Reproduced in accordance with s. 30 Copyright, Designs and Patents Act 1988, circuit diagrams were in the public domain prior to publication.
The circuit designer will have a supply voltage, varying in amplitude, and of uncertain polarity. To obtain a usable DC voltage (since the polarity of the voltage on the telephone line may swap over in various modes of use) all that is required is a simple bridge rectifier circuit, as seen in Figure 4.4, to ensure correct polarity at all times [Figure 4.4 omitted – if you don't know what a bridge rectifier looks like, you suck.] By using a bridge rectifier circuit, no matter which way around the device is connected to the telephone line, or regardless of the polarity of the voltage of the line, the output terminals of a bridge rectifier will be correct. Since the voltage on the line is d.c. when the telephone is actually in use, no smoothing is required in a low component count design. Any alternating voltage, or ripple, on the telephone line, is the audio signal, which is used to modulate the transmitter. Although a bridge rectifier device is shown in use in the following circuits, to keep construction small, four individual rectifiers are actually used in construction.
A simple series telephone transmitter is shown in Figure 4.5, with typical construction on stripboard shown in Figures 4.6a and 4.6b. The actual construction of the unit is not at all critical. By using small components, and soldering a few components underneath the board, this circuit can be installed inside a two-way plug-in telephone adaptor with ease, although the aerial wire would only be a few centimetres in length. Although only two breaks in the copper strips are shown, it may be prudent to cut any lengths of unused copper strip to prevent unwanted capacitive effects being introduced. Breaks in the copper strips can be made either by using a tool that is commercially available for this purpose, or by cutting the copper away with a small drill bit held with the fingers. With the exception of a few components, the circuit of the transmitter is the same as that of the simple voice transmitter in Figure 3.3. TR1 forms the oscillator. Modulation is derived from the audio on the telephone line, which is then superimposed on the supply voltage lines of the circuit.
Figure 4.5 Simple series telephone transmitter
Figure 4.5a Simple series telephone transmitter component layout
Figure 4.6b Simple series telephone transmitter – underside of stripboard
Component listing for the simple series telephone transmitter,
Figure 4.6a
Resistors: R1 = 330R, R2 = 100K, R3 = 330R
Capacitors: C1 = 1nF, C2 and C3 = 5p6
Semiconductors: TR1 = BC547, ZTX300, etc. BR1 = bridge rectifier made from 4 x 1N4148 diodes
Inductor: 7 turns 22 SWG, 6mm diameter
To enable tuning of the transmitter frequency, the coil may be iron dust slug type, or C3 may be of the trimmer type. The aerial wire should not be more than 150mm or so, since because there are no RF chokes in the supply of the circuit, feedback may cause the device to stop transmitting. The simple transmitting device may be greatly improved upon by adding RF filtering and improving the modulation method, as seen in Figure 4.7. The two inductors, L1 and L2, along with capacitor C6, will stop RF re-entering the circuit. Resistor R4 and capacitor C4 improve audio modulation of the transmitter.
Figure 4.7 Series telephone transmitter with RF filtering
Component listing for telephone transmitter with filtering, Figure 4.7
Resistors: R1 = 680R, R2 = 22K, R3 = 330R, R4 = 22R, R5 = 22K
Capacitors: C1 = 10 uF, C2 = 5p6, C3 = 5p6, C4 = 4.7uF, C5 = 1nF, C6 = 0.1uF
Inductors: L1 as Figure 4.5, L2, L3 = 10 uH
Semiconductors: D1, 2, 3, 4 = 1N4148, D5 = 6V zener diode
A further improvement may be made to the circuit shown in Figure 4.7. For greater transmission range, a power amplifier/buffer may be added after the oscillator circuit, as shown in Figure 4.8. Two turns of enamelled wire are wound on top of the tank coil, to act as a secondary winding. This circuit may prove difficult to tune, since when C8 and L4 are tuned for maximum output, the frequency of the VFO will have to be readjusted to compensate. While the circuit in Figure 4.7 can have a transmission range of around 200m, this circuit will have a much greater range.
Figure 4.8 RF filtered series telephone transmitter with RF amplifier
Whenever designing a surveillance device for use on a telephone system, care must be taken when either drawing excessive current from the telephone system, or introducing too large a resistance in series with the existing equipment, so as not to cause a fault condition.
Component listing for higher powered series telephone transmitter, Figure 4.8
Resistors: R6 = 27K, R7 = 4K7
Capacitors: C7 = 10nF, C8 = 5p6, C9 = 10nF
Inductors: L2 = two turns of enamelled 22 swg on top of tank coil, L3 = 4.7uH, L4 as Figure 4.5
Semiconductor: TR2 = ZTX300, etc.
A simple parallel connected telephone transmitter circuit is shown in Figure 4.9. The circuit is almost identical to that of the simple series telephone transmitter shown in Figure 4.5. The circuit is powered by a PP3 9V battery and because the device uses a power source that is independent of the telephone line, RF filtering should not be required as in the case of a leech device, provided the aerial wire is not too close to the telephone feed wires. The amount of RF power produced by a self-powering unit is only dependent on circuit design and power source, therefore very high powered, long range devices are often available. With reference to the circuit shown in Figure 4.9, the capacitor C5, with a value of around 1 nF, blocks the DC. from the telephone line but allows audio to reach the base of TR1. Resistor R1 limits the amount of audio to the base of TR1, and will typically be 100K. This resistor should also protect the base of the transistor from high voltage spikes that appear on the telephone lines.
Figure 4.9 Simple parallel telephone transmitter
A slightly more complex parallel telephone transmitter, with better design regarding deviation and higher 'invisibility' to line checks is shown in Figure 4.11.
Telephone surveillance transmitters, if installed outdoors, can be rainproofed by enclosing the circuit board in a suitable plastic potting box, which is then filled with potting compound, epoxy resin glue or even car body filler for a cheaper alternative.
Component listing for parallel transmitter, Figure 4.9
Resistors: R1 = 100K, R2 = 15K, R3 = 220R
Capacitors C1 = 1nF, C2 = 5p6, C3 = 5p6, C4 = 1nF, C5 = 1nF
Inductors: As Figure 3.3
Semiconductor ZTX300, BC109, etc.
A further circuit for a parallel telephone transmitter, using a FET device between the telephone line and oscillator, is shown in Figure 4.10.
The design in Figure 4.11 may be altered, by using component changes, to operate on either a 1.5V or a 9V supply. Component changes for the circuit using a 9V supply are shown in brackets in the following components listing.
Figure 4.10 Parallel telephone transmitter with FET input
Figure 4.11 Parallel telephone transmitter
Component listing for parallel telephone transmitter, Figure 4.10
Resistors: R1 = 100K, R2 = 270R, R6 = 2M2
Capacitors C6 = 1uF
Semiconductors: TR2 2N3819 or equivalent
Other components as Figure 4.11 (9V version).
Component listing for parallel telephone transmitter, Figure 4.11
Resistors: R1 = 10K, R2 = 10K, R3 = 470K, R4 = 72K (470K), R5 = 15K (27K), R6 = 27K, R7 = 68R (470R)
Capacitors: C1 = 1uF
Semiconductors: BR1 = bridge rectifier
Other components as Figure 4.9.
[Countermeasures not included because it sucks. Read The Myth and Reality About Eavesdropping instead. Thus concludes the review.]