A transistor works as an automatic switch when a small base voltage controls a larger current. A potential divider with a sensor sets that base voltage.
This lesson is part of SPM Physics electronics. It uses the divider from solving series and parallel circuits.
How does the base voltage get set?
Two resistors in series share the supply voltage. The base is connected between them. The voltage at the base is the share of the resistor below the connection.
Base voltage = supply voltage × R(bottom) ÷ (R(top) + R(bottom)). If it reaches about 0.7 V, the transistor turns on, and the lamp in the collector circuit lights.
Worked example: a lamp that lights in the dark
A 6 V supply has a 2 kΩ fixed resistor on top and an LDR at the bottom, with the base connected between them.
- In the dark the LDR is 10 kΩ. Base voltage = 6 × 10 ÷ (2 + 10) = 5.0 V, so the transistor is on and the lamp is lit.
- In bright light the LDR is 100 Ω. Base voltage = 6 × 0.1 ÷ (2 + 0.1) = 0.29 V, which is below 0.7 V, so the transistor is off and the lamp is out.
The LDR sits at the bottom because a high resistance in the dark has to raise the base voltage.
The mistake that loses marks
A student places the LDR on top for a lamp that should light in the dark. In the dark the top resistance is large, so the base gets a small share of the voltage, and the lamp stays off.
Check the placement with numbers, as above. If the dark case gives a base voltage below 0.7 V, swap the positions.
Check yourself
A fire alarm must switch on when hot. The fixed resistor is at the bottom and an NTC thermistor is at the top. Explain whether this works.
Answer
When hot, the thermistor’s resistance falls, so the top resistance is small. The fixed resistor at the bottom then takes a larger share of the supply voltage, and the base voltage rises above 0.7 V. The transistor turns on and the alarm sounds.
So the thermistor at the top is the right position, and the circuit works.
What to study next
See how signals are displayed in explaining electron-beam and electronic-device principles. Test the circuit reading in the practice set.
The simple circuit calculation tutor helps check dividers. If you want a teacher to go through transistor circuits with you, see online one-to-one Physics tuition.