lasertripwire

Building it:

Power

Our project in counter mode can be run off of 3 Volts, or when in alarm mode from 5 Volts. An extra power cube transformer I had which was rated at 5.2 Volts and 400 milliamps was used to power the project most of the time, while a 3 Volt AA battery holder was used for the general counter mode. Both were attached to a 9 volt battery connector for ease of use.

Light-to-Voltage Sensor

The main component in the project is the 3 pin LTV or Light-to-Voltage sensor. This little guy detects light that is incident upon its little bulbous front and converts that lights intensity to an output voltage. So when more light of the appropriate wavelength is shining on it the higher the out put voltage, the less light the lower the voltage. These things come in sensitivity ranges between 300 to upwards of 1000 nm on the lights wavelength. Mine is in the mid-high range around 740nm, so that would be the color red to just outside the visible spectrum. It can run off a red LED so the output of the laser I use (between 630 & 680 nm) worked great for this little guy.

The white thing with 3 pins in the middle of this shot is the LTV. The pinout for it can be seen in the top picture, and from left to right goes: Vcc, GND, output. This is looking into the tin I inserted the project into once it was completed. When the tin is closed a hole is drilled through the lid directly in the path of the LTV to allow the Laser beam to shine on the LTV.

3906 PNP Transistor

The LTV output is fed directly into the base of a 3906 PNP transistor. Working pretty much opposite to that of a 3904 NPN transistor, a LOW on the base will cause a current between the emitter and collector. Hence when the Laser beam is in contact with the LTV the transistor will be switched off, but when the beam is broken the transistor will be turned on.

555 timer - Monostable Mode

Now that we have the LTV and PNP transistor figured out we just need to decide what we want them to turn on and off. Initially I would connect a buzzer to the transistor giving me a little noise every time the beam is broken. This is pretty much useless for defensive purposes and such a slight noise would not alert the most fastidious of guard dogs to an intruder’s presence. If the buzzer is going to work it needs to go on for at least a few seconds. That’s where the 555 timer circuit comes in.

The 555 timer will be operating in mono-stable mode, meaning that once you ground pin #2 the timer will start and output a high signal on pin #3 for as long as you set it to do so. Setting up a 555 timer is pretty easy, and for mono-stable mode look on the internet or any number of electrical/hobby books. Or just look at the right half of the schematic given above, everything to the right of the LTV is part of the monostable circuit. For a general schematic replace the buzzer with any device you are using, and replace the transistor with any type of switch. Here is the schematic I used in my digital circuits class to build a car lockout mechanism:

(Note that the 3904 should be a 3906, there is no Diode comming from pin 3 and that the Controller is just a buzzer for this project.)

I chose a 10 micro Farad capacitor for this setup as it allows me to choose between times of 1/10^th of a second and a minute and a half by changing only a single resistor. What’s nice about mono-stable mode is that it is started by grounding pin #2 (giving it a Low signal) on the 555. So all we need to do is hook pin #2 to ground through our PNP transistor. In summary; if the beam is broken the transistor turns on grounding pin #2 turning on the 555. This causes a HIGH signal to be sent out on Pin #3 while our chosen capacitor discharges through a resistor for a predetermined time period. Once this cycle has run its course the 555 is automatically reset ready to run again as soon as the beam is broken. The HIGH signal on pin #3 is connected to ground through a buzzer which sounds the entire time the 555 is set for.

(Here is the 555 soldered onto the board we'll use.)

Counter Mode

As I was retiring one evening I noticed a pedometer I had been given that was going unused sitting on my bedside table and I thought “It would be cool if I could hook that up to my laser-trip-wire and have it count the number of times the beam is broken.” Much to the chagrin of my wife I was soon back in my computer room opening the back of the pedometer up to see what I could do with it. After a moment I saw two little insulated posts that were touched by a piece of metal attached to a spring every time a step was taken.

I pulled off the insulation soldered on two little wires and tapped them together and watched the pedometer count touches in the hundreds. So I put a 1k Ohm resistor between them and it started counting using ones each time I made the connection. To change to counter mode UN-connect the pin #2 of the 555 timer from the collector of the 3906 Transistor (the coiled orange wire) and plug in one of the pedometers leads through the resistor. Then connect the other lead to the common ground (other side of the transistor) and whenever the beam is broken the pedometer will count upward by ones, once for each broken beam.

After Project Notes: