Elenco Circuit Maker Skill Builder 125 - Manual (Page 53)

Project #105 The Dark Dimmer
Project #106 Lagging Light
This circuit shows how capacitors
can be used to provide a delay.
This type of circuit is used in house
lighting to fade out the lights when
you turn the switch off.
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Turn on the slide switch (S1) and push 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 470mF 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.
Build the circuit and turn on the slide switch (S1). The LED (D1) is
bright but slowly gets dark as the 470mF capacitor (C5) charges up.
The LED will stay dark until you push the press switch (S2), which
discharges the capacitor.
CM-125_Manual_031514.qxp_CM-125_Manual_031514 4/2/14 12:20 PM Page 53