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Simple Low Cost Square Wave Generator & Tester Schematic Circuit Diagram

Versatile Square Wave Generator and Tester

This square wave generator and tester utilize the TBA820M audio amplifier chip. It’s presented as a foundational design that invites further refinement through experimentation with component values. The circuit encompasses five frequency ranges, spanning from <0.1 Hz to >70 kHz. Its utility extends to testing various electronic components and equipment such as: Cables, communication devices, electrical interfaces, speakers, headphones, bulbs, transformers, LEDs, couplers, inductors, buzzers, and ultrasonic devices. Essentially, it provides a controlled signal ranging from below 0.1 Hz to 25 kHz and beyond. The circuit handles heavy capacitive or inductive loads effortlessly and can deliver up to one watt of output power.

Simple Low Cost Square Wave Generator & Tester Schematic Circuit Diagram

Versatile Square Wave Generator and Tester: Frequency Ranges and Outputs

Frequency Range Selection and Output Protection:

The frequency ranges of the generator are determined by switches S1-S4, with the option of using a DIP switch with four positions. The smallest capacitor, C1, is always connected, but an additional fifth switch can be added. Keeping C1 at approximately 10 nF and a total resistance of 15 kΩ between pins 5 and 3 of the TBA820M IC results in an output frequency of around 70-100 kHz. However, for reproducibility, it’s advisable to limit the frequency to below 50 kHz. Diode D5 and resistor R11 serve as a discharge path and a protection resistor for larger capacitors. Switch S5 controls the DC component flow, while S6, when closed, eliminates undesired high-frequency oscillations when driving heavy reactive loads. Potentiometer P1 finely tunes the output frequency.

Individual Outputs and Load Testing:

  • OUT1: This main output, protected by stopper resistor R9, is used for testing unknown circuits with large capacitance or inductance or circuits with a few volts. R9’s value can range between 22 to 100 ohms at a dissipation of 0.5 to 2 watts, depending on the application.
  • OUT2: This direct output from the IC tests components like loudspeakers, transformers, or lines with no voltage. Its power output depends on the supply voltage and is approximately 2 W into 8 Ω at VEE = 12 V.
  • OUT3: Controlled by potentiometer P2, this output is safeguarded by resistor R12, varying between 22 to 220 Ω based on the application. It’s ideal for testing headphones, audio lines, small loudspeakers, transformers, amplifiers, buzzers, LEDs, etc.
  • OUT4 and OUT5: Primarily used for amplifier, line, and headphone testing, these outputs find various applications. OUT4’s amplitude is one-tenth of OUT2, while OUT5’s amplitude is 1/100th. All outputs are protected against short circuits to the ground. Diodes D2 and D3 provide under voltage and over voltage protection for each output.

Resistance Range and Supply Voltage:

R2 and P1 together create a resistance range of 15 kΩ to about 250 kΩ. While the generator can function with a 1 MΩ pot, the higher parasitic capacitance, inductance, and noise make it less advantageous. For enhanced frequency adjustment accuracy, an additional linear potentiometer with a value equivalent to 5-10% of P1 could be included in series with P1.

Wide Supply Voltage Range:

The generator operates within a relatively broad supply voltage range of 4-16 VDC (VEE connection). It’s especially suitable for use with a 12-volt car battery, which typically maintains a voltage of around 13.2 V.


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