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How to Use a Unijunction Transistor

A transistor is an electronic component used to switch electrical currents. A unijunction transistor consists of two pieces of silicon: a small blob placed more than halfway along a larger chunk. As current rises through a unijunction transistor, voltage falls, reaches a minimum, then rises again. This response makes it useful for relaxation oscillator circuits. In this type of oscillator, a resistor feeds current to charge a capacitor. As the voltage across the capacitor rises, the unijunction transistor triggers, discharging the capacitor and continuing an endless charge-discharge cycle.

Things You'll Need

2N2646 n-type unijunction transistor
Solderless breadboard
47-ohm 1/4-watt resistor
9-volt battery clip
.01-microfarad 50-volt capacitor
470-ohm 1/4-watt resistor
100K-ohm 1/4-watt resistor
9-volt battery
Oscilloscope

Instructions

- 1
  Set the unijunction transistor into the breadboard so its three pins all go into separate columns on the board.
- 2
  Insert one lead of the 47-ohm resistor so it connects to the base two lead of the transistor. Insert the resistor's other lead into a free column. Set the black wire of the 9-volt battery clip into this column.
- 3
  Connect one lead of the capacitor to the column joining the black battery wire and resistor lead. Insert the capacitor's other lead so it connects to the transistor's emitter.
- 4
  Insert one lead of the 100K-ohm resistor so it shares the transistor's emitter with the capacitor lead. Insert the resistor's other lead into a free column. Insert one lead of the 470-ohm resistor so it connects to this column. Insert the resistor's other lead so it connects to transistor's base one lead. Set the red wire from the 9-volt battery so it shares the connection between the 470- and 100K-ohm resistors.
- 5
  Snap a 9-volt battery into the battery clip.
- 6
  Turn the oscilloscope on and set its horizontal sweep to 1 millisecond per division. Connect the oscilloscope's channel one probe to the capacitor lead that connects to the 100K-ohm resistor. Connect the probe's ground clip to the other capacitor lead. Observe the sawtooth-like waveform on the oscilloscope screen.

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