On When Off - The Transistor Inverter

So far all of our examples have been using a transistor to turn a device, such as a motor or a light bulb, on when a voltage is applied. You can also use transistors to turn a device off when a voltage is applied. This is called inverting because the state of the output is opposite to the state of the input.

We will use the example of a trip wire. Lets assume that you have a trip wire stretched across a path connected to a buzzer. You want the buzzer to sound when the trip wire is no longer present (someone has tripped over it). This is a perfect problem to solve with a transistor.

First we have to introduce transistor types. All transistors come in two different types: P Channel and N Channel .

N Channel FET	P Channel FET
On when voltage is applied to Gate	Off when voltage is applied to gate..

The only difference in the symbol is the direction of the arrow.

Up until now all of our examples have all been with N Channel transistors. N channel FET's tend to be dominant because they are cheaper to manufacture. However, for our trip wire example we want to use a P channel FET.

When the wire is removed, the buzzer will sound.

Remember that P Channel FET's are off when voltage is applied to the gate. So this circuit will just sit there doing nothing as long as the trip wire is in place. When the wire is removed, the FET will turn on and the buzzer will sound.

As long as the trip wire is in place, the large battery is not doing anything so it will not be depleted. However, the small battery is doing a tiny amount of work providing voltage to the gate of the FET and over time will be depleted. When this happens, the FET will turn on and the buzzer will sound. However, it is such a tiny amount of work required to turn the FET off that a typical battery in this setup will last years.