Theory — Transistor Switching Circuits

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A transistor switch lets a tiny input from a sensor switch a much bigger current on or off. It builds straight on the voltage divider.

Concept 1

The transistor switch

Key words

Transistor switch
a small input that switches a bigger current on.
Base
the transistor's control input; it turns on at about 0.7 V.
Voltage divider
two parts in series that set the base voltage from a sensor.
Output device
the lamp, LED or relay the transistor switches on.

You should be able to give a short "how it works" description of a transistor switch — and recognise its circuit diagram. All resistors are drawn the UK (BS) way, as a rectangle.

Transistor switch. A sensor — an LDR for light or a thermistor for temperature — sits in a voltage divider that feeds the transistor's base through a base resistor. When the base voltage rises to about 0.7 V, the transistor turns on and lets a much larger current flow through its output device (a lamp, LED or relay) — so a small input switches a bigger current. Swapping which arm of the divider holds the sensor sets whether it switches on for high or low light/temperature:

High-light transistor switch: LDR voltage divider feeding an NPN transistor that drives an LED
High-light switch: the LED switches on when light is bright.
Low-light transistor switch: LDR voltage divider feeding an NPN transistor that drives an LED
Low-light switch: the LED switches on when it gets dark.
High-temperature transistor switch: thermistor voltage divider feeding an NPN transistor that drives an LED
High-temperature switch: the LED switches on when it gets hot.
Low-temperature transistor switch: thermistor voltage divider feeding an NPN transistor that drives an LED
Low-temperature switch: the LED switches on when it gets cold.
✍️ How to say it — transistor switch

A sensor in a voltage divider feeds the transistor's .

When the voltage there reaches about V, the transistor turns .

This lets current flow through the (for example a relay or lamp).

See a full answer
A sensor in a voltage divider feeds the transistor's base. When the voltage there reaches about 0.7 V, the transistor turns on. This lets current flow through the output device (for example a relay or lamp).
Practice 1

An LDR-based transistor switch turns an LED on in the dark. Where in the voltage divider is the LDR placed, and what is the base voltage when the transistor switches on?

Answer
The LDR is placed in the divider so that as the light falls the base voltage rises (the LDR's resistance goes up in the dark, raising the voltage fed to the base). The transistor switches on when the base voltage reaches about 0.7 V, letting current flow through the output device.

✅ You can now…

  • describe a transistor switch: sensor → voltage divider → base → output device.
  • explain how swapping the divider arm switches on for high or low light/temperature.
Interactive

Transistor-switch simulator

Build the switch, drag the sensor's surroundings and watch the base voltage reach 0.7 V — the moment the transistor turns on. Then take the four design challenges to earn the 💡 Bright Spark badge.

Explore

Choose a sensor and which arm of the divider it sits in, then drag the slider.

Sensor
Sensor position
Dark Bright
Transistor switch: a sensor in a voltage divider feeds the base through a base resistor; the transistor switches an LED on its collector. +5 V 0 V R LDR Rb T LED Rs

Design challenges scored

For each brief, pick the right sensor and the arm it belongs in. Get all four correct to earn 💡 Bright Spark.

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Check

Check your understanding

Answer the multiple-choice questions, then mark yourself with the RAG self-check.

RAG self-check

Tap Red / Amber / Green for how confident you feel.