Hey All,

We have a fluid flow-meter that I need to monitor. The flow absolutely cannot be shut off, so I cannot install a simple flow-switch. There is already an in-line flow meter that looks something like this. I am thinking of rigging up a simple beam-break circuit to monitor the flow-meter, kind of like what I have attached here.

I think the circuit is pretty self-explanatory, but I'm not sure that I can spec LEDs properly for it. The circuit will be in a (nominally) lit laboratory. I was thinking maybe infrared LEDs for it, but now I'm thinking I may need something more like a laser LED so that the beam is highly directional. The bead in my flow-meter has a diameter around 4.5 mm, and so I'd need the beam diameter to be less than that. Can anyone recommend a reasonable laser LED in any spectrum? Also, if I do a visible LED, do I need to add background subtraction to my circuit?

Thanks!
- Aws

 

Here's a bright 8 degree led, color red.
http://www.superbrightleds.com/more...d-8-degree-viewing-angle-12,000-mcd/283/1210/

 

Awesome. I am planning on taping the detector and the transmitter directly to the meter. I just worry that the beam would diffract around the small bead in the meter and would still hit the detector. Do yous guys think this is likely?

In other news, I'd forgotten that LEDs were spec'd by viewing angle rather than beam diameter -- I guess I'm thinking lasers already. The body of the meter is about 30mm wide, and I'm thinking I'd like the beam diameter to be less than 5mm -- either at the center or across the whole thing. If I do a quick calculation, I get that I need a viewing angle less than 10°(Phi = 2*ATan[D/2L]). Sounds very doable. Does my math look right, also? D is desired beam diameter, L is the width of the meter, phi is the viewing angle.

Thanks!

 

Since the detector is only about 3 mm wide (maybe less), you can use the 8° LED since you are interested in if the ball blocks the beam. You can adjust the brightness of the LED so that if the ball is above the sensor, the sensor is on. If the ball partially blocks the sensor, the sensor decreases the current. If the ball is fully in the way and there is still a current, then you need to adjust the low end of the circuit to register there is no flow.

 

Since the detector is only about 3 mm wide (maybe less), you can use the 8° LED since you are interested in if the ball blocks the beam. You can adjust the brightness of the LED so that if the ball is above the sensor, the sensor is on. If the ball partially blocks the sensor, the sensor decreases the current. If the ball is fully in the way and there is still a current, then you need to adjust the low end of the circuit to register there is no flow.

Excellent! Thanks. I've ordered parts and I'll give it a go. Thanks for all the thoughts!

 

Last year I made a drop detector using a laser & 5 SM photo transistors that would detect an object 3 mm falling anywhere in a 1 cm window, for just in case present plan is not satisfactory.