Picture this: It’s the end of the year, and a few hardy souls gather in a hackerspace to enjoy a bit of seasonal food and hang out. Conversation turns to the Flipper Zero, and aspects of its design, and one of the parts we end up talking about is its built-in 125 kHz RFID reader.

It’s a surprisingly complex circuit with a lot of filter components and a mild mystery surrounding the use of a GPIO to pulse the receive side of its detector through a capacitor. One thing led to another as we figured out how it worked, and as part of the jolity we ended up with one member making a simple RFID reader on the bench.

Just a signal generator making a 125 kHz square wave, coupled to a two transistor buffer pumping a tuned circuit. The tuned circuit is the coil scavenged from an old RFID card, and the capacitor is picked for resonance in roughly the right place. We were rewarded with the serial bitstream overlaying the carrier on our ‘scope, and had we added a filter and a comparator we could have resolved it with a microcontroller. My apologies, probably due to a few festive beers I failed to capture a picture of this momentous event.

A Nasty Circuit That Just Has To Be Made

Here on the Morning After the Night Before, I’m sitting thinking about 125 kHz RFID, as this is honestly the first time in many decades playing with radio I’ve given one of these a look. (Though we’ve pondered its 13.56 MHz cousin.) An evil thought forms in my mind; would it be possible to make a single-transistor, self-oscillating 125 kHz RFID reader? It would be an extremely nasty circuit and there is no possible need for one to exist, but it’s the electronic engineers equivalent of an earworm. I know how I would approach it but I don’t know whether my idea would work. I’m thus going to set it as a New Years exercise for you readers.

So, how would I approach this? One of the first electronic projects I made over four decades ago was a regenerative radio. This is a one-transistor receiver for AM radio which applies positive feedback to an RF amplifier to the point at which it’s almost but not quite oscillating. This has the effect of narrowing its bandwith hugely, to the extent that it makes a passable narrowband radio receiver. I would approach the RFID reader using a variation on this circuit; a single transistor regenerative receiver which is just oscillating at 125 kHz, but whose oscillation is quenched momentarily every time the RFID tag loads its coil to indicate serial data. I should thus be able to pull a DC voltage from my emitter resistor, filter it, and return something that could be turned into a square wave. I think something like this could work but I stand ready to be proved wrong What do you think, would this circuit function?

Every Contest Needs a Few Rules:

Your circuit must use only one transistor, no ICs. Diodes and RCL passives are allowed, but also no vacuum tubes, tunnel diodes, or other active components, you lateral thinkers. It must demonstrably read a 125 kHz RFID tag placed within its range, and output something capable of being resolved by a 74 series logic gate of any family, thus decipherable as the serial payload by a microcontroller etc. That doesn’t have to be a TTL-level-compliant gate, and can be a Schmitt trigger. Otherwise it’s up to you. If you do a write-up somewhere, I’d even write it up for Hackaday. So go on, have a go at this one. I’d love to see what awesome awfulness you come up with!