Designing a UJT-Based Audio Tone Generator Circuit

Designing a circuit that uses a Unijunction Transistor UJT as an audio tone generator is a fun and simple project.

It creates a cool electronic musical instrument that can play different sounds.

This circuit works based on a relaxation oscillator which is a key feature of the UJT.

You can use different pushbuttons to change the oscillation frequency allowing you to create various musical notes.

The sound is then amplified and sent to a speaker so you can hear it.

When you press a pushbutton it sets a specific tone in the circuit causing the UJT 2N2646 to oscillate at a frequency that matches that tone.

The transistor boosts this frequency producing sound that comes out of the speakers.

This type of circuit is great for schools, electronics enthusiasts and anyone interested in experimenting with sound.

To power the circuit, you can use a 9V battery or a 9V DC power supply to make it work.

Circuit Working:

Parts List:

Above is a diagram of a cool Audio Tone Generator that uses a special part called the Q1 UJT 2N2646.

When you press any of the buttons, you can create different sounds.

You can even set each button to play a specific tune that you like!

The 2N2646 UJT works like a relaxation oscillator making regular pulses that control how high or low the sound is.

Here is how the circuit works:

When you press a a on/off switch S1–S8 a connected preset resistor VR2–VR9 decides how long it takes for a capacitor C2 to charge up.

Once C2 gets enough charge, it hits a certain voltage that makes the UJT turn on for a moment letting C2 release its charge quickly.

This happens over and over creating a sound frequency based on the RC time constant.

By changing the preset resistor VR2–VR9 you can change how fast C2 charges which gives you different sound frequencies.

Each buttons S1-S8 is linked to a unique musical note.

Since the sound signal is pretty weak we use a Q2 2N2222 transistor to make it stronger.

The boosted signal then goes to an 8Ω speaker, which produces the sound you hear.

Formulas with Calculations:

Below are the formulas with calculations for Designing a UJT-Based Audio Tone Generator Circuit:

Frequency Calculation for UJT Relaxation Oscillator:

The frequency of oscillation is given by the formula:

f = 1 / ( R * C * ln(1 / (1 – η)) )

where,

• f is the frequency in Hz
• R is the resistance in ohms Ω
• C is the capacitance in farads F
• η is the Intrinsic standoff ratio of UJT for 2N2646 which is around 0.6 – 0.8
• ln is natural logarithm function

Example Calculation for 2N2646 UJT

Using the values:

R = 250kΩ (250 × 10³ Ω)
C = 0.1µF (0.1 × 10⁻⁶ F)
η = 0.65

Substituting into the formula:

f = 1 / ( (250 × 10³) * (0.1 × 10⁻⁶) * ln(1 / (1 – 0.65)) )

f = 1 / (250000 * 0.1 × 10⁻⁶ * ln(2.857))

Approximating values:

ln(2.857) = 1.05

f = 1 / (250000 * 0.1 × 10⁻⁶ * 1.05)

f = 1 / (0.02625)

f = 50 Hz

Each preset resistor (VR2-VR9) changes the frequency generating different musical tones.

How to Build:

For Designing a UJT-Based Audio Tone Generator Circuit following steps should be followed:

• Gather all the components mentioned in the above circuit diagram
• Connect the emitter of UJT Q1 to one end of C1 capacitor and other end of C1 to GND.
• Connect B1 pin of UHT Q1 to positive supply through resistor R3.
• Connect the B2 pin of UJT Q1 to one pin of VR1 pot and other pin of VR1 to GND.
• Connect resistor R4 between B2 pin of Q1 UJT and one pin of pot VR1 and GND.
• Connect base of transistor Q2 to one end of capacitor C1 and other end of C1 connect to pot VR1.
• Connect one end of resistor R1 between base of transistor Q2 and capacitor C1 and other end of R1 to positive supply.
• Connect emitter of transistor Q2 to GND.
• Connect the collector of transistor Q2 to one end of 8 ohms speaker and other end of speaker connect to resistor R2 and positive supply.
• Connect each switch S1-S8 and adjust the corresponding preset VR2 – VR9 resistor to fine tune the musical notes as shown in circuit diagram

Conclusion:

Designing a UJT-Based Audio Tone Generator Circuit with the 2N2646 shows how a Unijunction Transistor works as a relaxation oscillator to create various musical sounds.

This circuit is a great way for beginners and electronics enthusiasts to learn as it highlights how to control frequency using RC time constants.

Using UJT 2N2646 is best option making it a dependable and simple electronic to generate various tone as per our likes.

References:

Datasheet UJT 2N2646