Month: May 2015

Motorized potentiometers

And other ways to remotely control existing dials.

Suggestions from


Motorized potentiometers

from p3america:

Screen Shot 2015-05-11 at 1.38.41 PM


From Online Controls:

 Motorized faders

Sparkfun motorized fader (like the kind used in DAW control surfaces)


Servo control and sensing

from electronics stack exchange:



The Potentiometer Handbook by Carl David Todd:

Google search: servo to turn a knob

Shower temperature control from SmithyTech:

Arduino HVAC Servo Thermostat/Controller by tikka308:

What kind of motor would I need to turn this central heating valve? – observations about the difficulty of using robots to turn knobs

Sous vide cooker with feedback control:


Homemade Electric Telescope Focuser:


Use your Raspberry Pi to move parts of a robot or control anything that can rotate – by Rob Zwetsloot

Substituting components in parallel

Resistance and capacitance in an AM radio.

A first test to find out if its practical to ‘piggyback’ external controls on to an existing radio. The reason for doing this is to leave an original radio intact by clipping the remote components to the leads of the existing controls.

For example a varactor would be connected in parallel to the variable capacitor already in the circuit. The existing capacitor would be set low. The capacitance of the varactor would then be added to the total, using the formula for parallel capacitors.

For potentiometers, its not as easy because parallel resistors are divided:


Formula:  Rtotal = R1×R2/(R1+R2)

For example if R1 is 10K, R2 would need to be 100K to get a total resistance of 9K. To get 99% of the existing resistance, the piggyback resistor needs to be 100 times the value of the existing resistor. 1 MegOhm if matched with 10K.

Practical considerations

What happens when the radio is not being controlled remotely?

  • For capacitance, the remote capacitor (varactor) should be set to 0.
  • For resistance, the remote resistor should be set as high as possible.

Conversely, how should the physical controls on the radio be set when operating remotely?

  • Variable capacitors should be set as low as possible.
  • Potentiometers should be set as high as possible. For a volume control this actually means turning the volume all the way down.


SPST switches can be considered as a form of potentiometer with infinite resistance. A piggybacked switch will only work if the existing switch is in the ‘off’ position. And vice-versa.

Double-Throw and Rotary switches present more difficulties as multiple states are maintained by the same device.

I don’t think multiple throw switches can be piggybacked. Two possible solutions:

  • mechanical connection to manual control (servo)
  • internal relays – requiring modification of the radio, so that the existing control and the remote control operate the same relays
  • Hybrid approach: Operate the switches manually while operating other controls remotely.


I piggybacked a tuning capacitor from an AM radio onto the tuning capacitor of a vintage Radio Shack Globe Patrol (regenerative receiver).



Voltage controlled variable capacitors

Also called varicaps. They are diodes operated in a reverse bias condition. As voltage increases the capacitance decreases.

Read this first

Tutorial by Phillip Atchely, KO6BB

More tutorials

Using Varactors by Stefan Hollos and Richard Hollos:

Tutorial by Ian Poole at Radio-Electronics 

Another tutorial from Radio-Electronics?:

Threads from The RadioBoard Forum:

Varactor tuned regenerative radio by Tony G4WIF “The Two Dollar Regen” : … ntid=28430 More information here: