Elenco Electronics 202 - Manual (Page 123)

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Project #290
OBJECTIVE: To use two transistors to make an SCR.
The transistors (Q1 & Q2) are connected so when the base of Q2 goes high, both Q2 and
Q1 turn on. They will remain on until the slide switch (S1) is turned off. Turn on the slide
switch and the LED (D1) should not light. Now press the press switch (S2) and the LED
lights. Turn the LED off by turning the slide switch off.
The two transistors act as an electronic device called an SCR (Silicon Controlled Rectifier). A
three-pin device that once its base is triggered, remains on until the current flow through it stops.
The SCR
Project #291
Light-controlled SCR
OBJECTIVE: To modify project #290 to use the photoresistor (RP).
Replace resistor R4 with the photoresistor (RP). The LED (D1) will only light when the
press switch (S2) is pressed and there is enough light on the photoresistor. Turn on the
slide switch (S1) on and place your finger over the photoresistor. Press the press switch
and the LED should not light. Remove you finger and press the press switch again, the
LED should light now.
Project #292
OBJECTIVE: To build a circuit that sounds only when there is
light in the room.
This circuit operates the same way as project #291, only now the
alarm IC (U2) will sound when the press switch (S2) is pressed and
the room is lit.
Light-controlled SCR (II)
Project #293
Light-controlled SCR (III)
OBJECTIVE: To modify project #292 to use the music IC U1.
Replace the alarm IC (U2) with the music IC (U1). Turn on the slide
switch (S1) and the music IC will sound when the press switch (S2) is
pressed and the room is lit.