Electronics for Kids: Play with Simple Circuits and Experiment with Electricity! by Oyvind Nydal Dahl

Author:Oyvind Nydal Dahl [Dahl, Oyvind Nydal]
Language: eng
Format: azw3
ISBN: 9781593277253
Publisher: No Starch Press
Published: 2016-07-08T04:00:00+00:00

Controlling an LED with a Transistor

Other projects in this book have used this simple LED circuit, complete with a resistor and a battery:

Based on what you’ve learned so far, what do you think would happen if you put a transistor between the resistor and the battery’s negative terminal?

With no voltage on the transistor’s base, or the controlling pin, no current flows from the base to the emitter. That means no current can flow between the collector and the emitter either, and the LED would be off.

But if you were to apply a small voltage to the base—for example, by connecting a small battery to it—the transistor would let current flow from the collector to the emitter, and the LED would glow. A transistor that allows current to pass is considered on; a transistor that doesn’t allow current to pass is considered off. The amount of voltage needed to turn an NPN transistor on is about 0.7 V, so a circuit like this one would allow current to pass through properly to light the LED:

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