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Circuit 143: Water level alarm with moving-coil instrument
Setup information
Install the two transistors and the four resistors as shown in Fig. 2.143. The ten jumper wires come next.
Last but not least connect the moving-coil instrument. Check the placement on the board again and only
then connect the battery.
Experiment
Even if everything is wired correctly, nothing will happen at first. But if you dip the two bare wire ends that
represent the sensor in water, the needle of the moving-coil instrument will deflect sharply. For example,
you could also check by means of the scale of the deflection, whether your potting soil is too dry and needs
some water, if you push the bare wire ends into the soil of a flower pot. The weaker the deflection the drier
the soil.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.143.
Components required:
2 x 2N3904 transis-
tors, 1 x 4.7 kΩ resis-
tor, 1 x 10 kΩ resistor,
2 x 220 kΩ resistors,
1 x moving-coil instrument,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.143: Water level alarm
with moving-coil instrument
169
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