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Thread: Building a light detector

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    Default Building a light detector

    Hi all,
    I'm just starting out in trying to get my head around electronics and circuits
    and thought I'd start on a small project. Basically I want to build one of these


    I cant make head or tail of the circuit diagram thats given and wanted advice
    on it. I was looking at a sensor like this one:


    to catch the red laser beam that will shine thru and hit the the sensor. I see that you can set the sensitivity by using a potentiometer (is that right ?) in a few different examples I've seen on other similar projects. I want to keep the circuit as simple as possible (one step at a time right) so I can send a signal back to a computer each time the laser is detected hitting the sensor. So the laser beam hitting the sensor will be quite bright and direct so sensitivity can be set quite low in this case (I'm guessing).

    Has anyone done anything similar to this or is planning to. Would love to hear any comments...
    Thanks,



Look Here ->
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    Ok... I can see what your up too.

    In this case there appears to be a hole in the meter disk, so pulses of light are let through. Count the pulses and you're counting watt-hours.

    So first question is, can you get access to the disk so you can shine a laser though it and does it have a hole etc ?

    You can monitor power usage with inductive coupling. A simple current based circuit running a VCO of some sort will do a similar thing on the output.
    There's more than one way to skin a cat.

    I'll check my own meter and see if it has any optical possibilities.

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    Yes - my meter is very similar to the illustrated one - i tested my laser pointer thru the disc and it should work fine - its just the electronics part that is new to me here - my watt usage is slightly different but I can just recompile the program with my figures to fix that...thats not the part that has me stumped - its all the electronics part. I really want to get one simple project under my belt and this seems to be a cool, practical one to do. On a side note I've got my water tank levels graphed here ( site should be up )

    So i want to do a similar thing with watt usage...
    Thanks,
    Peter

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    Ok... mine is the older style black meter. The disc does appear to have a hole in it, but it is well towards the center where I could not shine a laser clear through it.

    I could line up a laser so that it made it in, but there wouldn't be much hope of directly recovering the beam. This applys to all of the disc, there isn't any way to get a beam directly through.

    It would appear that I have two options. The edge of the disk has a black spot as well as the top near it. Laser light could be made to reflect off the edge of the disk and when the black spot passes, hope that thre is enough attenuation to trigger the circuit.

    A phototransistor is very much like a normal NPN transistor, except that the base is controled by photons instead of electrons.
    If you have a cro that can do a slow sweep, you should be able to see the black spot pass by with the phototransistor.
    An IR pin diode is a little more sensitive that a phototransistor, but works much the same way. You could use any IR LED and a lens or a pin hole with similar results.

    There is one other option that sprang to mind. It's a little more complex in circuitry, but the results really do speak for themselves.
    Use a small CCD camera.
    It might sound complex to interface a camera to any circuit, but there are nothing more complex that an analog serial signal all precisely timed.
    All you have to do is count horizontal lines from a vertical reset, then count microseconds across the line. White and black levels are 1Vpp. A pic chip can quickly pick out a point in the field of view. Then just carefully point the camera and look for the pulses on the output of the chip.
    Sync seperator chips are available off the shelf, but you can also make them with a 4049 and a hand full of resistors and capacitors.
    Just point the camera and shoot. This can be used for counting all kinds of weird shit passively even at great distance.

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    Those other options sound quite interesting - but probably over my head at the moment with my level of knowledge. I think I'll stick with the original plan and wonder down to the local jaycar / richard smith and get a few components so I can play on my breadboard to see if I can get a rudimentary detector going based on the laser light shining on a sensor...Will report on my 'findings' soon I hope...

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    I'm staying at a friends place tonight and sure enough, he has the same meter as me, but it's clear and I can see the internal workings, the disk with the hole in it.

    Ok, well we'll keep it simple then and let you worry about getting light to the sensor. Just buy a garden variety phototransistor from Jaycar or DSE.

    Ah.. I can get access to the jpeg tonight, I was having trouble accessing it last night. His circuit doesn't use a phototransistor, it uses a Cadmium Sulphide Light Dependant Resistor (LDR). Jaycar sell them too.
    His circuit is over complicated for purpose, I looks like he's just using the Parrallel port for data collection.

    You should have no trouble finding a circuit that outputs a "TTL" 5V levels for a light sensor. Really, if you wanted to cheat like hell, DSE sells a Funway 2 kit which is a light switch. It's an LDR circuit with a relay output, so you might like to replace it with a solid state relay if the click-click click-click gets too anoying.

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    I've been paying around with some parts on my breadboard and think I am getting somewhere ( I could be wrong ) but this is the results so far...
    This is very simple - thats the way i like at this point..so excuse any dodgyness on my part - as I said this is my first circuit.

    My DC power supply is pushing 7.12 V across its 2 terminals (thru the circuit when I switch it on).

    I have a LM311N voltage comparator


    I am using a BPV11 NPN photo transistor as a light detector


    pin 1 - not connected
    pin 2 - connected to the output (emittor leg of BPV11 NPN photo transistor)
    pin 3 - connected to the output of a potentiometer
    pin 4 - connected to -ve
    pin 5 - not connected
    pin 6 - not connected
    pin 7 - the output - connected to voltmeter at the moment for testing
    pin 8 - connected to +ve

    The output from the LM311N when no light is directed to the sensor
    is always 0.91 Volts. This seems to be the low point (ie off) - I thought it would be 0 volts, but looking at some of the output charts on the net from this device it always seems there is some 'residual' voltage coming thru before it switches high(on)...any comments on that one ??

    Then when I shine the laser on the BPV11N it then jumps up to about 6.4 Volts , laser off - then back to 0.91 Volts, laser on - 6.4 volts etc.
    So thats where I am up to now. I'm just scratching my head now - how do i turn this 6.4 volts into a logic pulse to pin 11 of my printer port ?.. I'm wondering if the residual 0.91 volts will cause any probs before it jumps to 6.4....any ideas on this one...hey this is quite fun..

    Appreciate any comments.

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    The low level isn't a problem, the high level may pose a problem.
    The standard for the levels you need are called TTL. It's usually pretty fussy about the voltage being 5V. You can test the response of your software with a simple potentiometer as a voltage divider.

    The low level is caused by the ambient light. Only in total darkness will it be zero and even then, the device still generates noise.

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    ok, so if i rig the output pin from the LM311N thru a potentiometer and tune it down to 5V and into pin 11 of the parallel port I should see it register in the C program....Now to find a parellel port on a linux machine...damn shuttles with no printer port

    My next question is powering the laser pointer with the same power supply as the circuit. I havnt got it to work yet. The laser pointer uses 3 x GPA76 watch type batteries off the shelf. These are 1.5V each so thats 4.5V total right.
    My power supply is rated at 5V and 350 mA.

    I just tested the mA used by the laser pointer when running on said batteries (as it dosnt state it on the pointer itself) and it looks like its using 17.8 mA when on - I connected the multimeter in series with the laser on and that was the reading.
    Interesting when i changed the multimeter from milliAmps to Amps I got a figure of -0.038 Amps .... not sure whats going on there ?? So am I to believe its 17.8 milliamps ??
    Anyhow I need to get this laser working from the power supply before I can move forward so any pointers (no pun intended) would be appreciated ...
    Thx, Peter

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    Current limiting. If in doubt, use a 3V lithium button cell with an ammeter.
    Once you know the current, calculate the required resistor for 12V etc.

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    Default

    I like your water level monitor, what was the cost of the aquaguage?

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    oops sorry for the late reply - better late than never ??
    Gee I cant remember what the cost of aquaguage was now - think it was around $150-200 all up..See
    should have current pricing...

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