Elenco Upgrade Kit SC300 to SC750 - Manual (Page 33)

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Project #386
OBJECTIVE: To light a bulb to indicate an open circuit.
Alarm Light
Project #387 Alarm Light (II)
This is another example of a alarm that activates when the circuit is
broken. Connect the jumper wire across points A & B and then turn
on the slide switch (S1). The lamp (L2) will not light until the jumper
wire is disconnected. Then the lamp will not turn off. Turn off the
switch to turn the lamp off again. This circuit remembers if there was
a break in the connection.
OBJECTIVE: To light a bulb to indicate an open circuit.
This project is similar to project #386, but uses a transistor (Q2). The
lamp (L2) will not light until the jumper wire is disconnected. The
jumper wire grounds the base of the transistor, keeping it off. Remove
the jumper and the voltage on the base rises; turning the transistor and
SCR (Q3) on, and lighting the lamp. Note, the adjustable resistor (RV)
is used as a fixed value. Once the SCR is triggered, it will light the
lamp even if the jumper wire is replaced. Turn the slide switch (S1) off
to turn off the lamp.