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Water detector with NE555

Discussion in 'Electronics Homework Help' started by INJAC, Feb 1, 2017.

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  1. INJAC

    INJAC

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    Feb 1, 2017
    Could someone help me with simple (if possible) modification of this circuit that i found online, as a water detector (between A and B is the water sensor) . I would like to add another LED that is lit up when A and B are not connected (when its dry), and its turned off when they are connected (when water is detected). Not very experienced so any help would be more then welcome. Thank you
    .[​IMG]
     
  2. Alec_t

    Alec_t

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    Welcome to EP.
    A LED in series with a 1.2k resistor, connected between pin 3 and pin 8 of the 555, would be one option.
    Assuming the usual situation is no water, then the added LED will draw ~5mA nearly continuously. If your 9V supply is a PP3 battery it won't last long. Is that ok?
     
  3. INJAC

    INJAC

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    Feb 1, 2017
    Thank you for the reply and for the welcome.
    I tried adding it as you said, it is lit when there is no water (that is good), but also when water is detected, the buzzer starts beeping, every time it stops, the LED lights up again, then when it starts beeping again it lights off and so on. Is there maybe a solution that when there is no water its lit up, and when there is, as the buzzer beeps it doesn't light up at all?

    Thank you.
     
  4. Alec_t

    Alec_t

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    Try this add-on. Success depends on the added transistor Q1 having a high gain, since R8 needs to have a high value to ensure the 555 can be kept in a reset state when there is no water. If the LED doesn't light you may have to replace Q1 with a Darlington transistor or a P-MOSFET.
    LED_addon.PNG
     
    Last edited: Feb 1, 2017
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  5. INJAC

    INJAC

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    Feb 1, 2017
    It worked perfectly, had a bit of trouble putting it all on a small breadboard but still managed to do it, thank you very much for you help and good guidance :)
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Since Alec is not a novice at this I must assume I'm missing something. Alec why couldn't he just connect the LED cathode to the Output pin and the limiting resistor to Vcc? After all the output pin sinks low when off.

    Chris
     
  7. AnalogKid

    AnalogKid

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    It also sinks low approx 50% of the time when the 555 is oscillating, as in post #3.

    ak
     
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  8. Alec_t

    Alec_t

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    Glad it's now doing what you want.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    That's why I prefaced that statement with "I must be missing something". As it turns out, I was! I didn't notice that the 555 was being used as an astable osc. I thought the 555 was being used for a specific on time (output high) after it's triggered. I saw the "Buzzer" on the schematic and ASSumed it was a self contained (DC operated) piezo osc. .... Sorry Alec.

    Chris
     
  10. Alec_t

    Alec_t

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    No probs, Chris. It must be a self-contained buzzer, since it's being pulsed at only about 1Hz.
     
  11. INJAC

    INJAC

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    Feb 1, 2017
    Sry for asking, but the other part of circuit with the 555 timer i understand, this par where u added the pnp, and those resistor. Can explain it a bit in some details about the currents and voltages, why does it exactly turn off the LED, when 555 is activated. Its for a university project and my knowledge isnt that so great, to know exactly what happends. Thank you.
     
  12. Alec_t

    Alec_t

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    In that case I'll give a hint rather than spell things out in full :).
    Consider R7,R8,R5 as a potential divider. Calculate/estimate the voltage at the base of Q1 (in my add-on), when Q1 (in your circuit) switches on/off. Sorry about the duplication of transistor designations.
     
  13. INJAC

    INJAC

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    Feb 1, 2017
    As far as my knowledge goes at the moment, these are my calculations for Q2 (in the 555 IC circuit) on/off stage.
    [​IMG]

    [​IMG]

    For Q1 and Q2 i didn't have the exact same transistors, i had BC557C and BC547B tried then and all the schematic worked with them. Now i'm not sure if i got the right resault for Vo2, and also if i took good hfe and Vbe from BC547B datasheet i browsed. Any suggestion on how to calculate all of these values would be a great help. I'm sorry if i ask some basic questions, hope i don't take too much of your time. And thank you very much :)
     
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  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  15. Alec_t

    Alec_t

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    Your maths looks about right (I haven't checked it fully). From the Vo2 values you can see why Q1 turns off when Q2 turns on.
    For this purpose you don't need to worry about actual hfe values (which can vary quite widely from 'typical' values quoted in datasheets). Assuming a notional 100 would be good enough. Likewise, 0.6V is a good notional Vbe.

    Edit: For 'notional' read 'nominal' :).
     
    Last edited: Feb 5, 2017
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  17. Alec_t

    Alec_t

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    Yes. Brain phart.:D
     
  18. TCSC47

    TCSC47

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    If this is an educational project, you may be able to get extra marks by mentioning electrolytic erosion if you have time. Look it up. When you pass a current from one electrode to another, the metal on one of the electrodes will be eaten away and deposited on the other. To avoid this in an industrial application, ac would be used to monitor the conductivity of the water, instead of dc as in your circuit.
     
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  19. INJAC

    INJAC

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    Feb 1, 2017
    Thank you all for help im almost done with my project, especially zou Alec_t :)
     
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It's fortunate that the electrode current is going to be <90uA otherwise you would be remiss if dismissing this post.

    Chris
     
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