There are a number of ways to read the position of the tuner indicator on an old radio. One versatile, but more involved way would be to read the capacitance of the tuner itself. Another would be to add a rotary encoder to the proceedings, or perhaps glue an encoder disk to the tuner's large wheel and read it optically.
Since we only need to measure a single position, and we don't need to know that state of the tuner (where it has been) we'll get away with using one of the most simple methods here: a hall effect sensor placed at the desired station number position that can read the presence of a magnet that travels with the indicator needle.
This is our circuit diagram for the Haunted Radio -- the Hall effect sensor has three connections:
- power to 3v3 on the Feather
- ground to GND
- digital out to pin 4
You'll also run a 10k resistor between legs 1 and 3 of the sensor.
Build the sensor harness with three of the jumper wires. You'll plug the sensor legs into the female jumper connectors and do the same with the resistor.
A bit of tape can hold the ends together nicely.
Make sure you match the orientation so that leg one is on the left with the "front" of the sensor (the smaller face) toward you, and goes into the power (red) wire.
Trim the legs of the resistor and then push them in to contact the sensor legs 1 and 3.
Plug the red power wire into the 3v3 terminal block, then screw it down.
Plug in and screw down the black wire to GND.
Lastly, plug the yellow wire into the pin 4 (marked SDA on the board) terminal.
You can plug in the battery to the JST connector on the Feather board now -- check that the switch on the Terminal Block Breakout Board is in the "on" position. You'll turn on the whole system with the radio's on/off knob.
Decide what frequency you'd like to use as your Haunted Radio's "cursed" frequency. Place the Hall effect sensor over this position (you may need to temporarily place the assembly back into the radio's case to determine the position).
Place the magnet on the dial -- we'll check for proper orientation later when we upload the code to the Feather. In case the dial isn't ferrous you can use tape or hot glue.
Tilt and bend the sensor legs so that the sensing face is nearest the magnet -- on many Hall sensors this is the larger face -- then affix the sensor to the radio with tape or hot glue.
The magnet should be easily read by the sensor at this distance, but you can fine tune it later.
Strip the ends of the radio's speaker wires, then screw them into one of the output pairs (either Left or Right) on the Music Maker FeatherWing.
Neaten up the whole package by combining the Feather stack and the battery using zip ties or rubber bands. I've placed a small piece of felt between them to keep the exposed terminal points on the bottom from poking the heat shrink of the battery pack.
Before fully reassembling the radio, let's upload the code to the Feather!