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Lesson: Chapter 11 — Electronics

The transistor at work 3 of 4

The transistor as a switch

A transistor can also act as an electronic switch, working in response to some sensor instead of a hand. In digital circuits that is mostly how transistors are used.

Activity 11.5

You will need: a 2SD313 transistor, a multimeter, a 2.5 V bulb, a 3 V battery, a 10 kΩ volume control VR, a 10 kΩ resistor, a project board and a switch S

  1. Build the circuit and turn VR fully clockwise so its resistance is least. Connect the battery with S off.
  2. Set the multimeter to 2.5 V DC and connect it between base (probe +) and emitter.
  3. Switch S on, then slowly turn VR anticlockwise, raising its resistance, watching the reading and the bulb.

Figure to be added

Activity 11.5 — the circuit: 10 kΩ and V_R as a divider setting the base voltage, a voltmeter across base and emitter, the bulb in the collector lead.

The observation

The bulb starts to glow when the reading nears 0.7 V and is brightest at about 0.8 V.

Below 0.7 V between B and E

No collector current IC flows.

The transistor is a switch OFF.

Above 0.7 V (about 0.8 V)

A maximum collector current flows.

The transistor is a switch ON.