2nd Attempt at a liquid level sensor

Discussion in 'The Projects Forum' started by scollinguk, Mar 3, 2011.

  1. scollinguk

    Thread Starter Member

    Oct 9, 2010
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    Hi All,

    I now finally have got a new liquid level sensor after the last one I was trying to use was DOA.

    This one works but doesnt seem to turn off when the sensor enters the liquid.

    I have taken pics of the circuit and the design it should follow in the instructions, can you check if it is right for me please?

    Thanks!!

    [​IMG]

    [​IMG]
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Are you trying to use that thing to control a pump directly? It says that the maximum current sink is 40mA - how much does the pump draw? (My guess would be that the pump draws very much more than that.)
     
    Last edited: Mar 3, 2011
  3. scollinguk

    Thread Starter Member

    Oct 9, 2010
    34
    0
    Hi Adjuster,

    Thanks for the reply.

    I havent a clue about the load for the pump, its an ebay special.

    The pumps run fine, completely A Okay, the problem is that the sensor should stop outputting when wet and this doesnt happen.

    Cheers!
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It seems very unlikely that any pump would use as little as 40mA. A few hundred mA to a few A would be more likely. It is possible that the overload from the motor has caused the sensor to burn out and short-circuit, so that it is now permanently on.

    Perhaps you could measure the motor current? While you are at it, you might want to check the polarity of the supply input to your sensor, unless you already have done.
     
  5. debe

    Well-Known Member

    Sep 21, 2010
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    The secret is probably the bottom circuit thats not shown using a relay to switch the motor on/off. I would think the motor has destroyed the sensor.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    That sensor=fried
     
  7. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    You could use it to drive a small reed relay. But as noted it's probably chicken fried part.
     
  8. scollinguk

    Thread Starter Member

    Oct 9, 2010
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    Bugger, oh well, we live and learn!
     
  9. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Your wee motor under load probably draws several hundred mA. The relay output in your above diagram (the part cut off) is probably what you want.
     
  10. Nate_Bro

    New Member

    Mar 2, 2011
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    may I ask what you are using this pump/sensor for? I'm looking at doing the same thing for my project.
     
  11. scollinguk

    Thread Starter Member

    Oct 9, 2010
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    Its for an auto topup system on a fishtank, I have been using two float switches but find them unreliable and get stuck closed a lot.

    Could I put the sensor (a new one anyway!) running with a relay powered by a 12v battery? How long would it last before needed to change the battery?
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
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    How long is a piece of string? Seriously though, it would depend on the currents drawn by the sensor, the relay coil, and the motor, assuming the latter was running from the same battery. You really have to know these currents in order to choose the right battery, or to get a safe rating for a mains adapter, if you chose to do that.

    Since the motor and relay coil will use more current than the sensor, the percentage of time that the pump would be operating would also be pretty key to the battery life.

    Get all that information together, and decide on what Amp-Hour capacity the battery would be, and you could estimate how long it would last. On the whole though, wouldn't this be better run from a mains adaptor?
     
  13. Nate_Bro

    New Member

    Mar 2, 2011
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    thats nice, I'm here also for my aquarium....

    I was making my own ATO system, and just broke down and got a Reef keeper Lite for now (paid $30 for two floats), until I get these circuits figured out to do my own, good luck!

    also what is the name/brand of your pump, I would love to put one of those on my system.

    also do you need a battery, or can you use a 12v power supply for your ATO?
     
  14. scollinguk

    Thread Starter Member

    Oct 9, 2010
    34
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    Hi Adjuster,

    I was more thinking of using a battery to run the sensor and the relay coil circuit and then using the existing adaptor to run the motor the other side of the relay.

    I would expect that as the sensor and the relay would only need a very small current that the battery would last a while, what would your thoughts be?

    Nate_Bro, the motor was an ebay special, just have a search for peristaltic pumps and similar ones should come up. I have had it running with float switches but have found them unreliable.
     
