Three Transistor Radio Variable Capacitor

Finding a cheap variable capacitor is always a problem these days, and here I am using a recycled one, which came from a broken radio. It consists of a pair of 130 pF stages, and a pair of 30 pF stages.

All I have done here is connect all the stages together in parallel to make one 320 pF variable capacitor. Remember that when the capacitors are connected in parallel, the capacitance adds together. The centre terminals usually connect to the shaft and are earth, whilst the terminal at each corner is for each stage.

With a cheap digital meter, you can measure the capacitance to make sure you have wired it correctly. The four screws are the trimmers, which are set with the digital meter. Rotating the trimmer screws, changes the capacitance reading on the meter. You need to set all four trimmers to achieve a minimum reading on your meter.

When you mount a variable capacitor, ideally you need to avoid placing any metal, especially any magnetic metal, nearby. In this build, I am using my hot melt glue gun, and a blob of clear plastic glue is good enough to robustly secure the variable capacitor.

This Article Continues...