Instructions

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Additional experiments:
Different series resistors are tested in the circuit. The smaller
the resistance, the greater the brightness. But does the switch
still work with the slightest resistance?
20 Alarm system with flashing indicator
You can build a simple alarm system with two LEDs. The
green LED lights up permanently, the red one not at first,
because the cable short-circuits it. Green means: Everything’s
ok! You can tie a piece of string to the cable and attach it to
the door. When someone enters the room, he will pull the ca-
ble out. The red and green LEDs will start to flash. The alarm
is triggered.
For this alarm system a large resistor with 22k Ω (red, red,
orange) is used. An alarm system must be in operation for
a long time. That is why it is crucial that the current flow is
very little. Here it is about 0.3 mA. A new 500 mAh battery
could run the system for about 1,500 hours – more than two
months. Real alarm systems are usually connected to the
power grid. But when it comes to pieces of great value in a
museum, a battery or a backup battery is used in addition to
the power grid in case of a power failure.