LaserTrigger Reaction Time

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello guys,

first of all I wanna thank you for helping me so much !
I finally built my first Arduino Laser Trigger with a LaserPointer and a LDR :slight_smile: I never could have done that without your help !

So I did some testing with the LaserTrigger and found out that it doesn't recongize fast moving objects going through the LaserPointer (for example if I go quickly over it with one of my fingers).
Which component does not allow it to recognize fast objects. Is it the LDR or Arduino itself ? Or asked in another way, is there a way to improve the quality ? :slight_smile:

Cheers
Clause

2

Your problem is probably the LDR.
The cadmium sulphide devices don't have very quick response times.
Google photodiode or phototransistor, or have a look in the Playground.

3

It's probably the LDR. Have you tried using a phototransistor (with a response range for the wavelength of the laser) on one of the digital inputs instead?

4

Thx for the quick reply :smiley:

I defently will try it out.

Cheers

5

I rigged up a trigger circuit like this and found that the LDR was pretty fast, but there were a few lags in the camera:

  1. I had to be at "half shutter" before trying to pop the shutter, and
  2. had to have the mirror locked up, since I was shooting with an SLR.
    Have you ruled out the camera being the source of the lag?
6

Kind off , cause i'm not actually going to trigger the camera with it just the flash. And I also just lit up an LED when it would trigger the flash.

Cheers

7

I am doing something similar and the Arduino is most probably not the problem. Unless your program is very inefficient. My current results are: (running @16 MHz): maximum lag: 0.5us, time resolution 1 us, frequency jitter <1us, precision better 100ppm. I am still working to get the crystal more precise. But the Atmels are definitely fast enough. The analog parts surrounding it give rise to much bigger delays. For example it is hard to find an affordable flash faster than 20us. Except you are willing to pay >1000 or you are going to build your own flash.

I would guess either the LDR is the issue, or you are polling the LDR. If you use analog in for detection you still have a sample rate of ~50 kHz (or 200us period time).

Udo

8

I've never tested this, but how about using a humble LED as the sensor? Preferably with the same color as the laser. I don't know how fast they are as sensors, but I would think faster than a LDR.

9

Don't get me wrong !
I am not 100% sure but I think your solutions would short the time btw the light beam beeing broken and the actual trigger, but I don't care much about this one.
But it's about that that it does not recognize fast movings.
And I guess that comes from the LDR's "long" signal falling time ?

Cheers

10

Yes, LDRs are slow. Their reaction time (best case) is measured in milliseconds. However if your instrumentation is not very sensitive to small changes then you may see reactions times in the seconds range.

Udo