Laser-Tripwire-and-Alarm-Using-Snap-Circuits.pdf

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Laser Tripwire and Alarm Using Snap Circuits
Laser Tripwire and Alarm Using
Snap Circuits
Written By: KRA5H
PARTS:
Base Grid (11” x 7.7”) # 6SC BG (2)
100K ohm Resistor # 6SC R5 (2)
1 0.02uF Capacitor # 6SC C1 (1)
1 Speaker # 6SC SP (1)
1 Battery Holder (2-AA) # 6SC B1 (3)
1 PNP Transistor # 6SC Q1 (1)
1 NPN Transistor # 6SC Q2 (2)
SCR # 6SC Q3 (1)
Pivot Stand Base # 6SC PSB (1)
Pivot Post # 6SC PSP (1)
Pivot Top # 6SC PST (1)
Photosensitive Resistor # 6SC RP (1)
Relay # 6SC S3 (1)
Single Snap Conductor # 6SC 01 (5)
Conductor with 2-snaps # 6SC 02 (6)
Conductor with 3-snaps # 6SC 03 (4)
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Laser Tripwire and Alarm Using Snap Circuits
Conductor with 4-snaps # 6SC 04 (2)
Conductor with 5-snaps # 6SC 05 (1)
Conductor with 7-snaps # 6SC 07 (1)
Jumper Wire 18" (Black) # 6SC J1 (1)
Jumper Wire 18" (Red) # 6SC J2 (1)
Jumper Wire 8" (White) # 6SC J3F (1)
Jumper Wire 4" (Blue) # 6SC J4 (1)
Laser pointer (1)
RadioShack.com has a green laser pointer similar to the one I use in this article. If you
happen to have the Spynet Laser Tripwire laser you may use it as your laser trip wire. I
used it in the demonstration video. It has two axes of rotation, horizontal and vertical,
and is much easier to aim at the photoresistor.
Magnetic Chip Clip (1)
Serves the dual purpose of clamping the on/off switch in the "on" position and as a stand
for the laser pointer--I got mine for free from an Indiana Fever game.
Object(s) of your choosing to stack under the laser pointer to aim it at the photoresistor
(1)
In this article I used a couple of packs of peanutbutter crackers.
SUMMARY
In this article you will learn how to build a laser tripwire and alarm out of Snap Circuits. You
will build two circuits: the laser tripwire circuit and the alarm circuit. The alarm is a very
simple two-transistor oscillator that is switched on by the relay in the laser tripwire circuit.
Once you have built the two circuits, you will then learn how to set up the laser pointer
where somone is likely to break the laser beam and trip the alarm. The Spynet Laser
Tripwire was the inspiration for this build. I wanted to see if I could build my own laser
tripwire. I also improved it by adding the relay to switch on an external circuit. I used the
two-transistor oscillator as an example, but one could use the relay to switch on any number
of external circuits such as floodlights, home alarm systems, etc.
Actually, the laser tripwire was an interesting problem to try to solve. In a standard
transistor-SCR (thyristor) circuit, you often see an LED where I put the relay.
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Laser Tripwire and Alarm Using Snap Circuits
The SCR is advertised as a latch-on device--once switched on, it is supposed to stay on, but
it don't work as advertised:
If you replace the relay with an LED and break the laser beam (or other light source), that is,
stop the light source from shining on the LDR (light dependent resistor) the LED switches on
as advertised. When you restore the light shining on the LDR, however, the LED switches
back off again.
So, I decided to insert the relay to switch on another circuit that would stay on even when
power was cut to the relay. Oddly enough, the relay somehow keeps the SCR switched on
but I'm not sure why.
In a single stroke (and by sheer accident!) I solved the problem and can now switch on any
external circuit via the relay without having to add any additional components to make the
externlal circuit remain on. And it will remain on until I switch the power off to the laser
tripwire circuit.
Wierd. But it may be useful to others working on any number of "electric eye" applications
All Snap Circuits parts used here come from two sets: Snap Circuits Extreme 750, and Snap
Circuits Green. Both sets are available at your local RadioShack store. Individual parts can
be ordered from
http://cs-sales.net/sncirepa.html
if needed
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Laser Tripwire and Alarm Using Snap Circuits
Step 1 — Overview
Snap Circuits is an educational toy
that teaches electronics with
solderless snap-together electronic
components. Each component has
the schematic symbol and a label
printed on its plastic case that is
color coded for easy identification.
They snap together with ordinary
clothing snaps.
The components also snap onto a
10 X 7 plastic base grid analogous
to a solderless breadboard.
All the kits include manuals printed
in color with easy-to-follow
diagrams to assemble the projects.
The illustrations for each project
look almost exactly like what the
components will look like on the
base grid when finished.
Because the electronic symbol is
printed on each electronic
component, once the project is
completed, it will look almost
exactly like an electronic
schematic.
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Laser Tripwire and Alarm Using Snap Circuits
Step 2 — Assembling the alarm circuit.
The alarm circuit is a very simple two-transistor oscillator circuit.
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