If you are into Radio Control Models or robotics chances are that you have an old RC transmitter laying around. This article describes how to create a motion control module for your RC transmitter, that will allow you to control your model or robot by simply tilting the transmitter case. That's right not sticks!
Testing one axis:
How it Works
The RC transmitter uses a potentiometer for each axis, it acts as a voltage divider sending a voltage of 0..5V (the middle position corresponds to 2.5V) to the analog input that is converted into a pule of 1..2ms that is sent over RF.
This module converts (amplifies and shifts) the accelerometer analog output , usually 1.65 +/- 0.4V to the same range of the potentiometer and sends it to the transmitter instead.
An op-amp in an inverting amplifier configuration is used. Vref is set manually by tuning the output to be 2.5V (or the PWM pulse to be 1.5 ms). However it is possible to calculate the theoretical value as follows:
Note that according to the rules of a feedback op-amp the voltage on it's inverting/non-inverting terminals tends to equalize so V(+) = V(-) and in our case = Vref.
Since no significant current enters the op-amp , the currents going through R1 and R2 are equal:
( V(-) – Vin ) / R1 = ( Vout – V(-) ) / R2
(Vref – Vin) / R1 = (Vout – Vref) / R2
solving for Vout gives us
Vout = Vref – R2/R1 (Vin – Vref) = Vref( 1 + R2/R1) – R2/R1 * Vin
now let's do some notations
G = – R2/R1
Vout = Vref( 1 – G) – G * Vin
According to our schematic G = R2 / R1 = – 5.12 , this will convert the accelerometer swing of 0.4 V to a swing of 0.4 V * 5.12 ~ 2V .
We want to make Vin = 1.65 correspond to a Vout = 2.5 so we have the equation
2.5 = Vref (1 + 5.12) – 5.12 * 1.65
from here we find
Vref = (2.5 + 5.12 * 1.65 ) / (1 + 5.12) = 1.78888 V
Well, this is the theoretical value , in practice we adjust the trimmer R3 until the output is 2.5 while the accelerometer is in laying in horizontal position (has an output of 1.65V).
How to Build
To build use a small proto-board following schematic. Part numbers are mentioned on schematic. Hook-up with the transmitter is described in images below and on the schematic. For accelerometer use Acc_Gyro or similar module, or build your own accelerometer break-out board.
The module is mounted in a free space under antenna using double-sided foam tape – best way to mount an accelerometer to avoid vibration. Note that we get +5V power for the module from the potentiometer contacts. You can test with a led that the power contacts can deliver at least 20mV, the module uses far less <5mA.
Here is a close-up of the module, as you can see I did my own accelerometer break-out board, but you can buy a pre-assembled one , there are many choices. You will need an analog accelerometer for this project.
Enjoy your new RC Tilt Transmitter. For any comments/questions use the comment form below.