Data Sheet

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104. Charging the Capacitor
Build the circuit shown on the left, press and hold the press switch (61). As the star
LED (70) turns on and fades out, the 470mF capacitor (74) is being charged. Release
the press switch (61) and the heart LED (69) will turn on and fade out.
The reason the star LED (70) turns on for just a short time and fades out when you hold
the press switch (61) can be explained by Kirchhoff’s Voltage Law. Initially, the 470mF
capacitor (74) has zero charge across it and thus the full 4.5V is across the star LED
(70). But as the 470mF capacitor (74) charges, the voltage drop across it increases,
which means the voltage seen by the star LED (70) decreases, which is why the star
LED (70) fades out. Release the press switch (61) and the heart LED (69) will turn on
for a short time and fade out. This happens because the charge on the 470mF capacitor
(74) is being discharged across the 1kW resistor (42) and heart LED (69).
105. NPN Transistor – a Current Switch
Build the circuit shown on the left and turn on the switch (62); the lamp (76) is still off.
Press and hold the press switch (61) and you will see the lamp (76) turns on after a short
delay. Make sure your 470mF capacitor (74) is fully discharged before you start this project
(you can discharge the 470mF capacitor (74) by placing a 4-wire (4) across the bottom of
the 470mF capacitor (74) module for a few seconds).
This circuit includes an NPN transistor (50). Transistors can sometimes be thought of
as switches where, in the case of the NPN transistor (50), a current owing into the
base (labeled with a “B” on the NPN transistor (50)) enables current to ow from the
collector (labeled “C” on the NPN transistor (50)) to the Emitter (labeled “E” on the NPN
transistor (50)).
106. Delay Circuit
Replace the lamp (76) with the motor (95) in the previous project; the motor (95) is still
off. Press and hold the press switch (61) and you will see the motor (95) turns on after
a short delay. Make sure your 470mF capacitor (74) is fully discharged before you start
this project (you can discharge the 470mF capacitor (74) by placing a 4-wire (4) across
the bottom of the 470mF capacitor (74) module for a few seconds).
The reason there is a delay before the motor (95) starts spinning in this project (and
before the lamp (76) turns on in the previous project) is because the 470mF capacitor
(74) initially has no charge across it and takes time to charge up. So initially the 470mF
capacitor (74) is holding the Base of the NPN transistor to ground (0V), and eventually
when the 470mF capacitor (74) charges up enough, there is enough voltage at the Base
of the NPN transistor (50) to enable current to ow in the Base and turn on the current
ow from the Collector to the Emitter of the NPN transistor (50), which enables current
to ow through the motor (95) making it spin.
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