Laser Sensor and Counter

have created a circuit which is supposed to detect light from a laser (using a photodiode) and increment a counter every time the light is blocked. It works partially, but sometimes the signal bounces and the counter jumps from 1 to say 4 and then to 5 then 7 etc... it's just unstable. If i flash the laser on and off very slowly, then it registers the count more or less properly, but i want it to detect fast changes, for example me waving my hand back and forth in front of the laser.
The not Gate is TTL so i don't need a pull up resistor
Here's a link to the diagram of my circuit. The counter circuit works perfectly, so there's no need to include it.


If you can't improve or point out the faults in my design, could you perhaps suggest an appropriate alternative circuit that would suit my purpose ?

If anyone here can help me i would be very grateful.
Thanks

The light may be blinking more than you think, or the circuit may be unstable at the threshold point.

To fix:

A simple RC filter on the input to the gate may help.

Change the gate to a schmidt trigger.

You might have to go all the way to a mono stable multivibrator or similar circuit to impose a minimum time on retriggering.

A scope ( if you have one ) might be informative.

Thanks for the suggestions. I'll look into each one and ask for more help if i need any.
I'd be happy to provide a scope, if i knew what you meant by that.
Do you want me to tell you what i need this circuit for etc ? what i'm going to use it as.. ?

Question
What program are you using for the diagram, I am using Autosketch but I couldn't paste my drawing to a post here.

This is excatly what I have been looking for. I know the principles but only just getting started no chance someone could make a shopping list for this please?

eptheta said:

I'd be happy to provide a scope, if i knew what you meant by that.

Click to expand...

He was referring to an oscilloscope, which is usually called a "scope". You'll need an analog storage scope (not many of those around anymore) or a digital storage scope that can capture the waveform (because it's a non-recurring type of waveform). You could use a video camera to capture a non-storage scope's screen, but this doesn't guarantee that you won't miss a fast-occurring event.

Unfortunately, scopes tend to be expensive devices and many folks don't have one. As an alternative, you might try using a video camera to capture what the photodiode is seeing. It might tell you that there are more trigger events than you think (but it might miss some too). Things like diffraction can cause spatial amplitude variations that might be causing what you're seeing. Collecting the light from a larger area with a converging lens (magnifying glass) might help a bit. Put the photodiode at the focal point of the lens. To determine the focal point, hold the lens up in front of a window and project the image onto a sheet of white paper. Move the paper in and out until you see a sharp image of the distant objects. The distance from the lens to the paper is the focal length, assuming you're imaging a distant object (e.g., many times the focal length away).

Check out the thread for "FAST HORSES" to see BMorses design for a laser detector that is extremely stable with very fast flashes of the beam being broken (13 hundredths of a second breaks) We originally built the curcuit with CDS but he has enhanced the curcuit to something else (it really works)