'Project' Temperature Circuit Help

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

  1. kbyrne

    Thread Starter Active Member

    Dec 10, 2011
    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


    Dec 23, 2011
    Let's start with:

    What is this circuit?
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    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


    Sep 9, 2010
    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 Active Member

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