Square Wave Generator

Two types of Square Wave Generator

Type 1: Basic electronic circuit, generating only square pulses

This is a type of device created based on the most basic square pulse generator circuit, the components only include capacitors, Diodes, Transistor (or IC 555) and resistors.

Simple Square Wave Generator circuit available in the market:

See details: 

• Basic electronic circuit, generating only square pulses
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Type 2: Built-in square pulse generator for universal pulse generator

Various types of pulses are emitted in this machine, including square pulses. This is a machine with a very complex internal electronic circuit, and has an additional screen to monitor the graph of voltage, current, and frequency.

See details: 

• Waveform Generator Signal
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• UNI-T UTG962E Function Arbitrary Waveform Generator Signal Source Dual Channel 200MS/s 14-bit Frequency Counter 30Mhz 60Mhz

Various types of pulses are emitted in this machine, including square pulses. This is a type of machine with a very complex internal electronic circuit, and has an additional screen to monitor voltage, current, and frequency graphs. This type of machine has many other names such as universal electrical signal generator, pulse generator, arbitrary wave generator, etc.

In addition to generating periodic square wave, It can generate sine, triangle, trapezoid, rectangular wave, etc.

Theory of Square Wave Generator

1. Circuit using Transistor

Basic circuit diagram:

Multi-harmonic circuit using Transistor NPN to create square pulses

Detailed diagram:

Detailed diagram of multi-harmonic circuit using NPN Transistor

Working principle of NPN multi-harmonic circuit generating square pulse:

When the power is first applied, all the capacitor plates of C1 and C2 are charged, one of the two transistors Q1 or Q2 works first (because in fact, even though the two transistors are of the same type, they are not exactly the same, will one transistor is more sensitive than the other).

We assume that Q1 is more sensitive, so it works first, which means that Q1 has a voltage Vbe greater than or equal to 0.6V (because the voltage at the B terminal of Q1 increases from 0 to 0.6V, before the voltage here equals 0.6V, Capacitor C1 cathode is still being charged), current can go from pole C to pole E and down to ground, so LED D2 is on, the positive pole of capacitor C2 is not charged because the current only goes down to ground. At the same time because Q2 does not conduct (doesn't work) so led D1 is not lit, the anode capacitor C1 will be charged, but will not be charged much. Because the current is now mainly flowing to the ground, the cathode of the capacitor C2 and the negative of the capacitor C1 too, can't charge much.

When Q1 is active, pole B is also considered to be connected to pole E to ground, so the current at leg B is passed through leg E to ground, meaning the voltage at B decreases from 0.6 V to 0V (cathode C1 discharges electricity). When the voltage at pin B discharges, Q1 stops conducting, led D2 turns off, to Stage 2.

Q1 stops conducting, the cathode C2 is loaded with voltage through the current passing through the resistor R1, when the loaded value reaches 0.6V, Q2 leads (due to Vbe >= 0.6V), the C pole of Q2 connects to the E pole. down to ground, the LED D1 is on, the positive pole of the capacitor C1 is discharged, and the positive pole of the capacitor C2 is charged because Q1 is not conductive. The principle is similar to stage 1, the cathode of the condenser C2 discharges voltage to the ground because the B terminal of Q2 is connected to the E pole, when the voltage discharges from 0.6V to 0V, Q2 stops conduction, led D1 turns off, then Capacitor cathode C1 is charged again, making the voltage at the B terminal of Q1 gradually increase to 0.6V, this voltage is equal to 0.6V, then Q1 leads again. These processes are repeated alternately, creating a multi-harmonic circuit with the voltage waveform at terminal C of the two transistors having the shape of a square pulse.

2. Circuit using IC 555

Basic circuit diagram:

Unstable multivibrator circuit can be called a free running multivibrator circuit. It has no steady state and continuously transitions between the two without the use of any external shocks. The IC 555 can be used to work as a multi-two non-stabilized oscillator with the addition of three external components: two resistors (R1 and R2) and a capacitor (C).

Pins 2 and 6 are interconnected so no external trigger pulse is needed. The circuit will automatically trigger the start-up and act as a free-running multi-harmonic oscillator.

The remaining connections are as follows: pin 8 is connected to the voltage source (Vcc). Pin 3 is the output pin and the pulse signal is taken at this pin. Pin 4 is the external reset pin. When this pin is active low, the timer will be reset. Therefore, when not in use, pin 4 is usually connected to Vcc.

The control voltage applied at pin 5 will change the threshold voltage level. But typically pin 5 is connected to ground through a capacitor (usually 0.01µF) to filter out the noise. Pin 1 is the ground pin. The output pulse width of the timer circuit is determined by the components R1, R2 and C .

Working Principle:

The schematic diagram below depicts the internal circuit of IC 555 operating in unstable mode. The RC timer circuit combines R1, R2 and C.

At first when the power is turned on, the flip-flop is RESET (and hence the timer output is low). As a result, the transistor discharges the saturating inductor (since it is connected to Q'). Capacitor C of the timer circuit connected at pin 7 of IC 555 will discharge through the transistor. The output of the timer at this time is low. The voltage across the capacitor is the trigger voltage. So during discharge, if the voltage across the capacitor is less than 1/3 VCC, which is the reference voltage to trigger the comparator (2 comparator), the output of comparator 2 will go high . This will SET the flip-flop and thus the timer output at pin 3 to go high.

A high level at this output will turn the transistor off. As a result, capacitor C begins to charge through resistors R1 and R2. Now, the capacitor voltage is the same as the threshold voltage (since pin 6 is connected to the capacitor resistance contact). While charging, the capacitor voltage increases exponentially approaching Vcc and the moment the voltage across the capacitor crosses 2/3 Vcc, that is the reference voltage to the threshold comparator (comparator 1), the output of the circuit goes high.

 

As a result, the flip-flop is RESET. Output of the timer goes low. This low output again causes the conduction transistor to provide the discharge path for the capacitor. Therefore capacitor C will discharge through resistor R2. And so the cycle continues.

 

So when the capacitor is charging, the voltage across the capacitor increases exponentially and the output voltage at pin 3 is high. Similarly, when the capacitor is discharged, the voltage across the capacitor drops exponentially and the output voltage at pin 3 is low. The shape of the output waveform is a sequence of rectangular pulses. The waveform of the capacitor voltage and output in unstable mode is shown below.

While charging, the capacitor charges through resistors R1 and R2. Hence the charge time constant is (R1 + R2) C when the total resistance in the charge path is R1 + R2. While discharging, the capacitor only discharges through the resistor R2. Hence the discharge time constant is R2C.

Applications of Square Wave Generator

The built-in square wave generator is very useful for radiation energy research. Because it indicates the voltage, frequency, of the voltage generated by electrical transients. Which transient voltage is the target to exploit to generate radiant energy in electronic circuits

Even a basic square wave generator circuit, along with some easy-to-find components on the market, makes it easy to create a Radiant Energy Generator for home.

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