Intro to Transistors Page 5
One Less Battery
We can actually optimize the circuit above a little bit by removing one of the batteries. Remember that with FET's a small signal to the gate makes a large change in the Drain to Source current. In the trip wire circuit, the signal to the gate is telling the buzzer to be off. If a small voltage is a little bit off, then a large voltage is a lot off. Really they are both just off.
With that in mind, we can use the same battery that powers the buzzer to turn the FET off. All we have to do is connect the trip wire to the large battery instead of connecting it to the small battery.
The only change that we have made is that now the FET gate is connected to the positive terminal of the large battery, and the small battery is no longer needed. The FET is off as long as the wire is connected. When tripped the wire is removed and the gate no longer has voltage applied. This turns the FET on, and the buzzer begins to buzz.
Note that in this case whenever the trip wire is in place the large battery is being depleted in order to provide voltage to the gate of the FET. Again the amount of power required to keep an FET off is so minimal that the battery should last for years.
In this guide we have talked briefly about transistors and how they are used to both switch and amplify. We specifically used an FET for our examples, but you could use a BJT in almost the exact same way. We also talked about N channel and P channel FET's, and how to make an FET inverter.