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Why won't it work? (sump pump project)

  1. Mar 30, 2016 #1
    So I'm a college student in electromechanical engineering technology and my lab partner and I are making a sump pump circuit for our class project. I got the circuit soldered and the alarm and light work but the relay won't turn on. We discovered that the resistor that goes into our transistor which from there it goes up into the diode in parallel with the relay and into an SCR for our alarm and light, the resistor stated gets 0 volts, the resistor leading into the SCR gets 12 volts and the diode for the relay gets .9 volts. The two of us cannot figure out why the first resistor stated gets no voltage yet other parts of the circuit do but not enough voltage goes up to the diode
  2. jcsd
  3. Mar 30, 2016 #2


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    Staff: Mentor

    Well, it's going to be kind of hard to help you without seeing your schematic and pictures of how you have hooked it all up. Can you use the UPLOAD button to the lower right to upload your documentation? Thanks.
  4. Mar 30, 2016 #3
    Yes, sorry bout that. forgot about the picture. I couldn't get a good picture of the circuit I actually had soldered that would've helped so just the schematic

    Attached Files:

  5. Mar 30, 2016 #4
    I'm not clear what you are trying to do with this circuit.

    However what you describe seems to be how it should work. Diodes D1 and D2 are safety diodes and shouldn't have current except to drain transient currents. The SCR is being used oddly, but lacking a way for the current to drain through the cathode, it's going to flow through the gate. Transistor Q1 is biased to prevent current from ever flowing (assuming the sump electrodes are + Vccish and groundish).

    Perhaps you could list the inputs and outputs you expect from the circuit?
  6. Mar 30, 2016 #5
    basically current is flowing down the electrode on the left and when the water in the container the electrodes will be in rises and reaches the electrodes current will flow into the electrode on the right which will then turn on the alarm and the LED and it's suppose to close the relay contacts and allow the pump to turn on and yes we have a secondary power source for the pump and I've tested it and got it working on breadboard before soldering it. The problem is the relay doesn't get enough power for the contact to close.
  7. Mar 31, 2016 #6
    Current forms loops. It does not flow down one isolated wire.

    Also, allowing current to flow through water is a very bad idea. It is dangerous and unreliable.

    You need a transducer to detect the water level (and an extra as a backup as well). Use the transducer current to trigger digital switches (transistors, SCRs) which in turn run any needed relays.
  8. Mar 31, 2016 #7
    I know it forms loops and I'm not adding anything into this circuit I have proven it to work the way it is on the schematic. Yes I know current flow in water is dangerous but the water is contained in a plastic water tight container that we're using for a simulated basement. The pump is suppose to turn on when water is touching the electrodes no certain level just any amount so nothing to detect the water level is needed
  9. Mar 31, 2016 #8


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    Science Advisor

    Clean water does not conduct electricity.
  10. Mar 31, 2016 #9
    If it works fine, then our job here is done.

    Of course it doesn't work or you wouldn't have posted. But at this point anything I add will only add to the danger. So good luck.
  11. Mar 31, 2016 #10
    Test it by putting a short across "electronics to sump" that'll test whether the problem is your method of sensing the water level.
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