Like this;
Uses of your circuit
1. As a Morse practice oscillator. Simply connect the two test leads to your
key and, each time the key is pressed, you should hear a note from the
speaker. The frequency of the note may be altered by putting a resistor in
series with the key. To do this, remove one test lead from the key and
select a resistor; connect one end of the resistor to the free test lead and
the other to the empty terminal on your key. Selecting the value of
resistor that you need will be a useful experiment in itself.
2. As a continuity tester. You can check fuses and lamp bulbs by connecting
them across your test leads. If the speaker remains silent, the fuse or bulb
has blown.
3. To indicate changes of resistance. Hold the ends of the test leads in each
hand; you should hear a low note, because of the high resistance of your
body. Squeeze the ends harder, and the frequency of the note should rise,
because you are now making better contact. Repeat this with damp
hands and the frequencies will be higher still.
4. As a thermometer. Connect the test leads to a thermistor (a device whose
resistance changes with temperature) and warm it with a hair-dryer, or
even in your hands, and you will hear the pitch changing with the
temperature of the thermistor. A suitable ‘bead’ thermistor is available
from Maplin (order code FX21).
5. As a diode tester. Use any diode, and connect the negative test lead to the
end of the diode marked with the ring. This is the cathode of the diode.
The other end, the anode, should be connected to the positive test lead,
and a note should be heard from the speaker. This does not necessarily
mean that the diode is working – yet. Reverse the connections and
nothing should be heard. If this is the case, the diode is working.
6. As a light meter. Use a photoconductive cell (a device whose resistance
changes with light intensity) connected between the test leads. A note
should be heard. Shading the device with your hand will increase its
resistance and the note should decrease in frequency. A suitable device is
the ORP12 cell from Maplin (order code HB10).
There are many more applications. Do not connect the test leads to other
circuits that are switched on. Your circuit, or the circuit you are connecting
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