Data Sheet

You will need:
39
Alarm!
Overflow!
In factories, there are often automatic filling systems
for tanks. So they need an electronic means to deter-
mine when a tank is full and to switch off the pump.
And in the basement of a house, it’s useful to have
an automatic monitoring system to sound an alarm
when rainwater gets in. A water level sensor can
handle tasks like those.
• CircuitfromExperiment38 • Drinkingglass
• Tape • Tapwater
Here’s how:
1. Tape the free ends of the wires to the rim of an
empty glass. The plugs should hang about 2 cm be-
low the rim, and there should be a few centimeters
of space between them.
2. Switch on the circuit and slowly pour water into
the glass. As soon as the water reaches the two
plugs, the LED will light up and the alarm will sound.
How it works:
The water makes a connection between the plugs
when it reaches them and the circuit responds.
43
You will need:
40
Low water level
alarm
Circuits like this one warn you when a liquid level is
too low, such as the gasoline level in a car’s gas tank.
• Batterybox • Redwire
• 10X-connectors • Bluewire
• 2I-connectors • 120kΩResistor
• 2L-connector • Drinkingglass
• 2Transistors • Tape
• RedLED • Teaspoon
• Selectorswitch • Tapwater
Here’s how:
1. Assemble the circuit and secure the free wire ends
to the inside of the glass. Switch on; the LED shines.
2. Fill the glass with tap water until both plugs are
submerged. The LED will go out.
3. Now take water out with the teaspoon. At the mo-
ment that the water no longer creates a connection
between the plugs, the LED will turn on.
How it works:
As long as the water creates a connection between
the plugs, the first transistor’s base is connected to
the positive battery terminal, so its C-E section is con-
ductive. That means that the second transistor’s base
is connected to its emitter and the negative terminal,
and the current is blocked in spite of the connection
with the positive terminal via the 120-kΩ resistor: The
LED remains dark. If the water level drops, the first
transistor’s C-E section cuts off, and the base connec-
tion via the 120-kΩ resistor ensures that the second
transistor is conductive. Then the LED lights up.
How it works:
How it works:
T1
T2
Electronics Learning Circuits Manual.indd 43 3/29/11 2:28 PM