Laser Sensor Timer




Posted in TechnologyElectronics

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



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My project starts from left to right but the same concept as yours. However mine will start from in between the break beams. Mine will start left on a pendulum type swing crossing the beam starting the timer. Then come back through that same beam, heading towards and crossing the second beam stopping the timer. MY QUESTION, will crossing over the first beam the second time stop the timer or will the timer keep counting til i reach the second beam. If so how would i fix this. Thank you.


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?

2 replies

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


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 :)

Hi how are you . How I contact with you.

What is the part next to the 555 timer and above the reed relays, is that the latching relay?

snip 1.JPG

Hey, if you don't mind I would like to ask you something. I'm planning to make this for my school project, but let me just say that I'm not exactly a genius when it comes to technical things. So first of all, if I'm absolutely sure that it won't be broken again, will I still need the 555 timer? Second of all, and I'm sorry if this sound stupid, but I'm really a noob in this, what are the reed relays for and are they absolutely neccesary or can I just leave them out. I've researched and stuff, but to be honest I don't get why all these ohm and microF are neccesary, so would you explain it to me, please?

Thank you for this information. Could you help me. I'd like to modify it to use just one laser to alternate the stopwatch between an on and off state.

hi, is there anyway of adding more lasers / sensors so it will cover from floor to about 2ft high.. thanks

1 reply

more lasers is unnecessary. Use mirrors.

Hi. Thanks for posting this. I am hoping to use this for an application in which the first laser beam may be broken 2 or more times before the second laser beam is broken without stopping the timer. As an example, if you timed a runner passing through the beam at knee level the first knee would break the beam, starting the timer. If I understand the circuit, because there is no latching function in the first module, if the beam then was unbroken for a moment while passing through the runner's legs and then broken again as the runner's back leg passed through the beam, the timer would stop. Is that correct?
I thought if I simply built 2 latching modules the first one would latch after starting the stop watch and the second one would still stop it. After building the 2 modules I find the first one starts the watch and latches but the second one won't stop it. With some experimenting I found if after breaking the first beam, I then pull out one of the stop watch leads going into the reed relay and then breaking the second beam, the timer stops. This leads me to believe that the latching function works by energizing the reed relay that controls the timer to the closed position and holding it closed until reset (kind of like holding the button down on the stopwatch) thus not allowing the second module to signal the timer to stop?
Does that sound right? Can you make any suggestions on how to make this work in a way that the first beam can only start the watch when first broken but not stop it if it's broken again before the second beam is broken?

Thanks for this awesome instructable.

1 reply

You are correct. The latching function in my circuit keeps the reed relay closed once it is set, and it doesn't open until the reset button is pressed. Thus it is like pressing and holding down the button on the stopwatch.

It now works the opposite way to the first schematic I posted. When the laser beam is broken and the transistor allows current to flow, the capacitor starts charging, during which time the relay closes. Once the capacitor is fully charged, the current can no longer flow and the relay opens and cannot be closed again until the capacitor is discharged via the reset button. Now, subsequent breaks of the laser beam will not affect the stop watch. Tried it out and seems to work perfectly.

In case this is any use to anyone, I made a slight modification and moved the capacitor to the other side of the relay circuit and put the reset button in the right place. Through trial and error I found that a 470uF capacitor works. I made two of these and hooked them up to a stopwatch and it worked!

photo-2014-12-13, 11:54 PM.jpg

You press the reset button which would charge the capacitor (I have no idea how to pick one, I would have to size it through experimentation). Then when the laser beam is broken, the transistor closes the circuit to the relay's coil, which is momentarily powered by the stored charge in the capacitor, momentarily closing the relay to start the watch, but then opening again as the capacitors charge is used up. Is that possible? Or do I not fully understand how capacitors work?