Circuit Notebook 103 - The Maplin Signal Generator MK105 (Order Code VX94C) (Velleman PMK105'1)

This is a really inexpensive bubble packed kit costing £4.99. The signal generator, when built, provides a choice of four audio waveform outputs, square, 'integrator' (between square and triangular), triangular and sine. The output frequency is fixed at a nominal 1 kHz, (my kit provided 970Hz).

Besides bring useful for general testing purposes, it would provide an audio line-up tone for the home station. When operating portable, it could be used to modulate the transmitter to assist in aligning beam aerials etc.

The kit contains a small PCB (40mm x 56mm) with all the components including a 555 timer IC and battery holder (a PP3 battery is not included). It can be built in about an hour and is shown assembled in Fig.1.

There is an output level control and the desired waveform is selected by a moveable link bridging two rows of pins. It would be possible to replace the pins with a 4-way wafer switch. A 9V PP3 battery fits into the battery holder can be bolted onto the PCB. No switch is provided, so switching on and off is done by fitting or removing the battery.

The circuit only draws about 9mA at 9V so a simple Zener diode regulator could replace the battery and would allow it to be fed from a 12V supply. A suitable diode would be a BZX79C (9.1V) with a 220R feed resistor.

The circuit of the Signal Generator is shown in Fig.2. The 555 timer IC generates a square-wave signal which passes through three simple low pass filters to provide, in addition to the square-wave signal, the 'integrator', triangular and sine waveforms. The final filter circuit employs a transistor connected as a Miller integrator producing a quite respectable sine wave. The maximum sine-wave output level is about 160mV rms. This could be reduced by the output level control and set to 77.5 mV (-10dBm) for testing/line-up etc. The maximum square-wave output is 600mV p-p.

The main features are its simplicity and low cost. It's useful little circuit that I wouldn't bother to build on Veroboard.

Fig.1. Assembled Signal generator

Fig.2. Circuit Diagram