Sensor Trouble

Discussion in 'The Projects Forum' started by joes238, Jul 29, 2009.

  1. joes238

    Thread Starter Member

    Jul 29, 2009
    21
    0
    I have another idea I want from the table I'm building that is a little hard for me to figure out so I thought why not post it here :D

    I'll have circles cut out of the table with Plexiglas over set in the hole so it will be flush with the top of the table. The Plexiglas will not be as thick at the actual table so there will be a small amount of space to work with. I want to put in a sensor that can detect if a cup is set onto that spot above it. Now for the electrical part. I want the circle to be lite up green when there's nothing there and when the sensor senses the cup I want it to switch off the green lights and then switch on circuit 2 (which will be a different color LED circuit) The problem I have is that I need a sensor that can go through Plexiglas (prob 1/16" thick) and sense the cup and not be affected by the LED's that are lite. I figure a single pull double throw switch would work best. I looked around on digikey.com and found this one. Its magnetic and I really don't want that but unless someone has a better (same price or cheaper) solution this I guess is my best bet. It's only 40 for 10 of them which I can handle. This is more of a DIY project to get me more familiar with electronics and give me some more hands on experience with different components and I figured making something like this table with everything I can think of in it would be the best idea. Also the cheaper the stuff the better because I'm paying for my college classes as well so every penny counts as my parents would say ;)

    As always I'm open to different solutions to my project so if you have any idea's they would greatly be appreciated.

    Thanks in advance,

    Joe
     
  2. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    I'd avoid that sensor, as I'd bet you could do things with cheaper components. Here are some ideas that popped into my head. Some are eminently unpractical and many may not meet your constraints you haven't written down. :)

    1. Use a microswitch under the table that isn't quite activated when the Plexiglas disk is resting on it, but when any additional weight is placed on it, the switch changes state.
    2. Use an infrared beam pointing towards the disk. Normally, the beam goes through the disk, but when something is set on it, the beam is reflected back and detected by an IR sensor.
    3. Since there will be an LED illuminating the disk, have the light go through a hole when the disk is in position. When a weight is placed on the disk, the disk moves slightly and the hole is occluded (this is an optical variation of the microswitch method).
    4. Use an ultrasonic transducer coupled to the disk. A sensor on the other side measures the amplitude. When a weight is put on the disk, the amplitude will change.
    5. Similar to the microswitch, but use a small pressure sensor. When the weight on the disk changes, the pressure sensor registers the difference. The disk is hydraulically connected to the sensor. (I know, this one is too complicated.)
    6. Similar to the microswitch, but turn it into a capacitive sensor that e.g. changes the frequency of a resonant circuit.
    7. A small radioactive source of alpha rays is put on the disk. When something is put on the disk, more alphas are reflected backwards, which gets detected.
    8. Design the disk so that it is composed of two conductive materials that are transparent and conductive. When something is set on them, they make contact. Or, encase the disk in a ring of metal that, when weight is put on the disk, contacts another metal ring, forming a switch.
    9. Use a piezoelectric sensor to sense when the weight of the plastic disk changes (i.e., weight the disk and what's on it). Similar thoughts can be made with a strain gauge. Or optically use the birefringence of plastic under polarized light to detect a change in loading.
     
  3. joes238

    Thread Starter Member

    Jul 29, 2009
    21
    0
    OK those are all pretty good ideas but could you maybe link a couple sensors you would suggest for me around that price range? I've been looking but really can't find any that I can use and are as cheap as the one I found.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    I'm retired -- I don't have time to do your searching... :)

    I'd suggest you check out All Electronics http://www.allelectronics.com/. They sell surplus/salvaged stuff and may have something you can use.
     
  5. joes238

    Thread Starter Member

    Jul 29, 2009
    21
    0
    Thank you for the site. The only site I had was digikey and that really wasn't working out:D

    I was just wondering. Is there a way to hook up just a spst switch and have it trigger something that can turn off one circuit and start the second one? The site you suggested from the looks of a quick search only have spst switches and I don't know how to make that work with 2 circuits. I'm pretty new to all this so if you could clear some of this up that would be great. I'll do my best to understand :D
    thanks,
    Joe
     
Loading...