Picture of 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.  
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Step 1: Basic idea

Picture of 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!

Picture of Materials!
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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

Picture of Hack it
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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)

Picture of 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

Picture of Laser sensor schematic
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Follow the schematic. After soldering everything in place, cut it out.

First picture= first sensor
Second picture= second sensor
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ElisaO14 days ago

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?

kjhdfadn1 month ago

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.

gdavies20062 months ago

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

ernie666 (author)  gdavies20062 months ago

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.
ernie666 (author)  ron.olmstead.563 months ago

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?
I was wondering if this would work:
photo-2014-12-11, 11:21 AM.jpg
ErikK13 months ago

Hi there, I built it and it works fairly well. One issue is that when the second laser gets broken I can hear that the reed relay switches, but it does not trigger the stop watch each time to stop the signal. I have tried resoldering the connections to the relays to make sure it wasn't a faulty connection. There has to be something in the signal for a longer time in order to stop the timer.

Do you know why this could be happening? Would it be possible to either enlarge the photocell surface or create some sort of amplifier to increase the signal to the stop watch to stop the timer?

ErikK1 ErikK13 months ago

Nevermind I figured out my mistake, I used a 100 ohm instead of 10 ohm.

Also everyone if you just build two of the first senors your timer will work in both directions.

sree.v.vicky4 months ago

hi is it possible to make one for racing and get it sync with my laptop so i can get to know the exact timing sitting in the pit

ernie666 (author)  sree.v.vicky3 months ago

Sorry, I'm not a computer expert. I don't know how to properly join computers with external interface. With enough effort you can usually find what you need on the Internet.

sree.v.vicky4 months ago

hi is it possible to make one for racing and get it sync with my laptop so i can get to know the exact timing sitting in the pit

JCondor6 months ago

My son wants to try to build this for school project. What are the specific form factors required for the resistors, capacitors and relays?

ernie666 (author)  JCondor5 months ago

all parts are fairly generic not special at all. resistors are 1/4W.

tconner858 months ago

I want to make one of these, but for running laps around a track. If I make two of the first circuit and put one of them on the lap button, and one of the on the Start/Stop button and not make the third circuit, it should work right? My hand would trigger the start stop and every time i run through the lap sensor, it would do my laps if i'm not mistaken.

ernie666 (author)  tconner858 months ago

that sounds like it would work! :)

Hi, I'm doing this for a project but it doesn't work well, I have connected the entire system but it works reversely, the transmitters send out a signal when I put the laser beam instead doing it when it's broken.

I changed the reed relays position (first I connected normally open all the reed relays) and now the second sensor doesn't do what I want, I mean It doesn't stops the stopwatch. I have tried many combinations to make the second sensor works but I didn't succeed in that task.

Can you tell me what I'm doing wrong? Is it related with the reed relays position? How did you connect them?

I'll be grateful if you could answer me as soon as possible.

ernie666 (author)  Anallely Tonks Doors8 months ago

The relays are normally opened. My only guess here is that you might have made a mistake when you connected the photoresistor. If you had swapped the positions of R1's and the photoresistors, then you would get a signal when light is shine upon and not when no light is shine upon, like what you had described. I hope this answers your questions, but if not, message me again.

mwagner634 years ago
Could you use this as a measure for speed? You could place them a foot apart and figure out feet per second
Yolo1927 mwagner6311 months ago

Actually seconds per foot

Yolo19271 year ago

Could I stick the ground for the sensor onto the positive terminal of the battery using an LED without messing up the circuit?

ernie666 (author)  Yolo192712 months ago

I'm quite confused, could you explain what you are trying to do a little better. (maybe with pictures?)

Yolo1927 ernie66611 months ago
(removed by author or community request)
ernie666 (author)  Yolo192711 months ago


Yolo1927 ernie66611 months ago

If the object hits the laser twice using the second circuit for the first sensor too would make it work, right?

hbrar111 months ago

I think it will not work in sunlight as photocell will be receiving light from sun. can u please explain??

Yolo1927 hbrar111 months ago

Simply wrap black electrical tape around a straw and stick it on the photocell. It should make it dark enough, and leave a straight path for your laser.

kd312711 months ago

I have tried to make this sensor. I am not savvy enough in electronics to put it together. I have all the parts and was wondering if you could assemble it for me. I'd ship you the parts and be willing to pay. if you would do this let me know how to get in touch.

mmac23 years ago
what is the resistor like with circle in it? thanks
Yolo1927 mmac211 months ago

A photoresistor

mraymo21 year ago
Do you know if there is a complete system that is commercially available? I would like to include a cost comparison in a write up on this project and am having trouble finding anything.
dbug101 year ago
I am wanting to do something similar to this we drag race boats and want to set up some kind of photocell in each lane to trigger a light to show the winner how would I do this ?
Mattdad12 years ago
How far apart can the sensors be? I'd like to build an electronic timing system to measure a 40 yard dash for the football team. Would you anticipate any problems from wire resistance or inductance from a long wire run between sensors?
fu12cme2 years ago
Has anybody tried to make this? I would like to try because i have a specific use for it. Unfortunatly, I know very little about building circuts.
samurai72 years ago
buddy, it doesn't work at all, can you explain the circuit of the second sensor, why the 555Timer???
ernie666 (author)  samurai72 years ago
If you look closely at the two circuits, you would find that the second circuit contains the first circuit, which is a light sensing circuit. The rest of the circuit with the 555 timer is a latching circuit. The latching circuit latches onto the time onto the recorded time on the stop watch so that you will not accidentally restart the timer and ruin your data in an experiment if you break the laser beams again on accident.
Could you share the model/stock no. for the photocells you used?

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