IMPORTANT Before connecting anything to the Raspberry Pi please be aware that incorrect connections could cause damage. Please take care.
Alarm System Circuit To demonstrate the use of a transistor, we will construct an alarm system that will sound a small buzzer when activated.
Circuit Description We will be using two outputs and two inputs. The first output is the familiar LED circuit that will be used to indicate when the alarm is armed. The second output is connected via resistor R2 to the base of the transistor; the resistor is necessary to limit the amount of current supplied to the base of the transistor so that there is just enough to switch it on. The collector of the transistor is connected to a buzzer so that it will sound when the output connected to the base of the transistor is set high. Although we could use the transistor to switch a higher voltage (for example 5V), to keep things simple we are using a 3.3V buzzer so that the whole circuit can be powered from a single voltage.
The first input is the same small push-button switch as in previous experiments, this will be used to arm and disarm the alarm. The second input is a magnetically operated reed switch. A magnet would be attached to a door and the switch to the door frame so that the switch will be opened and closed when the door is opened and closed. If you don’t intend to use the circuit on a real door and prefer not to buy a reed switch for the purpose of experimenting feel free to modify the circuit slightly to use another small tactile switch or whatever sort of switch you have to hand in place of the reed switch.
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