Altoids Tin FM Radio

…and now for something completely different! I got interested in super-regenerative receivers a few weeks ago, investigating ways to do simple, cheap radio links. Turns out it’s REALLY easy to do a few hundred feet with only a few parts. The secret words are “Super Regenerative”. A single transistor acts as an oscillator and detector and everything else, all in one shot. It’s an incredibly clever circuit, but it’s also old- older than even the super heterodyne. I won’t go over how it works- I will leave that up to Wikipedia.

My design isn’t 100% original, I poked around on the net for superregen circuits and then cribbed some ideas and came up with this circuit. It uses a second oscillator to quench the RF oscillator, which performs the detection of the signal. I used a schmitt trigger for this, setting it up as a simple RC oscillator, then used the approximate sawtooth produced on the capacitor to push the RF oscillator in and out of oscillation. This quench frequency is around 50-70KHz depending on the setting of the quench frequency. (This is adjusted so that it doesn’t cause a beat note with the 19KHz stereo pilot)

So, that’s what it is. Everything ended up fitting very neatly into an altoids tin. I soldered a piece of metal from the top of an RF shield to act as a battery holder. There’s a few wire loops soldered to the sides to hold wires, and I used some molex connectors to make a neat “install”. Unfortunately, the pictures aren’t very great.

Oh yeah, almost forgot. The coil is 4 turns on a 1/4″ form of #22 or so wire, tapped at 1 turn. Tap is near the ground end. Other than that, building it shouldn’t be difficult. If you cannot locate a suitable varactor diode, a variable capacitor will work… around 60pf max would be good. It’s hard to see, but I built it on perfboard, with the solder pad side up. The blank side of the perf is flat against the bottom of the tin for good shielding of the circuit. I used some resistor ends to attach the PCB to the bottom of the tin by soldering it. (Altoids tins are super nice to solder to, it takes the solder readily and easily due to the thinness of the metal). I used a mix of SMD and through hole depending on what I had to jump. The 74C14 is soldered in SIP fashion which ended up working very well.

View of the inside.

Another inside view.

Aaand the schematic. I forgot to make the part designations a darker colour. Sorry ’bout that. I will eventually get a better version.

2 thoughts on “Altoids Tin FM Radio”

  1. The headphone wire acts as the antenna. The 10pf capacitor from the RF amplifier filters off the audio leaving nothing but the RF, and the ferrite beads from ground and the LM386 block the RF from getting into the rest of the circuit (which usually just gets shunted to ground through the low impedance of the amplifier’s output or ground itself for that matter). This is why I said be careful of the jack you use- If you attach it to the altoids tin and it has a metal barrel (the threaded portion of the jack), then it will cause two problems: first only one side of the headphones will work (the way it’s wired puts both headphone speakers in series). Second, it will ground out the headphones which will also ground out the RF we need to power the radio. I used a plastic body connector for my radio.

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