1 transistor fm radio
DESCRIPTION
Transistor Fm RadioTRANSCRIPT
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Build A One Transistor FM Radio
Click on a picture to choose which design to build!
Build this one transistor FM
radio
(my design)
Enlarge: [medium] [large]
or Build this onetransistor FM radio
(Designed by Patrick
Cambre )
Enlarge: [large]
My Design
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".
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 thelate 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 enough
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audio drive for an 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. Acomplete parts list with stock numbers is listed below. The circuit board is available
through FAR Circuits. The variable capacitor is available through Electronix Express.
layout
Because this is a superregenerative design, component layout can be very important. The
tuning capacitor, C3, has three leads. Only the outer two leads are used; the middle leadof C3 is not connected. Arrange L1 fairly 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 tuningvery difficult.
winding L1
L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for
super-regeneration. Although it has many important jobs, it is easy to construct. Get any
cylindrical object that is just under 1/2 inch (13 mm) in diameter. I used a thick pencilfrom 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 windings
apart from each other so the whole coil is just under an inch (2.5 cm) long. Find themidpoint and solder a small wire for 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 tokeep 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 iswhat you want to happen. Tighten the screw just enough so it stays put as you tune the
capacitor. The resulting arrangement 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
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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 certaincomponent values are critical. The RF choke should be in the range of 20 to 30 uh,
although values from15 to 40 uh 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. Q1 can be replace with any high-frequency N-channel JFET
transistor, but only the 2N4416, 2N4416A, and J310 have been tested. 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.
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Printed circuit board
The printed circuit board for the One Transistor FM Radio is available through:
FAR CIRCUITS
Printed Circuit Boards
18N640 Field Court
Dundee, Illinois 60118
(847) 836-9148 Voice/Fax
email: [email protected]
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Some wiring notes:
• Unless you have experience with super-regenerative radios, I highly recommend
using the FAR Circuits printed circuit board.• Connect the two sections of the variable capacitor (C3) in series to linearize the
tuning somewhat. That is, use the connections on either end of C3 and don't use
the middle lead.
• L2, the RF choke should not be near a ground. The same is true for L1.
Capacitance to ground will disturb the feedback.
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 anexternal antenna (that would require and extra transistor).
•
You can drive a speaker if you add an external audio amplifier.
• If you want a little more audio gain, or you cannot locate a TL431CLP chip, you
can use some other audio amplifier in the circuit where pins 1 and 2 of D1
normally connect. You can use an LM386 or a TDA7052 audio amplifier. Quasar DIY project kit #3027 is a complete TDA7052 audio amplifier kit and it works
fine in this application.
Parts list
All parts except the RF tuning capacitor can be obtained from
Mouser Electronicswww.mouser.com [email protected]
1-800-346-6873
The RF tuning capacitor can be obtained from
Electronix [email protected]
1-800-972-2225
In New Jersey 1-732-381-8020
Part designator Part description Vendor stock number
C1a,C1b10 pf, 50 v, ceramic disc
capacitor 140-50N5-100J
C222 pf, 50 v, ceramic disc
capacitor 140-50N5-220J
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C3 RF tuning capacitor N14VCRF10-280P
C4330 pf, 50 v, ceramic disccapacitor
140-50P2-331K
C5,C80.001 uf, 50 v, ceramic disc
capacitor
140-50P2-102K
C6 0.22 uf, 50 v, film capacitor 140-PF1H224K
C70.0047 uf, 50 v, ceramic disc
capacitor 140-50P5-472K
C922 uf, 16 v, electrolyticcapacitor
140-XRL16V22
D1TL431AIZ voltage controlZener (shunt regulator)
511-TL431AIZ
EPH1 High impedance earphone 25CR060
L2 22 uh RF choke 542-70F225Q1 2N4416A JFET transistor 510-2N4416A
R1 470K, 1/4 w, resistor 291-470K
R2, R3 1K, 1/4 w, resistor 291-1K
R4 10K, 1/4 w, resistor 291-10K
R5 1M, 1/4 w, resistor 291-1M
R6 100 ohm, 1/4 w, resistor 291-100
S1 Small SPST switch 10SP003
screws for C3
screws for mounting C3 (2
needed) 48SS03
nylon screw#4 nylon screw used for
tuning C3561-T0440037
battery
connector mini battery snap 12BC025
Note: schematic shows slightly different capacitor values. Either will work.
Please feel free to send me questions and comments at [email protected]
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Last updated 5 July 2002
Andrew R. [email protected] circuits, text, photographs, and other graphics are copyright (c) 1998-2003 LTJ Designs.