The current draw for this tracker is 3.7mA, so the 1.5V button cell will last awhile. What the heck am I suppose to hear you ask? When your circuit is working you should see the LED flash quite fast. Take your FM radio and search for the low-beat ‘humbe-humbe-humbe-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). Anyways, play with it and learn. It is a nice project. The 12-inch antenna can be anything, it is not really that critical. I used a piece of 22 gauge flexible wire. I haven’t checked the range but will do that shortly.
* For stability, use a NPO types for C2 & C4.
* Resistance tolerance for R1 should be 1 or 2%.
* Frequency range is the usual 87-109Mhz on the FM dial.
* The coil is made from 22 ga ‘hookup’ wire, like the solid Bell phone wire. Leave the insulation on.
* The LED is the ‘High Brightness’ type for maximum illumination.Parts:C1= 100uF electrolytic capacitor
C2= .01uF disc capacitor
C3= 4 to 40 pF trimmer capacitor
C4= 4.7 pF trimmer capacitor
L1= 0.1 uH, 6 to 8 turns of 22 gauge hookup wire close wound around a 1/4″ diameter non-conductive core, such as pencil
IC1= LM3909 LED flasher
LED1= Red LED
Q1= 2N3904 NPN silicon transistor
Antenna= 10 to 12 inches of hookup wire