Elenco Circuit Maker Skill Builder 125 - Manual (Page 58)

Project #117 Touch of Light
Project #118 Change & Charge
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Build the circuit on the left. You’re probably wondering how it can work,
since one of the points on the NPN transistor (Q2) is unconnected. It
can’t, but there is another component that isn’t shown. That
component is you.
Touch points X & Y with your fingers. The LED (D1) may be dimly lit.
The problem is your fingers aren’t making a good enough electrical
contact with the metal. Wet your fingers with water or saliva and touch
the points again. The LED should be very bright now. Think of this
circuit as a touch lamp since when you touch it, the LED lights. You
may have seen such a lamp in the store or already have one in your
home.
Turn the slide switch (S1) on and connect points A & B with a 2-snap
wire. The LED (D1) will flash and the 470mF capacitor (C5) will be
charged with electricity. The electricity is now stored in the capacitor.
Disconnect points A & B. Connect points B & C and there will be a flash
from the 2.5V lamp (L1).
The capacitor discharges through the resistor to the base of the NPN
transistor (Q2). The positive current turns on the transistor like a
switch, connecting the lamp to the negative (–) side of the batteries
(B1). The light will go out after the capacitor discharges, because there
is no more current at the base of the transistor.
CM-125_Manual_031514.qxp_CM-125_Manual_031514 4/2/14 12:20 PM Page 58