typically the PPM signal can be found on a surface mount integrated circuit
on the receiver board. Sometimes these ICs are not silk screened so it’s difficult to tell
what chip is what. To find the PPM signal, we must use trial and error by testing each
pin on all the IC’s of the receiver for the correct signal. The correct signal may be found
using an oscilloscope. To confirm it is the correct signal you should see different pulse
width changing as different channels on the transmitter are varied.
Once the correct signal is found, we solder a small gauge wire (~30ga) to the appropriate
pin and apply medium to thick cyanoacrylate around the pin to avoid wear and tear. If
needed, shielded wire can be attached to this wire and run to the autopilot board.
Procedure:
1. Open the receiver to expose the circuit board
2. Connect the receiver to a battery.
3. Connect the negative terminal of the oscilloscope probe to one of the negative
servo pins.
4. Use the probe to look for the correct pin to attach a wire to.
5. When the pin is found, attach a small wire to the pin with solder and CA.
6. If you wish to use the receiver case you may need to drill a hole to run the new
wire out of the receiver.