A circuit for a telephone “bug” is shown in Figure . Again, the RF portion is identical, but the method of powering the circuit and the audio input circuitry is different. This unit is designed to be directly powered by the voltage available from the telephone line, with audio being fed to the circuit directly from the telephone line. How this is achieved can be seen in Figure 3-3. The circuit consists of a basic VHF oscillator tuned to a frequency in the FM broadcast band. In most telephone systems, a two-wire loop that connects the subscriber to the central ofﬁce is used, with the loop current around 20 mA and the loop voltage usually 48 volts DC. When the phone is on the hook, it appears as an open circuit, and the full 48 volts is present on the line. (The actual voltage may vary somewhat.)When the telephone is taken off the hook, the voltage across the phone drops to a much lower value, around 5–10 volts, and a current of nominally 20 mA will ﬂow. Superimposed on this line are audio and tone signals. In addition, an AC ringing voltage of around 100–120 volts at a frequency of 20 Hz may be supplied by the central ofﬁce to actuate the ringer in the phone when a call is received. This “bug” can be placed in series with the hot leg of the telephone line. The DC line current may be used to derive a voltage for powering a small circuit such as the transmitter shown.R1 and a diode bridge D2 to D5 are inserted in series with the line, ensuring correct DC polarity. R1 shunts excess current around the circuit and passes 8–9 mA, about half the normal off-hook line current. C1 is an RF bypass that prevents strong RF signals accidentally picked up by the phone line from being detected in bridge D2 to D5, and thus appearing as audio on the phone line.
The voltage drop is regulated by zener diode D1 to 9.1 volts and ﬁltered by C2. R5 is placed in series with the DC supply to sample the audio signal on the line and should be as low in value as possible. Line audio voltage drop across R5 is fed via C2 to the oscillator circuit, which is similar to that used for the wireless mike and is also tuned identically. This voltage causes frequency modulation of the oscillator operating in the 88- to 108-MHz FM broadcast band. Bridge rectiﬁer D2 to D5 causes about a 1.3-volt drop. The total voltage drop from this circuit is 11–12 volts. This is about one-quarter of the available open circuit voltage and should not cause any problems in most instances. This “bug” can be easily detected by a knowledgeable technician because of the high off-hook circuit voltage present on the phone line. This voltage is approximately 16–22 volts, which is 11–12 volts higher than that normally encountered (5–10 volts); however, this circuit has the advantage of not having a battery that must be replaced periodically. The antenna can be a short (6-inch) wire, or the telephone line itself can be used as an antenna.