Enter image description here

I want to build this circuit, but I don't have the proper transistors. Can general purpose transistors like 2N3904, 2N4401, BC547, 2N2222, etc. be used instead of the BC109 in this circuit?

The circuit is a lightning detector.

  • \$\begingroup\$ BC109s are not very good transistors by today's standards. I'd give it a try but be prepared to adjust R2 I'd say. It looks like a receiver with mixer and local oscillator built around TR1 and TR2. Tr3 looks like an AM detector. Trouble is with mixers is that they are very temperamental to the type of transistor they used and likely R2 may need to be increased (or lowered?) to get TR1 mixing and oscillating at the same time. \$\endgroup\$ – Andy aka Sep 6 '13 at 23:34

Direct replacement for the BC109 would be the BC549 rather than 547 (yes there was a BC107!). And the gain grades (A,B,C) line up too so it's really the BC549C you want to replace the 109C (gain about 400 or higher)

BC107 was aimed at switching, BC109 was aimed at low noise amplification, and the distinction lives on in the BC54x series (and their BC84x surface mount replacements)

2N series probably contains a satisfactory equivalent but I'm not so familiar with American transistor series. Look for gain or hfe of 400 or more.


For the most part, yes. The BC109 is not particularly high current (200mA), high gain, or high frequency, and the circuit does not demand any of those. A 2n3904 or 2n2222 would work just fine. These are all general purpose small signal transistors.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.