Elenco XP&trade - Manual (Page 26)

Project #A25 Capacitor Photo Control
Project #A26
Modify the preceding circuit
to match the one shown
here. It works the same way,
but the red LED (D1) stays
on longer and very little light
is needed on the
photoresistor (RP).
Capacitor Photo Control
with Slow Shut-off
Adding a second transistor makes the
circuit more sensitive to current from the
capacitor and photoresistor, so the LED
stays on longer. Transistors allow very
small currents to control large currents.
-25-
Turn on the slide switch (S1)
and press the press switch
(S2). If there is light on the
photoresistor (RP), then the
LED (D1) will stay on for a
long time after you release
the press switch.
The energy stored in the 470
μ
F capacitor
(C5) keeps the controlling current to the
NPN transistor (Q2) on even though the
press switch was turned off. If it is dark,
the high resistance of the photoresistor
shuts off the current to the transistor.