615611 Electronics Workshop 2 Manual - Thames & Kosmos

Figure 167. Good for the weather station: Humidity in the air is converted
into light. Stronger effect with a layer of salt on the sensor.
122
Figure 168. Layout for Figure 167.
121 . Dissolve a teaspoon of table salt in half a glass
of lukewarm water . Coat the sensor surface with
121 a few drops of this solution, and let the sensor dry
in a horizontal position . The resulting coating of
salt crystals is an excellent humidity detector . We
have tested it with a resistance meter: more than 20 MΩ at
low humidity, less than 200 kΩ when you breathe on it . Now
things get fun: Without touching the surface, move your
fi ngertip close to the coating . In this high-humidity local
microclimate, the LED lights up brightly! After you remove
your fi nger, the light fades . This sensor is not appropriate
for use in the kitchen or other rooms with a lot of water
vapor, of course .
122 . Let’s expand the circuit according to Figure
169, and we’ll even be able to change the response
point .
Figure 169. Adjustable sensitivity: Provides a point of reference.
Figure 170. Layout for Figure 169.
15 More than a spool of wire
In Chapter 6, we already had the earphone reveal its inner
life to us (Insider Knowledge on page 33) . We also learned
there what an alternating current is . Alternating currents
with frequencies between around 20 Hz and 16 kHz can
be rendered audible through things like as earphones or
speakers, which work by a similar principle . A magnetic fi eld
forms whenever current fl ows through the coil – in a circle
around the wire and therefore bundled in the coil’s interior,
with a north and a south pole .
Alternating currents have certain diffi culties with coils,
caused by a mysterious value known as self-inductance . Selfinductance
is something that wants to keep everything as
it already is . If we want to send a current through the coil,
a countervoltage arises and tries to obstruct it . But since it
only arises when the size of the current changes, it can’t really
succeed . Ultimately, as much current fl ows through the
coil as is permitted by Ohm’s law: applied voltage divided by
direct-current resistance of the wire .
In the coil, there is magnetic energy, and a magnetic
fi eld forms . This fi eld is no more directly visible than the
electric fi eld in the capacitor .
INSIDER KNOWLEDGE
With direct current, as it moves through the spool
the current always ultimately reaches a highest value
determined by the ohmic resistance of the wire and the
applied voltage . With alternating current, that isn’t the
case . The faster the alternation, the smaller the chance
that the current actually reaches this ultimate value .
This behavior is actually really interesting: If an alternating
voltage is applied to a coil at its highest value,
the current is zero . As the voltage drops, the current
rises! Just at the moment that the voltage reaches zero,
the current attains its highest value . At the negative
apex of the voltage, the current is zero again and subsequently
becomes negative .
All in all, everything happens to the current a
quarter oscillation later . Since you can compare a complete
oscillation to the completion of a full circle (360°
or 2π), people also say that the current lags behind
the voltage in a coil by 90° . Or, using different terms:
Voltage and current are phase-shifted relative to one
another by 90°, or π/2 . Remembering the behavior of
voltage and current in the capacitor, the exact opposite
relationship can be deduced for that .
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