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44
Project #120
Auto-Off Night-Light
OBJECTIVE: To learn about one device that is used to delay
actions in electronics.
Use the circuit from project #120 shown above.
When you rst turned on the slide switch (S1) in project
#120, the LED (D1) came on and very slowly got dimmer and
dimmer. When you turned the slide switch (S1) off and back
on after the light went out, it did NOT come on again. The
470mF capacitor (C5) was charged and everything stopped.
This time turn the slide switch off. Then press the press
switch (S2) for a moment to discharge the 470mF 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.
Project #121
Discharging Caps
OBJECTIVE: To show how capacitor delays can be
repeated by discharging the capacitor.
Project #122
Changing Delay Time
OBJECTIVE: To show how the size of the capacitor
effects the delay time.
When you turn on the slide switch (S1) the rst time the LED (D1) will
come on and very slowly get dimmer and dimmer. If you turn the slide
switch (S1) off and back on after the light goes out it will NOT come
on again. The 470mF capacitor (C5) has charged up and the NPN
transistor amplier (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 (B1) even if left on all night.
Use the circuit from project #120 shown above.
Change the 470mF capacitor (C5) to the 100mF capacitor (C4).
Make sure the capacitor (C4) is fully discharged by pressing
the press switch (S2) before closing the on-off slide switch
(S1). When slide switch is turned on, notice how much quicker
the LED (D1) goes out. Since 100mF is approximately 5 times
smaller than 470mF, the LED will go out 5 times faster. The
bigger the capacitor the longer the delay.
In electronics, capacitors are used in every piece of equipment to
delay signal or tune circuits to a desired frequency.
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