Breadboard

40 Hacking Electronics

Figure 3-13 A bipolar transistor

This transistor has three leads: the emitter, the collector, and the base. The basic principal is that a small current flowing through the base will allow a much bigger current to flow between the collector and the emitter.

Just how much bigger the current is depends on the transistor, but it’s typically a factor of 100.

Breadboard

Figure 3-14 shows the schematic diagram we will build on the breadboard. To understand this circuit, let’s consider two cases.

Case 1: When It’s Dark

In this case, the LDR R1 will have a very high resistance, so you could almost imagine that it isn’t there at all. In that case, current will flow through R2, through the base and emitter of the transistor, allowing as much current as it needs to flow through R3, the LED, and T1 into its collector and out through the emitter. When enough current flows into the base of a transistor to allow current to flow from the collector to the emitter, this is called “turning on” the transistor.

We can calculate the base current using Ohm’s law. In this situation, the base of the transistor will be at only about half a volt, so we can assume there is more or less the full 6V across the 10kΩ resistor R2. Since I = V / R, the current will be 6 / 10,000 A or 0.6mA.

Figure 3-14 Using an LDR and transistor to switch an LED