Useful test and sound effects oscillator with a frequency range of over 20:1 and a built-in speaker

This useful tone generator covers the range between 150Hz to 3.5kHz approximately with a single, continuously variable control; this range can easily be extended if required. The unit has a built-in, high-impedance speaker but an output socket is also provided for connection of an additional speaker having an impedance of 25 ohms or more, or some form of signal processor. The circuit can also drive a low impedance speaker, 8 ohms giving 1 watt RMS or 3 ohms giving 2 watts RMS, but will require a power supply capable of 100 to 200mA. The output of the tone generator is a good quality square wave.

The Circuit

The circuit diagram of the tone generator is given in Figure 1. As with the Metronome project in this issue, the TDA2006 power amplifier is used as an oscillator but in this case it runs at audio frequency and generates an output waveform with equal length 'high' and 'low' states (a square wave). R3 provides feedback to the non-inverting input of IC1, introducing hysteresis and causing the output to be in one of two states, high or low. When it is high, C2 is charged until pin 2 reaches the threshold set by R1-3, and the output goes low. C2 is then discharged until the lower threshold is reached; IC1 then reverts to the high state. This action repeats, generating a square wave having its frequency controlled by the charge rate of C2 and hence the setting of RV1.

Output socket JK1 is a 3.5mm jack having a break contact which disconnects the internal loudspeaker when a plug is inserted. The output signal level is approximately 7 to 8 volts peak to peak.

The current consumption of the unit is 25 to 30mA when using the internal loudspeaker, and about 12mA with no load or a high impedance load across the output.

Construction

The Veroboard layout and wiring of the tone generator are shown in Figure 2. Construction of the component panel is simple as there are no breaks in the copper strips and it is only necessary to drill the two 3.3mm diameter mounting holes (which take M3 or 6BA fixings) prior to soldering the components into position. The preformed leads of IC1 should be bent apart slightly before fitting.

The maximum frequency can be increased by reducing the value of R4, the lower frequency limit can be reduced by increasing the value of RV1.