Elenco Electronics 202 - Manual (Page 79)

Project #180 Discharging Caps
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Project #179
OBJECTIVE: To learn about one device that is used to delay
actions in electronics.
Auto-off Night Light (III)
OBJECTIVE: To show how capacitor delays can be repeated by discharging the capacitor.
Use the circuit from project #179 shown above.
When you first turned on the slide switch (S1) in project #179, the LED
(D1) came on and very slowly got dimmer and dimmer. When you
turned the slide switch off and back on after the light went out, it did NOT
come on again. The 100
μF capacitor (C4) was charged and everything
stopped. This time turn the slide switch off. Then press the press switch
(S2) for a moment to discharge the 100
μF capacitor. Now when you
turn the slide switch back on the delay repeats. Shorting a capacitor
with a low resistance will allow the charges on the capacitor to leave
through the resistance. In this case, the low resistance was the press
switch.
When you turn on the slide switch (S1) the first time the LED (D1) will
come on and slowly get dim. If you turn the slide switch off and back
on after the LED goes out, it will NOT come on again. The 100
μF
capacitor (C4) has charged up and the NPN transistor amplifier (Q2)
can get no current at its input to turn it on.
This circuit would make a good night-light. It would allow you to get
into bed, and then it would go out. No further current is taken from the
battery so it will not drain the batteries even if left on all night.
This circuit is just like project #50, which stayed on longer because of
higher capacitance. In electronics, capacitors are used in every piece
of equipment to delay signal or tune circuits to a desired frequency.