This is a tiny transmitter that consists on a one colpitts oscillator that derives it’s power from the phone line. Since the resistance it puts on the line is less than 100 ohms, it has no effect on the telephone performance, and can not be detected by the phone company, or the tappee. Since it is a low-powered device using no antenna for radiation, it is legal to the FCC. (That is it complies with part 15 of the FCC rules and regulations).
It, however is still illegal to do, it’s just that what you’re using to do it is legal. This is explained later in part 15… “no person shall use such a device for eavesdropping unless authorized by all parties of the conversation” (then it’s not eavesdropping is it?). What this thing does, is use four diodes to form a “bridge rectifier”. It produces a varying dc voltage varying with the auto-signals on the line. That voltage is used to supply the the voltage for the oscillator transistor. Which is connected to a radio circuit. From there, you can tune it to any channel you want. The rest will all be explained in a minute….
Item – description
C1 – 47-Pf ceramic disk capacitor
C2,C3 – 27-Pf mica capacitor
CR1,CR2,CR3,CR4 – germanium diode 1n90 or equivalent
R1 – 100 ohm, 1/4 watt 10% composition resistor
R2 – 10k, 1/4 watt 10% composition resistor
R3 – .7k, 1/4 watt 10% composition resistor
L1 – 2 uH radio frequency choke (see text)
L2 – 5 turns No.20 wire (see text)
Q1 – Npn rf transistor 2N5179 or equivalent
L1 may be constructed by winding approximately 40 turns of No. 36 enamel wire on a mega-ohm, 1/2 watt resistor. The value of L1 is not critical. L2 can be made by wrapping 5 turns of No. 20 wire around a 1/4 inch form. After the wire is wrapped, the form can be removed. Just solder it into place on the circuit board. It should hold quite nicely. Also be sure to position Q1 so that the emitter, base, and collector are in the proper holes. The schematic should be pretty easy to follow. Although it has an unusual number of grounds, it still works.
The odd thing about this Wireless Transmitter bug that we haven’t encountered yet, is that it is put on only one wire (either red or green) so go to the box, remove the red wire that was ALREADY on screw 1 and attach it to wire ‘A’ of the bug. Then attach wire ‘B’ to the screw itself. You can adjust the frequency which it comes out on the FM channel by either smooshing, or widening the coils of L2. It takes a few minutes to get to work right, but it is also very versatile. You can change the frequency at will, and you can easily record off your radio.