Pull down resistor for coupling of water level sensor with 7408

Discussion in 'Digital Circuit Design' started by ali_786, Aug 30, 2016.

  1. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    Hi! everyone...I am designing an automatic water level controller using digital Ic's like 7408,7432,7404. I dip a wire +vcc(8V) in water and voltage come out from water at the inputs of Ic's are 4V approx. I have connected a pull down resistor of 1K with Ic pin but my Ic does not picking this voltage level. can you tell me how to determine the proper value of pull down resistor.???its urgent...........plzzzzzzzzzzzzzz
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    The conductivity of clean water is fairly low so you will probably need some circuitry between the water your TTL logic.
    Something like this:
    upload_2016-8-30_16-10-34.png
     
  3. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    thanks. can i use 2n3904 instead of 2n2222A
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    Yes, almost any NPN transistor will do.
    If your electrodes in the water are very small and the water is very clean, that circuit may not work and you will need a darlington transistor to get it to work.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,566
    2,379
    Try a high impedance input, 2N7000.
    Max.
     
  6. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    the circuit that you have attached provides output LOW when water sensor dips. but i need HIGH output when sensor dips. can i use this this circuit in common collector configuration.???
     
  7. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    You can use your 7404 to invert the signal at Q1 collector from low to high...
     
  8. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    thank u so much. i used this circuit in common collector configuration and its working well....thanks again
     
  9. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    Hi everyone! I want to drive a 5V circuit from sensor voltage 2V. plz help me to find the solution...thanks
     
  10. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
    You haven't provided enough information to even begin to help you find a solution.

    What kind of sensor is it? Can we see a spec sheet?

    What are you going to do with the 5 volts once you get it? Actuate a relay? Drive a motor? Provide an input signal to an Arduino input pin? Light an LED? How much current must the output source (or sink, or both)?

    Is the 2 volt input signal a digital signal? Or an analog signal? If digital, what are the logic levels? 0 volts and 2 volts?

    If analog, what is the input signal voltage range? 0 to 2 volts? Or some other voltage to 2 volts? How do you want the input range mapped to the output range? How accurate must it be?

    The people here need to know all these things if they are to help you.
     
    ali_786 likes this.
  11. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    thanks. Actually i want to drive 7404 hex inverter IC from sensor voltages (DC 2V).
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    Do you already have a 5V supply for the 7404?
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,067
    3,837
    What is the max amperage of the sensor output?

    Are you driving only from the sensor "output" or do you want to supply 2V power to the sensor and use that same 2V power supply to feed a chip that boosts voltage up to 5V?
     
  14. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    hi ali,
    Is this for the water level sensor probe voltage as discussed in your earlier thread.?
    E
     
  15. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    yup..i am directly driving the TTL from 2V
     
  16. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    yes you are absolutely right eric. i am talking about water level sensor probe. actually i have dip a 7 volt wire in water tank. but as water is a poor conductor so maximum voltages are being dropd across water and just 1.8~2V are coming from sensors. so i want to drive TTL to control the motor. but these 2V are not able to trigger the TTL
     
  17. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Do you have any reason to start another thread, but it has the same problem with the previous one?
    Pull down resistor for coupling of water level sensor with 7408

    If there is no any good reason then I will merge this thread to the previous one.

    I saw two of your threads, but I still can't get the info for the sensor, do you have any info or link for it?

    Why do you want to use the TTL(7404, 7408,7432) for this application, can you use other ic?

    Could you draw a block diagram and label all the function and voltage?
     
  18. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    hi ali,
    TTL technology is not the best choice for your sensor, consider using high impedance input logic, such as CMOS.
    A HEF4049 CMOS inverter would be a good choice.
    Use caution when handling, avoid risk of static discharge damage of the 4049.
    If you need guidance, please ask.

    E
     
    Last edited: Sep 4, 2016
  19. ali_786

    Thread Starter New Member

    Aug 5, 2016
    14
    0
    thanks eric....your suggestion is valuable but the problem is that i have made hardware of my project so now a large change in circuit is impossible so therefor i m trying to play with this circuit. plz see the attached file. thanks
     
  20. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    So what happened to "thank u so much. i used this circuit in common collector configuration and its working well....thanks again"?
     
Loading...