The larger device is looking for a 0 to 10k ohm pot - I don't know if it is looking for a voltage drop or current.
Then it's probably just an analog voltage input. The input voltage needs to between 0V and some voltage determined by the device. It's really important to understand that voltages are all
relative measurements. You can't hook up 2 different circuits with their own power supplies and assume that 0V on one circuit is also 0V on the other circuit.
That's where the potentiometer comes in. If you were to use a potentiometer (as the device calls for), you would need to hook up 3 wires from the device to the pot: Voltage supply (I'll call this V+ from here on), ground (Vo), and a variable voltage signal line (Vinput):
Code:
Vo ---\/\/\/\/\/\/\/---- V+
^
|
Vinput
Voltage at one end of the resister of the pot is a full V+, while at the other end, it is Vo. The Vinput connection to the pot is the part that sweeps across the resister. With a linear pot, the voltage changes linearly across the resister, so the voltage reading of Vinput will be directly proportional to the position of the pot. What the device is really looking for is a difference between the input voltage and the device's Vo, and it can't read anything higher than the device's V+.
What's great about this 3-wire potentiometer setup is that there's no worrying about making sure the voltage is in the correct range. It's automatically guaranteed to be exactly within the range, because the device is providing the lower and upper bound reference voltages to the potentiometer.
Without knowing enough details about your project, it sounds like you have a completely separate circuit that you want to interface with this device. If, by chance, both devices are powered from the same power supply, then you can directly connect your variable voltage to the device's input. One way to accomplish this is to piggyback your circuit onto the device, so that your curcuit is connected to the device's Vo and V+ for power. Any voltage output of your circuit is now guaranteed to be within the correct range (unless you do some kind of voltage shifting/amplifying). Your circuit would simply take the place of a potentiometer.
Warning: being powered off of the same battery/electrical system does not necessarily equate to sharing a power supply. For example, an electronic device powered by our bike's electrical system probably has its own voltage regaulator to run on something like 5v rather than the electrical system's 12v. If some other device operates directly off of the 12v and provides a signal to the 5v device, bad stuff will happen. However, if both devices operate off of 5v regulators, then it would be safe to directly interface them. They both share the same reference point for 0v. Due to variations in voltage regulators, they might not both be running on exactly 5v (relative to that shared 0v reference point), but it will be close enough.
If both circuits share a 0v reference, but operate on different voltages, then there's still probably simple ways (circuitry) to scale your circuit's variable voltage to be within the expected range of the device's input. A mechanical servo and potentiometer is way over complicated. I suspect the digital potentiometer approach is even way over complicated.
