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53
Lasting Doorbell
OBJECTIVE: To build a doorbell that stays on for a while.
Build the circuit at left, note that there is a 4-snap wire on layer 1 that is
not connected to a 3-snap wire that runs over it on layer 3. Turn on the
slide switch (S1), then press and release the press switch (S2). There is
a doorbell sound that slowly fades away.
When the press switch is pressed, the transistors are supplied with
current for oscillation. At the same time, the 100mF capacitor (C4)
is being charged. When the press switch is released, the capacitor
discharges and keeps the oscillation going for a while.
Project #144
Place the fan on the motor (M1) and turn off the slide switch (S1).
Press the press switch (S2) and listen to the motor.
As the motor shaft spins around it connects/ disconnects several sets
of electrical contacts. As these contacts are switched, an electrical
disturbance is created, which the speaker converts into sound.
Turn on the slide switch and push the press switch again. The fan
spins just as fast, but the sound is not as loud. Capacitors, like the
470mF capacitor (C5), are often used to lter out undesired electrical
disturbances. If you replace C5 with one of the other capacitors in your
set then the sound will not be changed as much.
Quieting a Motor
OBJECTIVE: To show how capacitors can lter out electrical
disturbances.
Project #142
!
WARNING: Moving parts. Do not touch the fan or
motor during operation. Do not lean over the motor.
Place the 10μF capacitor (C3) on top of the whistle chip (WC). Press and
release the press switch (S2). It makes a clicking sound that repeats for a while.
Project #143
Lasting Clicking
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