User Manual
Build the circuit shown, but initially leave the red & black jumper wires
outside the cup. Turn on the slide switch (S1); nothing happens.
Place the jumper wires into a cup of water and an alarm sounds!
Variants:
1. Change the sound by using a 1-snap wire and a 2-snap wire
to make a connection across points A & B, or A & D, or B & C.
2. Remove the NPN transistor (Q2) and instead connect the black
jumper wire at point B.
Don’t drink any water used here.
Project 120
Human Alarm
Use the preceding circuit, but instead of putting the
red & black jumper wires in the water, touch the metal
ends of them with your fingers. You may have to hold
them tightly or wet your fingers to make this work.
Your body is mostly water, so it
also has some electrical
resistance, but much less than
air.
Project 121
Draw an Alarm
Use the circuit from project 119, but omit the cup of water and leave
the loose ends of the jumpers unconnected for now. There is one
more part you need and you are going to draw it. Take a pencil (No.
2 lead is best but other types will also work). SHARPEN IT, and fill
in the shape below. You will get better results if you place a hard,
flat surface directly beneath this page while you are drawing. Press
hard (but don’t rip the paper), and fill in the shape several times
to be sure you have a thick, even layer of pencil lead.
Press the metal ends of the jumper wires on the shape and move
them around over the drawing. If you don’t hear any sound then
move the ends closer together and move over the drawing, add
another layer of pencil lead, or put a drop of water on the jumper
ends to get better contact with your fingers.
The black core of
pencils is graphite, the
same material used in
resistors (electronic
components that limit
and control the flow of
electricity).
Water has some electrical resistance, but much
less than air. The NPN transistor acts as an
amplifier, to help overcome water’s resistance.
You could use longer wires and lay them on your
basement floor, if your basement floods during
a storm, then this circuit will sound an alarm!
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Project 119
Water Alarm