Elenco Circuit Maker Skill Builder 125 - Manual (Page 32)

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Project #53 Auto-Off Night Light
Project #54 Auto-Off Day Light
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 would make a good day light. It would quickly turn off
the lights when you opened the shades and let the sunlight in.
Build the circuit to the left. 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 LED
goes out it will NOT come on again. The 470mF capacitor (C5) has
charged up and the NPN transistor amplifier (Q2) can get no current at
its input to turn it on. To discharge the capacitor (C5) and reset the circuit,
press and release the switch (S2).
Cover the photoresistor (RP) and turn on the slide switch (S1). The
LED (D1) is bright, but it will very slowly get dimmer and dimmer as the
470mF capacitor (C5) charges up. If you turn the slide switch (S1) off
and back on after the light goes out it will NOT come on again. Push
the press switch (S2) to discharge the capacitor and reset the circuit.
If you uncover the photoresistor and to let light shine on it, then the
LED will get dark quickly. The photoresistor has much lower resistance
with light on it, and this lower resistance allows the capacitor to charge
up faster.
CM-125_Manual_031514.qxp_CM-125_Manual_031514 4/2/14 12:19 PM Page 32