Digimac 1+1 encoder for conversion to 2.4gPhil_G

This project was initially a one-off to convert an early 1970s Macgregor Digimac 1+1 transmitter bought off ebay recently for £14, a really clean example and almost all of the decals are intact.

Since most of the circuitry is identical, the Digimac 1+1 encoder is based on the Single Channel Emulation Encoder PCB with a few modifications – some components are omitted and an extra resistor inserted. The software of course is completely different.

Physically the new encoder operates exactly as the original 27mhz Digimac 1+1 did, giving smooth, proportional rudder and a sequential 3 position throttle. This is really nice way to fly Single Channel!

My Digimac uses a Frsky module, details & photos here:

That thread covers a lot more of the same thing if you're interested in these conversions.

Several of these have been completed to date, one of which is a Waltron 1+1 replica.

This diagram shows the wiring to the stick pot, throttle button and the RF module. The button is connected between neg (black) and the button input (white) exactly as per the Single Channel encoder:

Please note:

The stick pot needs to be adjusted to give the correct voltage at neutral stick.

With the stick (and trim if available) at neutral, the wiper voltage needs to be three-eighths of the 5v supply voltage. In practise, most 5v regulators sit somewhere just below 5v so about 1.85v is precise enough. Its preferable to measure the pot supply across the pos & neg and work out precisely 3/8 of that voltage, ie between the black and the red in the photo above should be almost 5v.

The neutral voltage of 1.85 volts measured between the negative end and the wiper, ie between black and white on the photo above. The stick voltage defines the pulse width, 1.85v = 1.5ms.

Extremes of movement will then give roughly 1.3 to 2.4 volts, but this isnt so critical.

To adjust, theres a grub-screw on the stick bush which clamps the pot shaft.

Whilst the voltage at full left or right might vary slightly between transmitters, the neutral value of 1.85v is critical to ensure the correct pulse widths are generated.

Servo Reversing:

Reversed rudder is available on rx channel 4, making servo-reverse unnecessary. This means if you have two models with opposite physical links one might be set up with its rudder servo plugged into channel-1 of its receiver, and the other might have its rudder servo on channel-4 of its rx, so theres no need to do anything on the tx when you fly either model.

On a new model, just try rudder on receiver channel-1, if it goes the wrong way, just move the rudder servo to receiver channel 4 which has reversed rudder. This saves changing the tx setup between models if the rudder happens to be opposite.

Update 21/01/2013:

I’ve added a throttle indicator LED. This will be included on all subsequent 1+1 boards.

It’s a Bi-Colour LED - Low throttle is red, high is green, the logic being green for go, red for stop.

At mid throttle the LED flashed alternately red/green. The flash-rate indicates the next throttle position, a fast flash tells you the next throttle position is fast, a slow flash says the next position is slow (or off).

Whether you choose to have the LED visible is up to you – I find it handy as I can never remember what the next throttle move is.

Any existing 1+1 users who’d like this feature please email me on as it involves changing the PIC chip for the new version with the LED output.