Elenco Upgrade Kit SC500 to SC750 - Manual (Page 14)

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Turn on the slide switch (S1), the lamp (L2) will be bright and the LED
(D1) will be lit. The diode (D3) allows the batteries to charge up the
470
μ F capacitor (C5) and light the LED.
Turn off the slide switch, the lamp will go dark immediately but the LED
will stay lit for a few seconds as capacitor C5 discharges through it.
The diode isolates the capacitor from the lamp; if you replace the diode
with a 3-snap wire then the lamp will drain the capacitor almost
instantly.
Project #521
OBJECTIVE: To show how a diode works.
Diode
Project #522
OBJECTIVE: To build a rectifier.
Rectifier
This circuit is based on the Trombone project #238. Turn on the slide
switch (S1) and set the adjustable resistor (RV) for mid-range for the
best sound. The LED (D1) will also be lit.
The signal from the power amplifier (U4) to the speaker (SP) is a
changing (AC) voltage, not the constant (DC) voltage needed to light
the LED. The diode (D3) and capacitor (C5) are a rectifier, which
converts the AC voltage into a DC voltage.
The diode allows the capacitor to charge up when the power amp
voltage is high, but also prevents the capacitor from discharging when
the power amp voltage is low. If you replace the diode with a 3-snap
or remove the capacitor from the circuit, the LED will not light.