Intro to Transistors Page 3
More Useful Circuits
Instead of switching the transistor on and off with a tiny battery, we can switch it on and off with other sources of voltage. Here are some example items that create voltage capable of switching a transistor on and off.
- A microphone responds to different noise levels with voltage output.
- A solar cell responds to different light levels with voltage output.
- A moisture sensor responds to different moisture levels with voltage output.
Notice that all of the above sensors respond to a different stimulus with a voltage output. By connecting that voltage output to the gate of your FET you can use the small voltage source to control a much larger device.
In this example we have a microphone connected to the gate of the FET, and the same light bulb connected to the current loop side of the circuit. As you yell into the microphone the light bulb will light up. The louder you yell, the brighter the light bulb will be. This is because the microphone is creating a voltage on the gate of the FET. When the FET senses a voltage on the gate, it allows a large Drain to Source current to flow.
In fact, in this circuit the FET is acting as an amplifier. The signal from the microphone is being amplified by the transistor. Amplification is another use of transistors.
Note: in this circuit we have actually used a speaker instead of a microphone, because speakers will generate a voltage when a loud sound is made near them.
Lets use the same circuit to instead control a motor.
This should look familiar. Now instead of using sound to control a light bulb, we are using sound to control a motor. The louder you yell into the microphone, the faster the motor will turn.