Introductory Electronics Unit
Messing Around:
ACTIVITY #4
Many security systems work by setting off an alarm when
a door is opened or a window is broken. In these systems there is a conductor
at these points that breaks, just like a N.C. switch. In this activity you
will be building a circuit that sets off an alarm when broken. To save your--or
your teacher's!--sanity, you may replace the buzzer with a light bulb.
You will be using a transistor to control this circuit. A transistor allows
a small current to trigger a larger one. BANG!! When there is no
current through the base (B) the buzzer is off, like an open switch.
When there is a small current to the base the collecter (C) / emitter
(E) part of the circuit turns on, and the buzzer buzzes. Study the
diagrams below and look at the two possible paths for the current with the
switch open and closed.
The resistor resists the flow of charge. It keeps electrons from running
willy-nilly around the circuit. Note that without the resistor there would
be a short circuit with the switch closed. Use your resistor code reader
to pick three values of resistance to try (ones that are far apart in value).
Circuit Diagram
Build this circuit on your circuit board, using your 'medium' resistor.
Try a higher resistance.
Q.1 Does it still work? Why or why not?
Try a lower resistance.
Q.2 Does it still work? Why or why not?
Q.3 Why did the circuit work with different resistors? Which resistor
makes the most sense to use and WHY?
Q.4 Indicate on a circuit diagram the part of your circuit that would
be placed around the window you wanted to protect. Explain why you chose
this.
Q.5 Indicate on the circuit diagram the spots a thief could cut if
she wanted to disable your circuit. What could you do to prevent this?
