Confusion on 7404 inverter inputs, in reference to "floating ground"

Discussion in 'The Projects Forum' started by prb22786, Dec 4, 2016.

  1. prb22786

    Thread Starter Member

    Sep 19, 2010
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    Hello, I'm working on implementing a circuit design I found on Electronics Hub. The schematic is attached. The circuit theory is that the two probes are placed in soil, which when dry will have high resistance and cause a large voltage drop. The output from the inverter will trigger the 555, which in turn will trigger the secondary 555 configured as astable monovibrator. The final pulse drives a PNP transistor which closes the relay connected to an electronic water pump. When the soil is wet, the probes conduct, and the circuit should open the relay.

    My confusion is that the schematic I am working off of does not have the 7404 IC connected to ground, pin 7 is connected to one of the probes. This gives a floating voltage on all of the gates, and the circuit does not work as intended. Now if I touch the probe to ground (in reference to Vcc), the cycle triggers and the relay closes for a few seconds, then opens. So the circuit appears to be working, I just can't wrap my head around this grounding. It seems to me that the IC needs to have some sort of reference. Am I on the right track here, or missing something?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You would be best to ignore that circuit since it violates the design specifications of a 7404 IC.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Try connecting the 7404's pin 7 and the right probe to common (ground) and see if that works.
    Otherwise you will need a different circuit to replace the 7404.
     
  4. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    Your 555 seems to be missing ground, (pin 1) too.

    It's customary to leave out the power rail pins on a circuit diagram, but sometimes they are in the notes somewhere.
    e.g. +12: U1(3); U30(5) etc.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    It has been common on schematics since the dawn of the IC era to omit Vcc and GND connections on parts with multiple functions per package. It was a gentlemans agreement with the PCB layout folks that they would take care of it. When the technicians would make wire-wrap boards they also would take care of Vcc and GND. You cannot begin to imagine the confusion and messiness of hand drawn schematics with that additional detail on a 400 IC equivalent CPU design.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    It is true that it is common to omit Vcc and GND in logic schematics.

    This diagram indicates that there are two additional connections to the 7404 gate which are labelled as pin-14 and pin-7, which are clearly Vcc and GND pins.

    Also, the circuit is powered from a 9V battery, exceeding the max 7404 supply voltage Vcc of 5.25V, absolute max Vcc of 7V.

    [​IMG]
     
  7. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Although it uses a 6V relay.
     
  8. hp1729

    Well-Known Member

    Nov 23, 2015
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    All very correct comments from the group. Yes, you can use ICs outside their specs and still have them work. They just will not be expected to work under rated conditions. Yes, the ORCAD 555 model shows VCC but GND is not shown. No problem.
    Yes, I would suggest a 74C04 instead and ground pin 7 as well as use it for the second probe. Also a 7555 or other CMOS 555 variation would be better for a battery powered application.
    PNP with no base resistor? Questionable.
    Relay? A big battery drain.
    I agree I would not likely give this design a passing grade.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Agreed. I would replace NE555 with LMC555, TLC555 or ICM7555 CMOS timer circuits for lower power consumption and less supply noise.
    The only time you need TTL NE555 or LM555 is if you need the 200mA output drive.

    I would also replace the 7404 input stage with a simple FET such as MPF102 which provides high impedance input.

    Output PNP transistor is missing a current limiting resistor on the base.
    I would replace the PNP transistor with an NPN or logic level N-MOSFET, after reversing the logic of the output.

    Like I said before, I would scrap the entire circuit and start from scratch.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 945 pulsing sprinkler.PNG
    Is there some reason you wanted to use the 7404? How about something like this?

    When the soil dries out Q1 is turned on, triggering the first 7555 for about 30 seconds. When it triggers the Discharge pin (open collector) is released allowing the second 7555 to oscillate at about a 3 second rate for that 30 seconds.
     
  11. prb22786

    Thread Starter Member

    Sep 19, 2010
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    Thank you all very much for the tips. I'm going to take a step back and re-evaluate how I want to go about this. This is one of my holiday gift ideas for my wife, so I still have a few weeks to figure it out luckily!
     
  12. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You describe what you think the circuit does, but it's no so clea what you wan to do.

    Do you just want to close the relay to turn on a pump when the soil is dry? What pump? What power?
     
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