The motion detector
Behind the 15th door you’ll find
another cable. You will be building
using an infrared motion detector.
The actual sensor is the piezo disc.
You already know that when the
temperature changes, it generates
an electrical voltage. And this also
works without direct contact if you
get close. It is even better if you
darken the silver layer of the disc
with a soft pencil. Your warm hand
radiates infrared heat. When it hits
the blackened sensor, it heats up a
little. This produces only a very small
electrical current. That’s why you
need a good amplifier, which here
consists of a Darlington circuit. In
addition, a very small base current
is needed, which the is supplied by
the yellow LED depending on the
lighting. In addition, there are buttons
for continuous light.
Wait a while until the red and green
LEDs shine evenly and dimly. With
a short push of the button you can
shorten the waiting time. Then hold
your hand about 5 cm away from the
piezo disc. After a few seconds, the
brightness of the LEDs will change.
Remove the hand and observe the
opposite change in brightness. The
two LEDs can therefore indicate the
approach of the hand. However, the
direction of the change cannot be
predicted. You can change them by
swapping both cables of the piezo
speaker. The LEDs should shine
brighter if you hold your hand closer
to the piezo disc. With this you have
built night light with a proximity
sensor.
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