One Transistor FM Radio Project

Build this one transistor FM radio

Click on photo to view enlarged picture

NEWS

A printed circuit board for this circuit is available through FAR Circuits.  Ask them for "Andy Mitz's One transistor FM radio printed circuit board".  The PC board includes one additional capacitor in series with the earphone.  It also has two sets of holes for C1 so you can put two caps in series.  A new schematic will be posted here soon.

Introduction

AM radio circuits and kits abound.  Some work quite well.  But, look around and you will find virtually no FM radio kits.  Certainly, there are no simple FM radio kits.  The simple FM radio circuit got lost during the transition from vacuum tubes to transistors.  In the late 1950s and early 1960s there were several construction articles on building a simple superregenerative FM radio.  After exhaustive research into the early articles and some key assistance from a modern day guru in regenerative circuit design, I have developed this simple radio kit. It is a remarkable circuit.  It is sensitive, selective, and has plenty of audio drive for the earphone.  Read more about theory behind this radio on the low-tech FM page.

Construction

parts source

Except the the circuit board and battery, all parts are from Mouser Electronics.  A complete parts list with stock numbers is listed below. I built the prototype on a perforated board with solder pads.  Radio Shack sells these boards for a reasonable price.  A plain piece of perforated board will also work.

layout

Because this is a superregenerative design, component layout can be very important.  The tuning capacitor, C3, has three leads.  If you orient C3 with the  tuning control facing you and the three leads pointed up, you can identify which leads to use.  The middle lead of C3 goes to ground and the left lead goes to L1.  The right lead is not connected.  Arrange L1 close to C3, but keep it away from where your hand will be.  If your hand is too close to L1 while you tune the radio, it will make tuning very difficult.  

winding L1

L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for superregeneration.  Although it has many important jobs, it is easy to construct.  Get any cylindrical object that is just under 1/2 inch in diameter.  I used a thick pencil from my son's grade school class, but a magic marker or large drill bit work just fine.  #20 bare solid wire works the best, but any wire that holds its shape will do.  Wind 6 turns tightly, side-by-side, on the cylinder, then slip the wire off.  Spread the winding apart from each other so the whole coil is just under an inch long.  Find the midpoint and solder C2 there.  Mount the ends of the wire on your circuit board keeping some clearance between the coil and the circuit board.  

a tuning knob for C3

C3 does not come with a knob and I have not found a source.  A knob is important to keep your hand away from the capacitor and coil when you tune in stations.  The solution is to use a #4 nylon screw.  Twist the nylon screw into the threads of the C3 tuning handle. The #4 screw is the wrong thread pitch and will jam (bind) in the threads. This is what you want to happen.  Tighten the screw just enough so it stays put as you tune the capacitor.  The resulting arrangement actually works quite well. 

Adjustment

If the radio is wired correctly, there are three possible things you can hear when you turn it on:  1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing.  If you got a radio station, you are in good shape.  Use another FM radio to see where you are on the FM band.  You can change the tuning range of C3 by squeezing L1 or change C1.  If you hear a rushing noise, you will probably be able to tune in a station.  Try the tuning control and see what you get.  If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much.  Try spreading or compressing L1. Double check your connections.  If you don't make any progress, then you need to change R4.  Replace R4 with a 20K or larger potentiometer (up to 50K).  A trimmer potentiometer is best. Adjust R4 until you can reliably tune in stations. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor.  Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM-25K).

Substituting other components

Many of the parts are fairly common and might already be in your junk box.  Only certain component values are critical.  The RF choke should be in the range of 20 to 30 uh, although values from15 to 40 might work.  The tuning capacitor value is not critical, but if you use values below 50 pf you should reduce or remove C1. The circuit is designed for the high impedance type earphone.  Normal earphones can be used, but the battery drain is much greater and the circuit must be changed.  To use normal earphones, change R3 to 180 ohms and place a 50 to 100 uf capacitor between the R3 and the earphone.  The positive side of the capacitor is towards R3.  Q1 can be replace with any high-frequency N-channel JFET transistor, but only the 2N4416 and 2N4416A have been tested.  The J310 transistor might work.  A MPF102 probably will not work. C2 is not too critical; any value from 18 to 27 pf will work. C7 is fairly critical.  You can use a .005 or .0047 uf, but don't change it much more than that.   

Schematic diagram

Click here for a nice PDF version of the schematic.

Printed circuit board

I am making arrangements for a printed circuit board, but here are some pointers if you want to design your own:

• You can connect the two sections of the variable capacitor (C3) in series to linearize tuning somewhat.  That is, use the connections on either end of C3 and don't use the middle lead.

• L2, the RFC should not be near a ground. The same is true for L1. Capacitance to ground will disturb the feedback.
  It is common to place a 100uf / 16v capacitor in series with the earphone to protect it. Otherwise, don't connect or disconnect the earphone without killing the power first.

• The gain is just enough to drive an earphone. If you live too far away from radio stations, you might have trouble hearing one.  There is no option here for an external antenna (that would require and extra transistor).

• You can drive a speaker if you add a LM386 buffer.
  

Parts list

All parts except the RF tuning capacitor can be obtained from
Mouser Electronics
www.mouser.com 
sales@mouser.com
1-800-346-6873

The RF tuning capacitor can be obtained from
Electronix Express
electron@elexp.com
1-800-972-2225
In New Jersey 1-732-381-8020

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Last updated 24 July 2001

Andrew R. Mitz
arm@gnode.org
All circuits, text, photographs, and other graphics are copyright (c) 1998-2001 LTJ Designs.