Help with LED indicator cicuits

Discussion in 'The Projects Forum' started by Sarge, Feb 9, 2011.

  1. Sarge

    Thread Starter New Member

    Dec 14, 2010
    Greetings all. I am designing a new PCB that is to be installed in place of others that have had catastrophic failures in the past. There are about 20 of these I have to do. They are part of the control circuits to old diesel powered generators.

    I have worked out the problems of the failures and have nearly completed the design

    As part of the new boards, I wanted to build into them a self check system. There are 5 circuits I wanted it to check, two of which I cannot figure out exactly how to do.

    The first system I need to check continuity. The circuit consists of a power source (12V DC) and a thermister. The value on the thermister can vary from 200 ohms to 27K ohms. All I need is an on board tactile switch, and a surface mounted LED. That is it. This thing is stumping me. I simply want to push a button on the PCB and have the LED light to tell me if there is continuity or not. With the variable resistance, I cant figure out exactly how to make it work.

    The second check I cant figure out is a voltage check. I don’t need a tactile switch here, but what it needs to do is go out and check a voltage source. On this circuit, there should be 10-15VDC, if it is, I would like for a surface mounted led to light up. If it is below (0-9VDC) I would like a different light to light up, or above (15-24VDC) a third light.

    The power for this circuit is a dirty 12VDC from batteries. The voltage I am checking can be as high as 24VDC.

    I have tried using a quad comparator for this, but my reference voltage is less than the upper limit of the acceptable level of the second voltage, so I cant figure out how to do that.

    I have also tried to do it with zeners, but I cant figure out how to keep the current out of the lower voltage zeners when the system faults to the higher voltages.

    I thought about an led driver, but that is a little over powered for what we need I think.

    I just ordered some shunt regulators, (LM431) to experiment with those.

    I have searched the site (and others) for anything that might point me in the right direction, but to no avail.
    I have also studied the heck out of Bill Marsden’s pages on here (and love them), but I just cant seem to get it.

    I am very flexible on how this can go, we can do it with only two LED’s or whatever system would show us that the voltage we are checking is within tolerance.

    The only restrictions on this project are board real estate and cost. They have to fit in a box on the unit, and it is already very crowded. I do have room for a 14 DIP and half dozen SMD LEDs or so, and the tactile switch, but that is about it. There are about 20 of these to do, and the investment thus far is huge, so cant add much more to the dollars.

    Power source - 12VDC automotive type batteries
    Leds - standard 20 ma SMD’s red, yellow, orange - 2v and green and blue - 3.2v.
    Resistance on thermistor - 200 ohms - 27K ohms
    Second voltage to check for fault 0-24VDC (10-15 is within tolerance)

    I am currently pursuing my AAS from CIE and I am liking it. But I am operating right at the edge of my skill level at the moment. :confused:

    Soon, I hope to be a contributing member and be useful around here.:)

    Thank you so much, let me know what you think.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    To cope with testing continuity over a range of resistance, apply a small test current (feeding via a large resistance from a fixed voltage would probably be sufficient) so as to develop a certain voltage with the highest acceptable circuit resistance. Use a comparator to detect whether this voltage is exceeded.

    The problem of scaling input voltages relative to a reference could most likely be solved by using resistive potential dividers. (You could scale the reference voltage down, if it happens to be too high) In my opinion, you should really have a clear understanding of basic concepts like voltage division before tackling this kind of project.

    On reflection.this may sound a bit harsh - perhaps there is some special difficulty in testing the voltages that I don't know about. Can you specify the limits on the voltages to be tested, as well as the reference value?
    Last edited: Feb 9, 2011
  3. wayneh


    Sep 9, 2010
    I agree. Everything the OP has described should be a snap with a quad comparator. You just need to establish reference voltages for each detection and then divide down the inputs as needed to compare meaningfully to those references. Use a transistor at each output of the comparator to drive the LEDs. (I wouldn't try to drive the LEDs directly with the comparator. It can work, but not well and driving more than one at a time is bound to cause problems.)

    BTW, the simple notion of dividing down the voltage under test, so that it's near the reference, is indeed simple. But the first time I needed to do it, I had the slap-on-the-forehead moment when that technique was suggested to me.