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