The circuit diagram of the relay driver and multivibrator is given in Fig. 6. Here TR1 and TR2, in modified super alpha configuration, drive the relay. The relay contacts are shown in the quiescent state. A 1W resistor, Rl, can be inserted in the “polarise” circuit to reduce the detector coil current and so cut down on battery consumption. It follows that the higher the value of this resistance the smaller will be the signal presented to the main amplifier, so the choice of value should be made when the unit is completed and tested; 4.7 ohms is a suitable value to start with. “Cut and try” methods should provide balance between a tolerable signal and battery economy.

The multivibrator, comprising TR3 and TR4, provides an equal mark-space output of three seconds duration at the collector of TR3. This is passed via S2, when switched to the “auto” mode, to the base of TR2.

Details for assembling and wiring this circuit module are
given in Fig. 7.