In this instructable I'll show you how to make a very accurate laser sensor timer. The timer latches the time automatically, making it easier to record. The latch has to be reseted before the sensors are ready to use again.

This was originally used for my science project, where I had to test the time it takes for my parachutes to land. They fell at about the same speed so I had to come up with something to time them accurately, and I came up with this. It worked out very well and I like to share it with all of you.

Step 1: Basic Idea

The lasers are going to act as the transmitters and the light sensing sensors are the receivers. When the laser beam is broken, the receivers loose connection with the transmitters and they send out a signal that starts/stops the timer. The first set of sensors start the timer and the second set stops the timer. The second sensor latches the timer

Step 2: Materials!

Materials:

digital stop-watch
toggle switch x 2
lasers x 2
9v batteries & clips x 2
AA batteries x 4
AA battery holders x 2
push-button switch
prototyping board
heat shrink (optional)

2N4401 x 2
555 timer
100k Ω x 2
10k Ω x 2
220 μF
0.01 μF
reed relays x 3
photocells x 2

Step 3: Hack It

Open your stop watch and locate the buttons; once you find them find their contacts. Solder one wire to each contact. You will only need to solder the wires to the start/stop contacts but I did it to all of them :)

Step 4: Lasers (transmitters)

Take your laser and solder the toggle switch to the negative terminal. Solder the other connection of the switch to your battery holder's black wire, then connect the red wire from laser to the red wire from the battery holder.

Repeat the steps above 1 more time (we're making 2!)
Add heat shrink if you want

Step 5: Laser Sensor Schematic

Follow the schematic. After soldering everything in place, cut it out.

First picture= first sensor
Second picture= second sensor

Step 6: Done!

I made mine! Did you make your's?

If your having trouble laying out the circuit or trying to figure a way to put everything on a board to save the most space, you can take a look at mine as an example.

Step 7: Use It

All you have to do to use the sensors is to shine a laser on the each sensor's photocell and put the batteries in! Once you do so, the timer will start timing if you break the laser beam.

Remember, use the first sensor(the none latching one) at starting line and the second sensor on the finish line.

Step 8: Final Thoughts

Here are some additional things that you can do after your done:

-add power switches
-make holders for the lasers (look above)
-connect the reset button on the second circuit to the stopwatch's reset button
-reflect the laser with mirrors to make the sensing area bigger