Download Setting up the stick pots for the Phil_G proportional encoders:

Proportional encoders: 2+1 and multi-channel
29/12/2012
Using the same printed circuit board as the Single-Channel Emulation Encoder, I’ve done a few
proportional encoders suitable for updating early proportional sets. Variations include:
1+1:
this gives proportional rudder via a single-axis stick, and a sequential throttle.
Receiver channels are 1=rudder, 2=fixed at neutral, 3=throttle, 4=reversed rudder.
See separate Macgregor Digimac 1+1 document on the Archive page.
2+1:
this gives proportional rudder & elevator via two single axis sticks or one dual axis, and a
sequential throttle. Suitable for upgrading a 2 channel set such as the Macgregor 2.
Receiver channels are 1=rudder, 2=elevator, 3=three position throttle, 4=on/off throttle.
Servo reversing is by holding over the stick whilst switching on the transmitter.
5:
this has four proportional channels and one switched and is suitable for upgrading an old 3, 4
or 5 channel set such as the Skyleader Clubman.
Receiver channels are 1=aileron, 2=elevator, 3=throttle, 4=rudder, 5=aux.
Servo reversing is by holding over the stick whilst switching on the transmitter.
In a future update this is likely to become a 4 channel encoder with the switch used to enable
an elevon mixer rather than a 5th channel.
All these encoder versions use the PIC analogue to digital converter in 10-bit mode. Stick pots need
to be set up to match the encoder, see section “Setting up the stick pots” appended later.
Here is the 2+1 encoder and its connections. The pos and neg feeds are daisy-chained to both pots
so effectively they are in parallel across 5 volts. Hence the encoder has only one pos & one neg
connection pin for both pots, and a separate wiper connection for each. (or more in the case of the
5ch encoder):
There is a small modification to the S/C board, to provide power to the stick pots. This involves
drilling one hole next to the voltage regulator output, into which a 33R resistor is fitted, the other end
going to the R2 hole nearest the regulator.
R2 is omitted, R3 is a 150R, a header replaces the LEDs and the buzzer can be omitted.
On the 5 channel (or 4 plus elevon mixer) version, the S/C button is replaced with a toggle switch.
Module and power connections are unchanged:
5 Channel Proportional Encoder
This is a simple encoder based on the same printed circuit board as the Single Channel encoder, but
with a couple of hardware modifications and different software.
The resulting set is similar in facilities to a DX5e or a Planet 5 – it isn’t what you’d call a ‘computer
radio’ but it is significantly better than the original 1970 sets which didn’t even have reversing!
These photos show the changes and the connections. The pos and neg feeds are daisy-chained to
all four pots so effectively they are in parallel across 5 volts. Hence the encoder has only one pos &
one neg connection pin for all the pots, and a separate wiper connection for each.
The Channel-5 switch (retracts, flaps, etc) has 2 wires, connected to the ‘button’ input and to neg.
Setting up the stick pots for the Phil_G proportional encoders:
29/12/2012
There’s no standard as to how stick pots (potentiometers) are set up – different manufacturers use a
variety of encoder circuits, which have the pots wired in different ways. They can be wired as true
potential dividers, with a stable supply across the track ends, or they can be simple variable resistors.
We are going to use the pots as potential dividers, with a 5 volt supply across the track, and the
wiper providing a variable voltage depending on the stick position. In a typical stick unit, the wiper
doesn’t traverse the whole track – only about a quarter of it. Whilst the pot is capable of rotating 240
degrees, the stick can only rotate it about 60 degrees out of the total 240.
From the 5v supply therefore, the wiper voltage variation will only sweep a quarter of the 5v supply.
The 60 degrees that the stick can move could be set to any one of the four quartile positions:
0-25% of full travel, 25% to 50%, 50% to 75% or 75% to 100%,
which gives to a wiper voltage variation of:
0 to 1.25 volts, 1.25v to 2.5v, 2.5v to 3.75v and 3.75v to 5 volts.
We dont want to use the first and last options as the stick could be physically pushed against its stop
at 0% and 100%, so we can choose either the 2nd or 3rd quarter. Either is good, and as it happens I
chose the 2nd quarter. Maximum end-to-end stick movement therefore needs to give a wiper voltage
within the range of 1.25v to 2.5 volts, and neutral-stick should be halfway between, at 1.875 volts, or
precisely 3/8 of the supply voltage.
