Elenco Projects 102-305 - Manual (Page 44)

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Project #225
OBJECTIVE: To learn about one device that is used to delay
actions in electronics.
Auto-Off Night-Light
OBJECTIVE: To show how capacitor delays can be repeated by
discharging the capacitor. OBJECTIVE: To show how the size of the capacitor effects the
delay time.
Use the circuit from project #225 shown above.
When you first turned on the slide switch (S1) in project #225, 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 470
μ F 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 470
μ F 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 #226
Discharging Caps
Project #227
Changing Delay Time
When you turn on the slide switch (S1) the first 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 470
μ F capacitor (C5) 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 (B1) even if left on all night.
Use the circuit from project #225 shown above.
Change the 470 μ F capacitor (C5) to the 100 μ F 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
100
μ F is approximately 5 times smaller than 470 μ F, 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.