'Project' Temperature Circuit Help

Discussion in 'The Projects Forum' started by kbyrne, Jan 21, 2012.

  1. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    I have a few questions on a LM35 and LM3914N temperature circuit that I need assistance finishing. Circuit is powered by 7VDC. It has 3mm LED"S and 20 of them. How do I figure out the adjustment part of the circuit?
    1 Trim R-3 for 3.074V @ Leg # 8 IC. #2
    2 Trim R-2 for 1.055V @ Leg # 4 IC. #1
    3 Trim R-1 for Pin #5 both IC'S 0.075V +or- 100mv/ degrees celcius
    @ T ambient.
    I am new to electronics and cannot get a different voltage reading with adjustments to R2 or R3. Could the reason be Carbon film resistors @ 5% instead of 1% resistors. Can that degree of change on schematic cause that problem or where else do I look. Voltage to both IC'S check out and LED'S light in a row about half. Thankyou.:eek:
    LM35 TO-92 Left side of gif then R1, R2, And finally R3.
     
  2. PaulEE

    Member

    Dec 23, 2011
    423
    32
    Let's start with:

    What is this circuit?
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    It's a slight variation of the Bar Graph Temperature Display schematic shown in Figure 15 (page 9) in the LM35 datasheet. http://www.ti.com/lit/ds/symlink/lm35.pdf
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    Are you saying that the voltage stays fixed regardless of the position of the variable resistor? What are you actually observing? A cheap pot might make adjustment difficult, but should certainly change the voltage.

    A photo of your build would be helpful also. Indicate where you are measuring your voltages.
     
    kbyrne likes this.
  5. kbyrne

    Thread Starter Member

    Dec 10, 2011
    84
    2
    Will go thru measurements and post shortly Thankyou.
     
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