Friday, October 7, 2011

BFO Metal Detector

I did some designing this week. Here's a spin-off of my experiments. It's a beat frequency operation (BFO) metal detector -- and a good one -- for BFO. It is able to detect an old Victorian penny at more than 100mm. It is based on a novel 555 LRC oscillator. Note that the 555 IC must be a CMOS version, such as the ICM7555 or the TLC555. Instructions for winding the coil L1 may be found at BFO Metal Detector [2]. The output is taken via a wire to a MW radio aerial, and the radio is suitably tuned, to find a good heterodyne. It's a nice, simple project for entertaining a child. One may experiment with the value of R1 to tweak sensitivity. OBSERVATION: This may easily be turned into a self-contained BFO metal detector. A similar circuit is twinned with this one, only with an 820ยต inductor for L1, and a 10k multi-turn potentiometer for R1. A crystal earpiece is wired between the two outputs.
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (

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