Components List:
R1 = 10K
C1 = 100uF/10V
C2 = 10nF
C3 = 4-40pF trimmer capacitor
C4 = 4.7pF
IC1 = LM3909
Q1 = 2N3904 NPN transistor
LED1 = Red LED/or another color as you like
L1 = 0.1uH, 6 to 8 turns of 22 gauge hookup wire close wound around a 1/4″ diameter non conductive core, such as a pencil
Antenna = 10 to 12 inches of hookup wire

Notes:

• For the purpose of stability, work with a NPO types for C2 & C4.
• Resistance tolerance for R1 have to be 1 or 2% (use metalfilm resistor).
• The coil is built from 22 ga ‘hookup’ wire, just like the solid Bell phone wire. Leave the insulation on.
• Frequency range will be the usual 87-109Mhz on the FM dial.
• The LED is the ‘High Brightness’ type for optimum illumination.

The current draw for this FM tracker is 3.7mA, so the 1.5V button cell will last a while. When your circuit is working you should see the LED flash quite fast. Take your FM radio and search for the low-beat ‘thumpe-thumpe-thumpe-etc’ equal to the flash of the LED (probably around the 100Mhz). Found it? If that position is interferering with a radio station you can fine-tune it with the variable capacitor. If you like to have the tracker around the 88Mhz you can do that by spreading the windings from the home-made coil just a bit (1/2 a millimeter or so). Well, have fun with it and find out. It is a good project. The 12-inch antenna can be anything, it’s not seriously that crucial. I made use of a piece of 22 gauge flexible wire.

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