Elenco Circuit Maker Skill Builder 125 - Manual (Page 55)

Project #109 Glow & Go
Project #110 Fading Siren
This type of circuit is used in alarm
systems to detect light. If an intruder
turned on a light or hit the sensor with a
flashlight beam, the alarm would trigger
and probably force the intruder to leave.
-54-
Build the circuit to the left. Turn on the slide switch (S1). The alarm will
sound, as long as light is present. Slowly cover the photoresistor (RP),
and the volume goes down. If you turn off the lights, the alarm will stop.
The amount of light changes the resistance of the photoresistor (less
light means more resistance). The photoresistor and NPN transistor
(Q2) act like a dimmer switch, adjusting the voltage applied to the
alarm.
First, place the 470mF capacitor (C5) across points A & B to discharge
it. Then build the circuit as shown on the left. Press the switch (S2), the
integrated circuit should make the sound of an up-down siren that gets
weaker with time. The fading is produced by the charging of the 470mF
capacitor (C5). After it is charged, the current stops and the sound is
very weak.
To repeat this effect you must release the press switch (S2), remove
the capacitor (C5), and discharge it by placing it across the snaps on
the bottom bar marked A & B. Then, replace the capacitor (C5) and
press the switch again.
CM-125_Manual_031514.qxp_CM-125_Manual_031514 4/2/14 12:20 PM Page 55