Slotted opto-switch multiplexed to analogue input

Hi everyone,

I've been trying to work out how to do something which *should* be easy, but is foxing me and I was wondering if you could give me a little guidance please?

I capture rainwater from my roof and store it in a water butt for garden irrigation. The tap is quite near the bottom so as to maximise the water I can take out of the butt. Every so often (about every 6 months) water suddenly stops coming out because silt and muck from the roof has risen up above the level of the tap. The problem is, I use this water for automatic irrigation and I don't have a sensor for saying that the water isn't running.

I decided a better way might be to have a number of sensors to monitor the build up of silt in the bottom of the butt so I started thinking about building a short length of pipe with maybe 4 slotted opto-switches on it so that and potting the whole lot. Then, I could put that length of pipe down to the bottom of the butt and as the silt rises, so it will cover each of the opto-switches in turn. I could use this to work out what level the silt was.

I've got a device which I've played with before that is a datalogger with a 10V range and a single analogue input. It also has the capability to uplink data (I've not done much with this right now) but I was trying to work out if there was a way to encode the output from the individual opto-switches (probably using some appropriately sized resistors) that would mean I could inspect voltage and decode which of the switches were covered. In my mind, if each resistor connected to the output of each opto-switch was half the resistance of the previous, I should be able to get a voltage that I can decode back into which on is on or off.... but I am struggling!

Any help or guidance or better suggestions about how to take multiple opto-sensor output and encode onto one channel would be much appreciated!

Many thanks in advance!

Neil

What opto sensors have you got and what voltages are available?

Mike.

And - what is the output voltage range you want to produce?

Voltage when there is zero silt
Voltage when there is some silt
Voltage when there is some more silt
Voltage when there is enough silt to block the output completely

ak

The opto devices I have are TT Electronics OPB800W51, and the input range in my ADC is 10V, it's a 10bit device so not massively high, but should be good enough I think...

Thank you for the replies so quickly!

Neil

A couple of ideas. Use a catch basin to catch some of the solids. The fine stuf (like 50 um) it won't catch.

A long sock filter could be used. Make water enter the sock and leave the top open which will act as an overflow.
My homemade laundry filter works like that, but the particle size was larger.

I tried a 50 um filter on storm water to filter clay silt and the bad clogged too easily. I definitely would need a bypass.
It's designed. but not implemented.

What supply voltage do you have?

You can connect all the input diodes in series with one series resistor to limit the current.
At the output you could use five resistors in series (say 50k ohm) as shown below (using a 12V supply).
The output will then have 5 distinct voltages, depending upon the number of optos that are on, with the maximum (all optos off) being 4/5 V1, and the minimum (all optos on) being about 2V (due to the on-resistance of the optos).

circuitmayhem said:

The problem is, I use this water for automatic irrigation and I don't have a sensor for saying that the water isn't running.

Click to expand...

Just a point for discussion...why custom build a silt detector if a readily available flow sensor will do?

DrG said:

Just a point for discussion...why custom build a silt detector if a readily available flow sensor will do?

Click to expand...

My thought exactly, a flow sensor on the output can actually measure the water passing through it, and easily detect when something goes wrong - either silt, or just an empty butt.

I'm somewhat dubious about inserting slot-sensors in water?, never mind how effective they might - it's also fairly likely they get blocked up with the silt, and require regular cleaning. Historically such sensors were used in VCR's (for detecting reel rotation and speed), and in what is a pretty clean environment still required cleaning and replacement.

Thank you all for the replies!

I have tried filtration before and although effective, as you have mentioned, it needs quite frequent emptying / cleaning. Some of the run off comes from the edge of field too which of course has quite a lot of mud!

I can indeed use a flow sensor to measure when the water isn't flowing.... but at that point, I have to deal with the problem rather than buying myself time. Having a series of sensors that I can use to monitor the increasing level of the silt means I have a reasonable indication of when I need to get in there and clear it out *before* it stops flowing! This is the appropriate time for me to clean the sensors too so hopefully they won't get too clogged up and stop working! Let's see!

@crutschow that is exactly the sort of circuit that I was thinking of, but I wasn't smart enough to do! Thank you very much for your help! I will try this with some of those opto-switches and let you know how I get on!

Thank you all again for your help!

Neil

Have never used one, but this looks kind of interesting at only a few bucks.

Or, if you happen to have US 5K burning a hole in your pocket, this one.

DrG said:

Have never used one, but this looks kind of interesting at only a few bucks.

Click to expand...

Now that's pretty cool

Here's the revised circuit to correct a typo, and also changed the resistor values to make the voltage steps relatively equal, as shown in the simulation graph:

Thank you Crutschow!
That's really helpful. Sorry for my late reply, I've had a busy few days!
I'm hoping to be able to get a few bits together and test this over the next couple of weeks! I'll report back!
Many thanks again
Neil