1. Unijunction Transistor Circuit Templates
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2. 1 pn junction 2 pn junctions n n + + pn junction p anode cathode
current
meter +
volt n p
pn junction
This diode is forward biased
anode
cathode
1 pn junction
current
meter
volt + n p
pn junctions
This transistor is in conduction
collector
base
emitter
2 pn junctions

3. 1 pn junction 2 pn junctions n n + + + pn junction p anode cathode
current
meter +
volt n p
pn junction
This diode is forward biased
anode
cathode
1 pn junction
2 pn junctions
current
meter
volt + n p
pn junctions
This transistor is in conduction
collector
base
emitter
Base 2 b2
emitter e + b2 n b1 e
UJT
Base 1 p b1

4. emitter
Base 1
Base 2 e b2 b1
UJT
emitter p n
current
meter +
base 1
base 2
Low voltage signal applied to emitter
OFF
Low current flow from base 1 through base 2
A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit. p n
Low voltage signal applied to emitter
OFF
current
meter

5. emitter
Base 1
Base 2 e b2 b1
UJT + +
Low voltage signal applied to emitter
OFF
Low current flow from base 1 through base 2
emitter p n
current
A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit.
base 2 p n
base 1
meter
High voltage signal
applied to emitter ON
emitter
base 1
current
meter
High current flow from base 1 through emitter

6. Always a small flow of current in this branch of circuit.
Current flows into this circuit branch until voltage at node becomes high enough to stop the flow. B2 + n + p n + + B1 +
capacitor
But not
enough to turn the light on.
Unijunction transistor in a light flasher circuit

7. Always a small flow of current in this branch of circuit.
Current flows into this circuit branch until voltage at node becomes high enough to stop the flow. B2 + n v
threshold + p
When node voltage reaches threshold value, the capacitor discharges current through the emitter to B1 circuit
(This charging current continues its flow into the capacitor until the capacitor becomes fully charged and node reaches the threshold voltage value.) n + + B1 +
Light is ON while capacitor discharge current is flowing.
capacitor
Unijunction transistor in a light flasher circuit

8. Always a small flow of current in this branch of circuit.+ n v
threshold + p
When node voltage reaches threshold value, the capacitor discharges current through the emitter-B1 circuit
Voltage across capacitor drops as capacitor current discharges n + + B1 +
Light is ON while capacitor discharge current is flowing.
Light is OFF when capacitor voltage is
capacitor
below UJT’s threshold voltage value.
Unijunction transistor in a light flasher circuit

9. Voltage across capacitor drops as capacitor current discharges n +
Always a small flow of current in this branch of circuit.
When voltage across capacitor drops low enough current starts following in this branch again and cycle repeats itself. B2 + n + p
Voltage across capacitor drops as capacitor current discharges n + + B1 +
capacitor
Unijunction transistor in a light flasher circuit

11. n p n + Unijunction transistor in a light flasher circuit capacitor B2