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You really do need to find out now much current the sensor draws. Guesses are not much use, but let's suppose it takes 10mA. That's what this sensor would consume at 12V - it looks a bit like the one you have.

    http://www.fluidswitch.com/pages/OpticalLiquidLevelSensorOS-900.htm

    If you used NiMh rechargeable AA cells for your battery, its capacity could be about 2500mAH - so each charge might give 2500mAH/10mA = 250H or just over 10 days. This ignores the extra 40mA or so for the relay, on the basis that it will only be on for a small fraction of the time. It might be different if the motor also worked from the battery, as its current is bigger.

    Why do you plan to use a battery in the first place - do you suspect that the mains supply output may not be stable enough? If it is 12V unregulated, that could be a valid concern - the output voltage could become much bigger at a low current drain. Have you checked that?
     
  16. scollinguk

    Thread Starter Member

    Oct 9, 2010
    34
    0
    Hi There,

    I have seen on the sheet that came with the sensor that is does in fact draw 10mA at 12v.

    I have seen a relay at maplin which has a 12v 400ohm coil and a max voltage of 24vdc on the contacts.

    The adaptor I have outputs out 13.2v and 300mA.

    The thinking of the batteries is so that I dont have to run two adaptors for the circuit.

    Would there be a way to run both sides of the circuit from the adaptor without frying the sensor again?

    Cheers!
     
  17. Adjuster

    Well-Known Member

    Dec 26, 2010
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    That would depend on what kind of adaptor you have. If it is not regulated, the output voltage may become too high when the motor was not running. The variation of the power supply voltage with load may also be a problem if the same adaptor runs the motor.

    It would be a good idea to measure the adaptor output voltage off load before risking connecting it up. If the output is more than 12V off load, do not use it to run the sensor: you do not want to fry another one.

    I am also worried that you have only a 300mA rated supply. This sounds too small for comfort. The fact that it has already been running for a time does not prove that it is safe, and if the transformer overheated it could go on fire, or its output could become live at mains voltage. It is particularly unsafe to use overloaded electrical equipment in proximity to water.

    The pump might take more than 300mA, and the total load would be greater still if it was powering pump, sensor and relay. If you don't know the pump current you should measure it - preferably while pumping water. You would then need to use a power supply rated for at least that current, and 50mA to allow for the sensor and its relay, plus a bit of margin. If you don't have a multimeter (or are not confident about using one) you would be very well advised to get help from a competent person.
     
  18. scollinguk

    Thread Starter Member

    Oct 9, 2010
    34
    0
    Hi Adjuster,

    Firstly can I say thank you for you help so far, you sound like a professional and I would assume have a huge amount of expertise to call from and probably have had or do have a career involving something to do with circuits.

    The main thing here is that the 'project' is exactly that, a 'project' from the things I have lying around. If I was going to go out and buy the perfect things then I would simply buy a mainstream auto topup system.

    I want to use a sensor to improve the system I have been using for the past 12 months, that is really it.

    I am happy with the adaptor running the pump, it has been for long enough for me to be comfortable with it, as with all forums I am hoping that someone can give me some advise on how to achieve my aim and no one will be responsible for the outcome except me.

    Now, back to the problem.

    I want to run the sensor without having to use two power adaptors, as stated above, because I only have one at my disposal.

    Is there a way to use this one adaptor to power both the circuit on the coil side of relay without killing the sensor and also power the pump on the contact side the relay?

    I would imagine the coil side needs some sort of way of reducing the power going through it so that it doesnt kill the sensor, while the pump side needs to have as much as possible to drive the motor at the highest speed.

    Is this possible?

    Cheers, Scott
     
  19. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Still lots of unknowns; my aquarium air pump rated 2.5W @ 115V AC would approx 200 mA @ 12V DC- just at a start- so 300 mA supply might work for every thing. Using 12V sens. add simple 12 V zenor diode- 100 Ω resistor regulator. Assuming wet = HI, ref black - to blue output. Use a P ch FET or NPN transistor to drive pump.
     
  20. scollinguk

    Thread Starter Member

    Oct 9, 2010
    34
    0
    Thanks for that, really appreciate it!

    Just so I am getting what you have said correctly; split off the positive, put a zener diode then a 100Ω resistor then the rest of the sensor/relay circuit as normal. I am not sure I understand the FET for the pump?

    Would you be able to quickly put it as a circuit diagram in paint so I can understand what you mean?

    Cheers!!!
     
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