Fototransistor/LED-Flipflop
You’ll find another cable behind door number
17. Now you are going to build a circuit with an
LED and a photo transistor that switches each
other on or off. Bend the wires to fit so that the
LED is mounted opposite the phototransistor and
illuminates it directly. For this reason, the switched-
on state is maintained even in the dark. But if you
put a sheet of paper between them, the LED will
turn off and won’t turn on by itself when you pull
the paper out. But it only takes a short flash of light
or the light of a lamp to switch it on again. You can
place the circuit in a dark room and after a while
find out if someone has switched on the light in the
meantime.
This circuit is also called a flip-flop circuit because
it can switch between the two states On (flip)
and Off (flop). In this case, switching is achieved
by lighting and shading. The resistance of 10 k
Ω
determines the brightness at which the flip-flop
switches to the on state. If you install a resistor
with 330 k
Ω
in its place, the circuit already reacts
to much weaker light.
17
