Elenco Electronics 202 - Manual (Page 33)

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Project #49
OBJECTIVE: To learn about one device that is used to delay
actions in electronics.
Auto-Off Night Light
Project #50
OBJECTIVE: To learn about one device that is used to delay
actions in electronics.
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
470
μF capacitor (C5) charges up. If you turn the slide switch off and
back on after the light goes out, it will NOT come on again. Press 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.
Auto-Off Night Light (II)
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
off and back on after the light goes out, it will NOT come on again. The
470
μF 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 press switch
(S2).
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.