This square wave generator and tester is based on audio amplifier chip type TBA820M. It is presented as a design idea for further refinement and optimizing of component values by way of experiment. The circuit has five frequency ranges covering <0.1 Hz to >70 kHz. It will be found useful for testing cables, communication equipment, electrical interfaces, loudspeakers, headphones, electrical bulbs, transformers, LEDs, couplers, inductors, buzzers, ultrasonic equipment, etc. or wherever you need a signal in the range from below 0.1 Hz to 25 kHz and up with controlled frequency, amplitude and drive capacity. For example, driving heavy capacitive or inductive loads is no problem, nor supplying up to one watt of output power.
The generator’s frequency ranges are selected with switches S1–S4. To be able to use a DIP switch with four positions the smallest capacitor, C1, is always connected but there’s no objection against adding a fifth switch. With C1 at around 10 nF and a total resistance of 15 kΩ between pins 5 and 3 of the TBA820M IC the maximum output frequency is of the order of 70–100 kHz. However with reproducibility in mind it is recommended to limit the frequency to below 50 kHz. D5 and R11 provide a discharge path and a protection resistor for the larger capacitors. Switch S5 allows the DC component from the TBA820M to be blocked or passed to the generator outputs. S6 should be closed when driving heavy reactive loads to eliminate undesired high-frequency oscillations. Potentiometer P1 controls the frequency of the output signal.
OUT1 is the main output protected with stopper resistor R9. It is used to test unknown circuits where large capacitance or inductances could be present, or circuits presenting a few volts themselves. R9 may be dimensioned to suit the application and can take a value between 22 to 100 ohms at a dissipation of 0.5 to 2 watts. OUT2 is the direct output from the integrated circuit. It’s used to test loudspeakers, transformers, lines sure to have no voltage on them, etc. The output power depends on the supply voltage and is around 2 W into 8 Ω at VEE = 12 V. The load does affect the OUT2 signal frequency and amplitude to a small extent but that’s not an issue in most applications. OUT3 is a level controlled output using potentiometer P2. This output is protected by R12 taking a value from 22 to 220 Ω depending on the application. The output is used to test headphones, audio lines, small loudspeakers, transformers, amplifiers, buzzers, LEDs, etc. OUT4 and OUT5 are used mainly to test amplifiers, lines and headphones but will find many other applications. The amplitude on OUT4 is one tenth of OUT2 and the amplitude on OUT5 is 1/100th. All outputs are protected against short circuit to the ground. Diodes D2 and D3 afford some under voltage and over voltage protection on every output. The slew rate of the output signal of the generator with no additional load is more than 20 V/μs and depends slightly on the brand of TBA802M used and the method of wiring it (PCB or free wired).
R2 and P1 together present a resistance range of 15 kΩ to about 250 kΩ. Although the generator will work with a 1 MΩ pot there is a little benefit to be gained due to the higher parasitic capacitance, inductance and noise. On the other hand, an additional linear potentiometer with a value equivalent to 5–10% of that of P1 could be included in series with P1 to achieve better accuracy of the frequency adjustment.
The generator’s supply voltage range is relatively wide at 4–16 VDC (VEE connection). The unit is particularly suitable for operation from a 12 volts car battery with a typical voltage of around 13.2 V.