Elenco Upgrade Kit SC300 to SC750 - Manual (Page 20)

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Project #341
OBJECTIVE: Turn on and off LED’s using a relay.
A relay is an electronic switch with contacts that are opened or closed
using voltage. It contains a coil that generates a magnetic field when
a current flows through it. The magnetic field attracts an iron armature
which switches the contacts. Contact #2 is normally closed,
connecting the green LED (D2) and the resistor across the batteries.
With the slide slide switch (S1) turned off, the green LED should light.
Now turn on the switch, contact #1 on the relay (S3) will switch to
contact #3, lighting the red LED (D1).
LED & Relay
Project #342
OBJECTIVE: To build a manual timer using a relay.
The transistor (Q2) acts as a switch, connecting the relay (S3) to the
batteries. As long as there is positive voltage on the transistor’s base,
the bulb (L2) will light.
Turn on the slide switch (S1) and hold down the press switch (S2). The
transistor turns on, capacitor C5 charges up, and the bulb lights.
When the press switch is released, the capacitor discharges through
the base, keeping the transistor on. The transistor will turn off when
the capacitor is almost discharged, about 7 seconds. The relay
contacts will switch and the bulb will turn off.
Change the value of the capacitor and see what happens.
Manual
7 Second Timer