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Hand-Held Digital Audio Oscillator

Do you need to test audio equipment, including amplifiers and speakers, in the field and in the workshop? If so, you could use this compact and inexpensive digital audio oscillator. It can produce sine, square, triangle and sawtooth waveforms in the frequency range from 10Hz-30kHz and features three output ranges: 20mV, 200mV & 1V.

Design by Darian Lovett, Words by Mauro Grassi

Specifications

Frequency Range: 10-200Hz in 10Hz steps, 200Hz-1kHz in 100Hz steps & 1-30kHz in 500Hz steps

Amplitude Ranges: 0-20mV, 0-200mV & 0-1V RMS (output amplitude adjustable within the selected range)

Waveforms: sine, square, triangle & sawtooth

Frequency Accuracy: ±4%

Total Harmonic Distortion + Noise: approximately 3%

Output connectors: 2 x RCA parallel mono outputs

Power supply: 9V alkaline battery

Current drain: 25mA

This compact hand-held digital audio oscillator will allow you to quickly test wiring and to diagnose faults in audio systems. It is ideal for testing amplifier and speaker set-ups and is portable and easy to use.

To use it, you simply select one of four waveforms – sine, square, triangle or sawtooth – and set it to a frequency between 10Hz and 30kHz. The digitally synthesised waveform is then available at the two RCA outputs. These two outputs are in parallel and are doubled-up simply for your convenience. It means you can test a stereo amplifier and speaker set simultaneously.

Turning to the front panel, there is a 4-position slide switch that selects one of three levels for the output signal: 20mV, 200mV and 1V. Each selected level can be continuously varied down to zero with the “Level” control.

There are also three pushbuttons on the front panel. The two on the right increase or decrease the frequency of the output waveform. The output frequency and the waveform type are shown on a blue backlit LCD screen.

Pressing the “Wave” button on the left while at the same time pressing the “Down” button on the right lets you scroll through the four different waveform types: sine, square, triangle and sawtooth. It’s that easy!

Circuit details

Fig.1 shows the circuit details. It uses an Atmel microcontroller (IC1) to implement most of the features.

The unit is powered from a single 9V battery. As shown, the +9V rail is fed via reverse polarity protection diode D1 to one pole of a 2P4T (2-pole 4-throw) switch, S4a. In three of the four positions, the switch feeds the resulting +8.4V rail on D1’s cathode to voltage regulator REG1.

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