EASY TRF AM RECEIVER (note: Quiet Sound)

For those of you who are fed up with DIY videos of "simple" radios that end up being 30-piece, superheterodyne receivers, I give you the one and only AM radio that uses 2N3904s. Elegant in its simplicity, this radio can give a clear (but faint) audio signal WITHOUT complicated circuitry. This receiver is based on ElectroBOOM's AC voltage detector, using multiple 2N3904s, a tuned RF filter, and a diode to power a pair of cheap, 32-ohm earphones.

If you end up building this project, I encourage you to post an "I made it".

***In the picture, one of the 9v battery leads was not connected to the power rail. I was too lazy to add a switch, so when I want to turn the radio off and on, I jam the wires into the power rail manually.

-1x antenna coil (I made mine by wrapping approx. 20 turns thicker wire around a ferrite rod, which can be bought online or salvaged from a broken radio. If you are using thinner wire, 80 turns should do it)

-4x 2N3904 NPN transistors (equivalents will also work, I found the BC547 works best)

-1x variable capacitor. (Mine was 140-60 picofarads, I recommend you buy a large metal 365 picofarad one like the kind used in homebrew ham radios as they are more durable)

-1x germanium diode

-1x 330 ohm resistor

-1x 100 kilo ohm resistor

-1x 10 nanofarad capacitor (marked 103)

-1x 9v battery and connector

-1x cheap 32 ohm magnetic earpiece

I added a picture of my antenna coil and variable capacitor in the photos above.

I stripped the wires of airplane earbuds and soldered them to thicker, stranded wire I removed from an old tube TV. Then, I taped the whole thing to a piece of cardboard.

TAKE CARE NOT TO BREAK THE EARPHONE WIRE. IT'S VERY THIN.

The antenna coil and variable capacitor pick up radio waves, and the germanium diode acts as a crude envelope detector, extracting the audio from the amplitude modulated signal. Once the audio is extracted, it is amplified by the transistors in Darlington configuration.

Time to assemble the radio. I wired mine up on a breadboard, only because my soldering skills are awful. Make sure you wire the transistors correctly, because they release magic smoke easily.

Stick the 9v battery connector wires into the power rail. The variable capacitor serves as the tuning dial. You should be able to get very faint sound from the earpiece. I picked up ESPN and a Kars 4 Kids ad.

This section is the most extensive part of the instructable. If you managed to get this device up and running on the first try, you're either a genius or incredibly lucky.

Check Connections: A common issue I encountered was with the transistors. Sometimes, the collector wasn't fully connected to the power rail, resulting in a continuous clicking sound in my earpiece.

Transistor Test: Touch the cathode of the germanium diode. If you don't hear a hum, it indicates that:

1. A) Your transistors are faulty.
2. B) Your transistors aren't properly connected.

Alternatively, you might be in an area with minimal EM interference.

Antenna Coil Adjustment: If the previous steps don't resolve the issue, consider rewinding your antenna coil on a larger core with additional turns. In an attempt to make my radio more compact, I sacrificed signal for small size when I wound my antenna coil on a miniature ferrite bar I removed from an unsuspecting Sony.

Capacitor Conundrum: If you have a variable capacitor with three pins, the first and third pins should be soldered together. I added a PDF to show how you should wire your capacitor. Sorry about the name.

I'm working on a radio with louder, higher quality sound using the TA7642. Don't forget to post an "I Made It".