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Project #225
OBJECTIVE: To learn about one device that is used to
Auto-Off Night-Light
OBJECTIVE: To show how capacitor delays can be OBJECTIVE: To show how the size of the capacitor
Use the circuit from Project 225 shown above.
When you first turned on the slide switch (S1) in Project 225, the LED
came on and very slowly get 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 470µF capacitor (C5) was charged and everything
stopped. This time turn the slide switch (S1) off. Then press press
switch (S2) for a moment to discharge the 470µF capacitor. Now when
you turn the slide switch (S1) 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 light 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 470µF capacitor (C5) has charged up and the NPN transistor
amplifier (U2) 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.
Use the circuit from Project 225 shown above.
Change the 470µF capacitor (C5) to the 100µF 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 (S1)
is closed, notice how much quicker the LED goes out. Since 100µF is
approximately 5 times smaller than 470µF, the light 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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