Elenco Basic Electronic Experiments - Manual (Page 62)

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EXPERIMENT #45: FINGER TOUCH LAMP WITH MEMORY
Instead of using the wire to flip-flop the LED you may also use your fingers as you did in Experiment 20, the Two Finger
Touch Lamp. We’ll use almost the same circuit here as in the last experiment. Remove the loose wire and replace the
right LED with a diode, because we don’t need two “lamps”. Wet two fingers and hold one on 9V (the (+) row of holes)
while touching the other to one of the transistor bases. (This is easy if you touch the metal leads of the resistors connected
to these points, or you may insert wires into the holes and touch the wires). But now you must touch the base of the “off”
transistor to make them flip-flop, not the “on” base. Do you know why? Your body has more resistance than the other
resistors in the circuit and cannot “short circuit” the transistor bases to circuit ground like the wire can. So instead we
connect the off transistor to the battery to turn it on.
But this uses two fingers and in Experiment 21 we also had a one-finger version, so can we do that here? Change your
wiring (add wires or move parts) so that metal from 9V is close to metal from the transistor bases. Wet a large area of one
of your fingers and touch it to 9V and a transistor base at the same time. Now we have a one-finger touch lamp with
memory!
LOOSE WIRE
+9V
3.3k
Ω
1k
Ω
100k Ω
33k Ω