Q2 is used as a voltage controlled resistor connected to a simple filter network based around Qi. The controlling voltage is applied to the base of Q2 and can be varied manually or automatically through the use of a triangle wave generator consisting of a standard integrator and comparator oscillator. The frequency of the sweeps is determined by the ‘speed’ potentiometer and can be altered from 0.5 Hz to about 10 Hz.
A 9 V supply was used so the unit can be fully portable if a PP3 battery is used.