"REVISED Laser Beam Alarm"

Revised June 21 2007

When I did the First Design of this project I used a 3 Terminal "2N5777". Later I found some manufacturers changed this part to a Two Terminal Device, But the 2 Pin Version is the same, just without that adjustment.

Additionally some persons have had troubles in getting the Relay to work.
"The Circuit IS Correct".

In Any Case, the New Design below uses an SCR to Acomplish the same thing as the Relay. And its Cheaper.

A Few persons have asked me How Long will this laser pen last when running continuous? I really can't Say, But I have now ran one for over 3 Months, Day and Night. It shows No sign of giving up. It Still works perfectly.

Here is the Revised Design.

Also, I Can NOW Supply the 2N5777.
This is the Origional, General Electric, 3 Terminal 2N5777 Photo Transistor.

This Alarm can be used to protect your property, Inside or Outside. And it can protect a Very Large area such as the complete perimeter of your yard. Be Aware: There are some Possible Drawbacks: Snow or Fog could Block the Beam, Giving a False Alarm.
No Alarm System is Perfect.

You may also want to Build More than one of these alarms, so as to detect intruders in Different area's, Making a More Refined System.

It uses the Cheap "Laser Pointers" (Slightly Modified) as the laser light source and they are found in "Dollar stores" for around One or Two Dollars. Since they are visable light, setup is easy as you can see where to aim the Laser beam.

These Laser Pointers typically run on 3 small Button Batteries. Not a very practical power source, But almost any adapter will run them.

On the Laser I used, it requires About 40 mA. So, using a 12 volt adapter and After Removing the Existing Resistor, A New Resistor can be easily calculated.

Using the formula below, A 225 ohm (220 Ohms, Close enough) resistor is suitable.

(Supply Voltage - 3) Resistance in Ohms = -------------------- Current in Amps

At the Same Time, you will want to Remove or Bypass the Switch.

This Circuit Features a short "On Delay" adjustment on the reciever to allow for Birds flying through the laser beam, as well as a "Delay Reset" for the alarm sound.

If Longer delays are required, a higher Value Capacitor in place of the 4.7 uF can be used. Or use Higher value potentiometers.

If a Very Short Delay is Preferred, Reduce this Capacitor to a .1uF.

The Reciever Assembly is fairly simple.

It consists of a Tube, about one inch diameter and 10 inches long. (This is to keep out Sunlight or other unwanted light sources)

A small piece of glass is Placed in front of a tube, to Keep out Bugs, Birds and Mice.

I attached this Tube to an opening in a small box, Containing the Photo Transistor. You May Just put it into the Tube. Also putting a Cheap Magnifying lens to Re-Focus the Laser Beam can be useful. I Didn't do this, But It would Increase Sensitivity.

The Laser Beam gets Aimed at the Photo-Transistor thats attached to the reciever.

Note that the inside of the Tube and Box MUST be painted BLACK, to Absorb Unwanted Light.
See "Alarm Layout" Below.

To have the beam go around an area, use Mirrors on a 45 degree angle. This will divert the beam by 90 degrees. Use one mirror at each corner.

Important Notes:

1) The Push Button in the Latching Circuit, MUST be a NC Type. (Normally Closed)

2) In the Non-Latching Circuit the Load should Not Exceed 200 mA. 2A) If this load is connected Supply to Pin 3, It will "Go ON", When the Beam is Broken. 2B) If this load is connected Ground to Pin 3, It will "Go OFF", When the Beam is Broken.

In my Proto I used a 2N5777 Darlington Photo Transistor, However any Photo Transistor that works in the Visible Light range of 630 to 680 nm should be fine. However Most Photo Transistors today are in the Ultra Violet range and Won't Work.

"Click On the Titles Below.

"GO BACK to My Projects Page."

"Schematic for 3 Lead, 2N5777.

"Schematic for 2 Lead, 2N5777.

"The Cheap Laser Pen I Purchased for this Project"

"The PCB"

"PCB Overlay Photograph"

"Typical Alarm Layout"

"A Picture of the 2 Different 2N5777 Photo Transistor."

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