In a nutshell, for the analogue to digital converter to work properly, the wiper voltage needs to
be three-eighths of the 5v supply voltage with the stick and trim at neutral. 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.
The next bit deals with setting up each pot so its wiper covers the 2 nd quarter of the pot track as
described above.
Some stick units, for example the Macgregor Digimac 1 and 1+1) have a grub-screw clamping the pot
shaft, and can easily be adjusted to give the correct wiper voltage at neutral-stick. This is the easiest
case!
Some manufacturers provide reversing by swapping the outer pot connections with a change-over
switch, which means that neutral-stick must be set to exactly half-travel on the pot track. Most
commonly the pots are 5k, so in this case, at neutral the wiper would be exactly 2.5k ohms from
either end, and switching the ends around will reverse the channel without moving the neutral offcentre.
Others have sticks which are mechanically fixed to the pot shaft and cannot be readily adjusted, In
this case we have to do a bit of testing before we can wire them up.
The first step in setting up the pots for all of the proportional encoders, regardless of channel count, is
to measure the value of the pots from end to end. For this we use a multimeter across the outer pot
connections.
It will probably read 5k ohms, though some are 10k. If it is much above 20k (eg old analogue sets)
then the stick unit might not be suitable for conversion.
Next we need to find out where the wiper sits on the track while the stick and its trim lever are at
neutral. We measure from either end of the track to the wiper which is the middle one of the three
tags.
If the stick was set up for reversing then from either end it will read half the track resistance, ie 2.5k
for a 5k pot. If that’s the case, the 5v pos and neg supply to the pot track ends can go either way
around, so we can skip the next paragraph.
If the pot wasn’t central, say for example the 5k pot reads 3k from one end to the wiper, and 2k from
the other end – then the end with the lowest resistance (the 2k end in this case) will go to negative.
The higher resistance end will go to positive. This ensures that the wiper voltage is below half the 5v
supply, which puts it in the right quarter.
At this point we need to power the pots from the encoder board, see the diagram above. Theres no
need to plug in the RF module at this stage. Connect pos to the higher resistance end of the pot, and
neg to the lower. The positive and negative are common to all pots in parallel.
With the meter negative lead on the pot neg, measure the wiper voltage. If its around 1.85v then its
job done, you dropped lucky! If, as is more likely, you need to adjust the neutral voltage, we use a
miniature 2k preset pot in series with the stick-pot supply. .
With the stick and trim at neutral, if the wiper voltage is above 1.85v, we need to add some resistance
into the positive feed, so remove the pos from the pot and reconnect it via 2k preset resistor (pot).
These have 3 connections - use the centre connection and one end, doesn’t matter which, leaving
the other end disconnected. Whilst measuring, adjust the preset for 1.85v at the wiper, or more
precisely 3/8 of the supply voltage as measured between the pot pos & neg.
With the stick and trim at neutral, if the wiper voltage is below 1.85v, we need to add some resistance
into the negative feed, so remove the neg from the pot and reconnect it via 2k preset resistor (pot).
These have 3 connections - use the centre connection and one end, doesn’t matter which, leaving
the other end disconnected. Whilst measuring, adjust the preset for 1.85v at the wiper, or more
precisely 3/8 of the supply voltage as measured between the pot pos & neg.
Whilst taking these measurements the meter neg should be on true neg, eg a mounting bolt.
I actually mount the presets by soldering them directly into the stick-pot tag. The pos or neg
connection is insulated with heatshrink.
This entire exercise needs to be repeated for every pot, (ie every proportional channel). It only has to
be done once, so its worth getting it right, and it is much easier to do than to describe!
If the pot wires are particularly long, its possible for them to pick up extraneous noise. This can be
fixed by soldering a 100nF capacitor onto the pot tags, between the wiper and the negative end of the
pot. This needs to be done individually for each pot:
When all the pots are set up, switch off, plug in the RF module and bind to a receiver. Check
everything works as expected. Experiment with the reversing method, and with the Aux channel
servo-slow. Then range-check and Fly!!!