Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 65)

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Instead of using the wire to flip-flop the LED you may also
use your fingers as you did in Experiment 19, the Finger
Touch Lamp. We’ll use almost the same circuit here as in
the last experiment. Just replace LED2 with the diode
(move the wire on spring 3 to spring 10, and the wires on
spring 4 to spring 11) because we don’t need two
“lamps”. Wet two fingers and hold one on battery spring
27 while touching the other to the transistor base springs.
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 19 we also
had a one-finger version, so can we do that here?
Connect wires between springs 41 and 32, between 15
and 30, and between 18 and 34. Wet a large area of one
of your fingers and touch it to springs 30 and 32 or 32 and
34 at the same time. Now we have a one-finger touch
lamp with memory!
EXPERIMENT #46: Finger Touch Lamp with Memory
Schematic