Laser Sensor Timer





Introduction: Laser Sensor Timer

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!


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


  • ScavangeR87 made it!


  • Remote Control Contest 2017

    Remote Control Contest 2017
  • Arduino Contest 2017

    Arduino Contest 2017
  • LED Contest 2017

    LED Contest 2017

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Please be positive and constructive.


Questions & Answers


please help me i want to present it in my college i had made the ckt bt its nt working i didnt know y and one more question which type of relay is used in this ckt which is being made by u

howd u get the laser beam 2 show up on ur pic?

I used smoke :)

mine ckt is nt wrking can u please help me which type of relay did u use in these project

I need a timing circuit with an accuracy of 1/2000 second for a research project measuring the speed at which needles of different tip geometries need to enter rather than just depress a target vein. Will the circuit described work with a more accurate stopwatch? If so, could you recommend one that is compatible with this circuit.

James Riopelle MD

Hey ernie how did you connect 2 wires of star/stop contacts in stopwatch to 4 contacts in 2 reed relays as shown above? Did you solder two extension wires in the same contacts in the first relay you soldered the start/stop contacts? Hope you will answer me :)

hope someone still not his thread. I'm wanting to build the above with a release switch so timer starts when a door is opened and the laser will be 200m away. Also I need multiple beam from 11inches high up to 25 inches and a distance from sensor of 3 meters


what kind of wire do I use??

Hi ! I'm working on it and got it working with old IDE conectors wire.
Cheap (you can get it from free in all computer store if you ask nicely) and with only 1 IDE connector you got all the wire needed for the project.

To connect each sensors and stop watch I used RJ45 connectors and wires. You can get this for cheap too (connectors from every old network devices, and wires are decently priced).

Hope it can still help you :)

Just registered to tell that I did this as my first electronic
project. As I was not really confident in my skills, I just made a
couple of the first sensor (w/o the latching) and got it to work fairly
easily. Thanks @Ernie666 !

I Made it to be used for Roller
skating speed slalom. The run lenght is about 28m (~92Ft) and we got 2
lanes side by side. So i had to build each sensor twice. Derivation worked just fine so i managed to get a "Start" case with both sensors and a "stop case" with both sensors too.
Then i built 4 Transmitters.

Since the lenght of wire would have been a pain to manage, I came up with the idea of a RJ45 connection. I had long enought wire available for free and get the RJ45 connectors from old computers motherboards. So instead of wirring my reeds straight to the stopwatches I soldered them to pin 1&2 for lane 1 and 7&8 for the lane 2. This way, i got only 1 wire by case for both lane. Then i Hacked 2 stopwatches, soldering start/stop button to a couple of RJ45 connectors and put everything in a nice case.

Eventually i wanted to wired up everything to my computer. A developer friend of mine, helped me to program a little software with specific needs of our sport. Here is how i got my sensor working with my computer:

I simply get an old usb keyboard and hacked it. Got ride of the plastic case and buttons and kept only the electronics part.

At the bottom of it, you'll get connectors. Each keys is a combo of 2 of them. Hook whats left of your keyboard and try each combo (IE in a notepad) till you identify the key binded to your start function in your software. Do the same with the stop function. Once you get the needed keys, just solder (can be tricky) your reeds on them instead of soldering them on a stopwatch.

In my project i soldered 2 RJ45 connectors so it can work with my setup.

To finish this , I'm really sorry for my english. It's clearly a mess and maybe everything isnt crystal clear, But if you have any question, feel free to ask me. I'll try to get things with differents words or pictures :)

PS: 1st picture: Sensors Wired to Stopwatches (Lane one stopwatch is missing on the picture x) )
2nd picture: The interface Sensors > Computer from an old Keyboard
Extra: A video of Sensors working with my software >