looking for the right IR emitter, receiver, and help with OR(ing)

Hi Cory,

If I understand what you want correctly, you need 40 light sensors with just one output - if any sensor loses light (interrupted beam) the output triggers.

If you could explain the geometry of how they are positioned hand how large is the interrupting object it might help us to optimize. If they are close together, you may get light from multiple sources hitting each detector. If this is outside, then sunlight may saturate all detectors -you might have to modulate the light to differentiate it from ambient light.

1) If you could have retro reflectors on the object and use light sources beside each detector - then the trigger would occur when a light was received instead if interrupted. Then you could simply use photo transistors or photo darlingtons (more sensitive) for detection and they can all be "ORed" simply by connecting all of their collectors to the same pull-up resistor. (parallel open collector configuration) If any photo darlington gets some reflected light the output will go low.

2) This could also be done with light interruption as well by connecting the output of each sensor to an open-collector transistor, but this requires wiring up an extra 40 transistors.

3) If the speed of the object is not too great, you could use cadmium sulfide detectors or cadmium selenium which is better for IR. Just connect them all in series (a single daisy chain = minimum pins and connections). Since they can be less than 1K ohm when lit up and 20K when dark, you just need a single pull-up resistor and a comparator to detect an "ORed" interruption trigger. You might even get away with using a transistor instead of a comparator - you could adjust the threshold using diodes or a zener.

Doug

The object is a ping pong ball. I am building a ping pong table that will be inside. Once I did all the math I realized I need 40 sensors on either side of the table and each would be about 1.5 inches apart so no mater where the ball lands on either side it will always trip a beam. I am looking for the ball to just be recognized that it hit the table on whichever side it does land on which is why i just need the l/0 output for either side.

Hi Cory,

Could you just use a simple impact sensor on each side of the table and which ever signal is bigger tells you which side was hit?

Doug

Unfortunately I have looked into that before and i figured that the light weight ping pong ball would not have enough force to trigger it. Also, unless if there is a one big plate i could have for the side then it would not be very effective because if there are a lot of small plates then the ball might hit the side of one and go off in the wrong course which would interfere with the game, but if there is a way for them to pick up on the small force of the ball hitting them and they could be flush with the table then that would work.

One thing I have thought of and it just worked is to have the outputs of the IR sensors going through diodes and the diodes going into the one pin. For now I know it works with 2 but I do not know what would happen with 40.

The diode OR sounds promising.

It just so happens I just finished building a sensitive shock sensor. In addition to an output it illuminates an LED when it detects a vibration.

It can detect a pin drop from a couple of mm.

To provide some indication of whether this could work for your application, I jammed the sensor on the underside of my dinning room table (I don't have a ping pong table) and bounced a ping pong ball on the table.

In the video you can see the circuit board powered by a USB battery at the far side of the table where the sensor is. And you can see the LED flash when the  ball hits the table. The sensor could be made more sensitive since there is an amplifier in my circuit - I just don't need more sensitivity for my application. Attachment to the table could also be improved, I just stuck it in a crack.

Note that this table is split in the center to allow extra leaves to be inserted. When I bounce the ball on the other side of the split the impact is not detected. This would be ideal for ping pong tables that have two separate halves.

Doug

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RE: looking for the right IR emitter, receiver, and help with OR(ing)

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Ping pong sensor

not sure but I would think there is a need for separation of sensors across the net, if the idea is to detect if a player indeed cleared the net and hit the table, bouncing on your own side may not count after it has just arrived ?

just a thought, so is it possible to have each side separated without a gap in the table ?

LOL, sorry Doug, for some reason I did not see the last line of your post, ignore me lol

Well this gets into the system design choices that depend on accuracy requirements, cost, complexity and some unknowns that may need to be tested. You could probably use either amplitude or propagation speed to determine which court was impacted.

With a single sensor in each court, you could determine which signal was larger to determine which court was impacted. There might be ambiguity near the net, so. this may need careful calibration for impacts near the net.

If propagation delay is used, the first sensor to trigger would "win" and indicate which court was impacted. Basically the first trigger would "lockout" the other one for some period.

yes i was planning on having a separation as my goal is to make this look as much like a ping pong table as possible. What is the schematic for the circuit you are using or where did you buy it? I will give that a shot as I said I have successfully managed to hook up 3 IR sensors in series to one pin using a diode for each sensor and so far it is working perfectly, but your idea of the vibration circuit is very interesting I could theoretically stick one to the bottom of each side, and that would be much cheaper than buying and wiring 40 IR emitters and 40 IR receivers since i only have to buy a few parts to make the boards.Here I show the 3 sensor test that I have set up now. It's not the cleanest I just wanted to see if it would work first.

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As for the rules, they would be standard so the system on which I detect the ball wouldn't matter. If the ball hits one side more than once than it is a point to the other person. if the ball hits the net and goes over then that would count. This is where the one of the only faults i see with the shock detector method. If the ball gently lands on the side( for instance it hits the net but gently falls over so it lands bounces maybe once then roles) then that may be a problem. I have thought about that and was going to add to the code to prevent a problem like that with the IR method anyways so it's not a big deal because that is still easier and cheaper.