Elenco Electronics 202 - Manual (Page 51)

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Project #93
OBJECTIVE: To learn about a device that is used to delay
actions in electronics.
Capacitor Photo Control
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 μF 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.
Project #94
OBJECTIVE: To learn about a device that is used to delay
actions in electronics.
Capacitor Control
Build the circuit and turn on the slide switch (S1). The LED (D1) is
bright but slowly gets dark as the 470
μF capacitor (C5) charges up.
The LED will stay dark until you press the press switch (S2), which
discharges the capacitor.