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

Comments

[Mattdad1]

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?

[fu12cme]

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.

[samurai7]

buddy, it doesn't work at all, can you explain the circuit of the second sensor, why the 555Timer???

[ernie666]

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.

[Arkasha]

Could you share the model/stock no. for the photocells you used?

Thanks.

[uaaero]

I am awful with electronics but I am making this for some research I am doing and i need to time things. But if you don't mind me asking what are the ratings on the laser and the reed relays. I do not know what amp relays to buy or if it even matters. Also, photocells have a rating, as long as you have a wavelength above that rating are you good?

[ernie666]

lasers are 5mw, relays are 5v reed relays, photocells are ordinary ones that you could find in solar garden lights, night lights, etc

[vidura23]

Hi mate Im trying to build this but im struggling to understand how the first circuit works. Please help me!

My understanding is that the stopwatch requries one impulse to start it and one impulse to stop it.

So the first circuit as the light shines on the photocell it generates current which is amplified by the transistor and delivered to the relay reed. This turn on the reed relay. When the light beam is cut it stops the current flow there by turning off the reed relay. Now what happens when then light beam comes back on after the object has gone through? This will cause more impulses to be sent to the stop watch so how does it work?...im a beginner so id really appreciate your help.

[ernie666]

sorry for replying so late.
The photocell is a photo-variable-resistor. In other words, when light is shined upon it, the resistance in it decreases. When light is shined upon the photocell, it allows more current to flow between the positive side and ground (negative). This creates a "short" (in a way), preventing the transistor from receiving enough current to turn on. On the other hand, when less light is shined onto the photocell (laser beam cut off), the resistance of the photocell increases, stopping current from flowing to ground and forming a "short"; current now flows only to the base of the transistor and turning the transistor on. Once the transistor is turned "on", current flows from the positive, through the relay, through the transistor, and then to ground. The contacts in the relay closes as current flows through it, sending a impulse to the stopwatch, telling it to START.

When the light beam comes back on, it simply re-opens the contacts in the relay; it doesn't send another impulse.

[mmac2]

what is the resistor like with circle in it? thanks

[mwagner63]

What is the dotted square with the squiggle in it? Is it the relay?

[ernie666]

If you're talking about the box in the schematic, yes it is a relay.

[mwagner63]

Could you use this as a measure for speed? You could place them a foot apart and figure out feet per second

[birdo]

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

[ernie666]

I used smoke :)

[birdo]

wow this is very cool. I always wanted something like this. oh and ur drawings make things so much easier to understand.