Elenco Green Projects - Manual (Page 71)

50mA
-70-
Project #116 Water Timer
LED or Bulb?
Install the battery eliminator in the radio and
build the circuit shown. Turn off the volume
knob on the radio. Set the slide switch (S5) to
position C, the meter (M6) shows the current
for the yellow LED (D5).
On most models, your radio has a light bulb.
Set the switch to position B, and push the
LIGHT button on the radio. The meter shows
the current, it should be high.
Your radio may draw current even when the
radio and light bulb are off. This is due to
circuitry in the battery eliminator, which
protects the radio from voltages over 3V.
Set the slide switch (S5) to position C. Place the loose ends of the red
& black jumper wires into a cup of water without the metal parts
touching. The red LED (D1) should light dimly and the clock (T2) should
run, but it depends on your local water supply. Add salt to the water if
they are off or to make them brighter.
Remove the water, and push the press switch (S2) to reset the clock.
Place the empty cup under a faucet or a gutter drain. When water goes
into the cup, the clock will start. If you go away and return, you can use
this timer to see how long ago water entered the cup.
Project #115
Compared the light produced and current used by the LED and light
bulb. LEDs need a lot less energy to produce the same amount of light
as ordinary light bulbs, and are being increasingly used to light homes.
LEDs also last longer. See projects 1 & 3 if you need to recharge the battery (B4).