Elenco Electronics 202 - Manual (Page 52)

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Project #95
OBJECTIVE: To build a circuit that uses a programmed sound IC.
Turn the slide switch (S1) on, a space war sound plays and the LED
(D1) flashes. Press the press switch (S2) to change the sound. If the
photoresistor (RP) is covered, then the sound may stop. Shine light on
it and action resumes. See how many sounds are programmed into
the space war sound IC (U3).
Photo Space War
with LED
Project #96
OBJECTIVE: To convert a changing voltage into a constant
voltage.
Build the circuit and turn on the slide switch (S1). The LED (D1) is
flashing and the speaker (SP) makes a siren sound. Now press the
press switch (S2) to connect the 470
μF capacitor (C5) to the circuit.
The LED is brighter and stops flashing.
The signal from the alarm IC (U2) to the speaker is a changing
voltage, which is why the LED was flashing. The 470
μF capacitor can
store electricity, and it combines with the NPN transistor (Q2) to make
a rectifier. A rectifier converts a changing voltage into a constant
voltage, so the LED stays on instead of flashing.
Alarm Rectifier