Please Help - LED Temperature Gauge using Bi or Tri-Color LED

Discussion in 'The Projects Forum' started by camaroz1985, Sep 2, 2010.

  1. camaroz1985

    Thread Starter New Member

    Sep 2, 2010
    I am looking to create a water temperature indicator for my motorcycle. I have a bi-color LED voltmeter now (red and green, and combined to make yellow), which has given me the idea. It is a 3mm LED, so I would like to make something similar for water temperature.

    Basically I want to tap into the stock temperature sending unit, which varies resistance with temperature. I would like to make it one color below the normal operating temperature range (yellow or blue), another while in the normal operating range (green), and another when above the normal operating range (red), and possibly flashing if overheating (again red).

    I would like to use a tri-color (RBG) led for this. I haven't found any tri-color in 3mm size though, so I might have to use 5mm or use a 3mm bi-color (red/green).

    Now for some constraints. It will need to be waterproofed, and an circuits should be able to be packaged inside a small project box (2"x2"x1" max, preferably smaller). It will run off of the motorcycle's 12V system, and should be a through hole LED that I will mount through the dash of the motorcycle.

    I realize I still have some things to figure out, but hope this is enough to get a concept (at least on paper) started.

    Any help would be greatly appreciated, and thank you in advance.
  2. camaroz1985

    Thread Starter New Member

    Sep 2, 2010
    The information I have found so far is:

    The sensor resistance is as follows.

    -4 DegF - 18.8KOhm
    32 DegF - 6.544KOhm
    104 DegF - 1.136KOhm
    212 DegF - 0.1553KOhm

    I want my display to be Blue for temperatures below 180 DegF, green between 180 and 240 DegF, and red over 240 DegF.

    Can I use an LM3914 tied to a 3 color LED where instead of having 10 LEDs I have 1-4 tied to the Blue, 5-8 tied to Green, and 9-10 tied to Red? (Haven't worked out the exact divisions, but I'm just trying to figure out if what I'm thinking is at all possible.)

    Thanks again.
  3. SgtWookie


    Jul 17, 2007
    How about a bi-color (red/green) LED that changed slowly from green (when cold) to yellow-orange (when warm) to red (when hot)?

    Somewhere recently I ran across an idea (I think it was from Dallas/Maxim) on using PWM to drive an H-bridge to vary the color emitted from an inexpensive Green/Red LED in 32 steps; but using analog ICs (like 555 timers, etc) the transitions would be very smooth.

    Need more details about the resistance of the sensor in the desired range. For example, what does it measure at 200°F and 240°F? You're showing a very non-linear response curve at the moment.
  4. camaroz1985

    Thread Starter New Member

    Sep 2, 2010
    That would work, but I would rather have definitive steps, as the actual value isn't as important as knowing it is working in a certain range. It is much easier to look down and see it is definitely red, green, or blue, and the change from one to the other is more likely to catch your eye.

    The values are from the manufacturer's service manual.I will have to take the sensor out of the bike to take some measurements as to actual values, but I would assume it is non-linear (probably exponential).

    Interpolating the known values my best guess for the switch points is as follows.

    180 DegF - .272kOhm
    240 DegF - .073kOhm