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Project Notes

#468 BreadboardBling/DuplexTuningCapacitor

Making a breadboard-compatible variable capacitor for radio circuits.



Duplex tuning capacitors aka radio or AM variable capcitors contain two sets of capacitor plates ganged together, with fine tuning adjustment screws for each capacitor. Total capacitance is typically under 300pF, but mulitple arrangements are possible as either a single or dual variable capacitor.

These components are typically used for radio tuning, and while building radio circuits on a breadboard can be a hairy proposition due to all the stray capcitance, a solid mounting for use on a breadboard can minimise the evils.

the specific part I’m using is a duplex 223p 60pf/140pf 2.1mm Hole Shaft Radio Variable Capacitor from a seller on aliexpress.

How They Work

The main tuning dial controls two variable capacitors (Co, Ca) that share a common connection G. Ca is typically around twice the capacitance of Co. Trimming capacitors are in parallel with each (Cto, Cta).

Ca and Co may be used as two linked/proportional variable capacitors with a common “ground”: connections G & O, and G & A.

Alternatively, O and A may be bridged for a single variable capacitor between O/A & G or maximum capacitance.

For my specific part (without trim), I’ve measured the capacitance range at around:

  • Ca: 18pF to 160pF
  • Co: 17pF to 75pF
  • or combined, from 35pF to 235pF



I’ve simply mounted the capacitor on a piece of protoboard large enough to raise the tuning dial off the deck, with two rows of header pins at the base and a little hot glue on the reverse to keep the capacitor secure.

This provides a very solid mount, so that when on a breadboard it can easily be adjusted without needing a second hand to keep things in place.

The O-G-A pins are unbridged and connect directly to the breadboard, so any combination of capacitance can be used by making the appropriate connections.



Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.