UJT Oscillator

Introduction: UJT Oscillator

About: We are a group of people that are designing analog electronic circuits. Our projects are funded by Renewable Energy Society.

UJT stands for Uni-junction transistor. This article shows you how you can make an oscillator form just one transistor.

For information on UJT oscillator design you can click here:





Parts: Uni-junction transistor (UJT), 10 kohm resistors - 3, 100 ohm resistors - 2, 470 nF pillow capacitor, 1 Megohm variable resistor, insulated wires.

Optional parts: 4.7 uF electrolytic capacitor, solder, box/encasement, masking tape, knob, 1 kohm resistors - 2.

Tools: USB Oscilloscope, pliers, wire stripper, hole puncher.

Optional tools: Soldering iron, audio input sound system (HiFi/computer), speaker/headphones.

Step 1: Make the Circuit

I used high power resistors but you can use low power resistors. We can calculate the power dissipation across the two 100 ohm resistors during transistor saturation.

P = Vs * Vs / (R1 + R2)

= 9 V * 9 V / (100 ohms * 2)

= 0.405 Watt

(this is not assuming the loading affect of the Vo2 output).

I twisted the component and wires together. I did not use a soldering iron for this circuit.

This is a description of the wires that I used:

1. Red - 9 V power supply.

2. Black - Ground.

3. Blue cable - 1 Meg variable resistor.

4. Yellow and White - Outputs.

The three 10 kohm resistors are used for output and variable resistor short circuit protection. In certain positions the variable resistor is a short circuit.

Step 2: Encasement

A box is a good idea because it will protect your circuit from damage.

You can use a hole puncher or a drill to make the hole for the variable resistor.

I attached an old black glue cap with masking tape (you can see in the photo) instead of using a professional knob.

Step 3: Testing

I used a USB oscilloscope to sample the data used to plot the graph that you see in the photo. I found that at certain positions of the variable resistor the oscillation would stop. This would happen for lower frequencies the the variable resistor was set to higher value.

You can try connecting a speaker to the output because the circuit has short circuit protection. You might find that the output signal is very quiet. You will need to connect to a high impedance load or reduce the values of output resistors. This is why I specified using 1 kohm resistor for output. Also, you will need a capacitor for eliminating the output DC component.

Output high pass frequency will equal to:

fh = 1/(2*pi*Ro2*Co2) = 1/(2*pi*(10,000 ohms)*(470*10^-9 F))

= 33.8627538493 Hz

Thus you can use 470 nF capacitor for Co2.

Calculating Co1 capacitor is beyond the scope of this article because both Co1 and Ro1 values will affect the oscillating frequency of the load resistance is below 10 Megohms.

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    Franks Instructables

    Good old UJT, such a useful device that works every time as an oscillator as opposed to phase shift oscillators that quite often can be very fussy about starting.


    Reply 2 years ago

    At certain variable resistor positions the oscillator would NOT oscillate due to control voltage/current being too high or too low. I never encountered the same problem/issue with phase shift oscillators.

    Franks Instructables
    Franks Instructables

    Reply 2 years ago

    Yes, a phase shift oscillator (such as a Wien Bridge) is a better choice if you want it to be a variable frequency oscillator. I was mainly talking about a fixed frequency oscillator, which when I used a UJT always found it to start up easily when set up within certain parameters.

    Maybe I have not seen it, but is there any video of the oscillator working?


    Reply 2 years ago

    The graph shown consists of samples acquired from USB oscilloscope. Also, please read my comments above.