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Thread: Using Autometer Electric senders to trigger warning lights

  1. #1
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    Default Using Autometer Electric senders to trigger warning lights

    I have both a electronic FP gauge and a pyrometer I would like to monitor. My though is I should be able to T a LED and SCR (perhaps a better idea for a gate?) to trigger a LED if one of the gauges fall below a certain level.

    i.e. fuel pressure, ideal level is 5-6 volts depending on boost if it drops below 4.8 I want to trigger a LED mounted next to the gauge to turn on.

    Any Ideas on how to accomplish this?

    The pyrometer would be a high side gauge so I think I can set this to trigger simply with an LED and a known threshold voltage level. its the below a level and turn on that I am having trouble with.

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    Just t off a resister with the voltage you want to light the light. how do you know your electric fp is accurate?
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    Quote Originally Posted by Kitz Kat View Post
    Just t off a resister with the voltage you want to light the light.
    The problem is, the signal is a positive voltage normally, low pressure is vower voltage. So using that method, the light would go off when the supply voltage fell below a certain threshold, I would prefer the opposite to happen.

    Quote Originally Posted by Kitz Kat View Post
    (snip) how do you know your electric fp is accurate?
    Gauge R&R with a known calibrated gauge for dyno measurements

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    Are you able to work with some microcontrolers? I think Atmel Atmega 8 is a great chip. It's cheap and has 4 D/A converters per unit.
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    a quad nand might do it but here is the problem i see (you can to using the truth table).

    Keep input 1 on nand 1 high with constant 5v
    wire parallel your fuel pres volage leg to input 2 of nand 1.

    with two high inputs, you should trigger an output which is not the desired effect so the below quad nand might not be what you need. At 40 cents a pop, these are exactly what the nand is called the universal gate. You can marrage up two chips to end up with the desired effect of any known gate available.

    If you want, i can post up the truth tables for all the gates. I think OR gates would be more appicable.

    the problem i have here is the reference voltage you are using. Any time the reference voltage input is different than the A/D input your want to monitor (weather its higher or lower) will cause the output to trigger.

    It can be done though with radio shack parts on the cheap.

    example,,
    if input 1 (ref leg) were kept at 5v and the circuit you want to monitor (wired to input 2 of nand 1) were to fall <>= the ref leg, you would have an output. again, it would take some thinking but maybe an OR gate would be best.
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    the more i think about this the more i feel a voltage divider or a simple transistor could be used to trigger the led.

    You might be able to wire up a common emitter NPN like the following.

    wire constant 5v to the collector
    wire your led off the emitter to ground

    the base would be fp input. Under normal conditions, the fp voltage would be higher than the collectors input voltage.
    as the fp volage dropped this would cause the tansistor to bias and allow current to flow from the collector to emitter and through the led to ground.

    just dont install the LED backwards or it wont light up...its a diode


    the problem i have with the above is that the transistor base input voltage might have to drop very significatly in order to forward bias the transistor.
    solution, - wire the collector up with 12v input. Leave the base wired to the parallel branch off the FP wire. With 12v up on the collector and the desired 5-6 volts on the base, that may be a perfect balance to keep the LED OFF. Decrease the input voltage to the base and determine what voltage will trigger the transistor to turn on the led.

    you may discover that you have to add a variable resistor on the 12v collector input to cut that voltage down just a little. (ie- calibrate it)
    A 5v led should be fine i think.
    A little brighter but could be the best if you have to adjust the collector input.

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    Quote Originally Posted by Trinom View Post
    Are you able to work with some microcontrolers? I think Atmel Atmega 8 is a great chip. It's cheap and has 4 D/A converters per unit.
    in his case, he needs ADIO

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    sorry for the mistake,, i made an error on the truth table.

    Now more than ever,a quad nand seems more applicable. With a constant ref V of 5v on input A, and with monitored circuit input B of approx 5v, you will have no light on the output Y

    If either input drops low, you will have a light on.

    if no inputs on either A or B, you will have a Y output.

    again, sorry for the mistake.
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    Last edited by jcassity; 12-24-2009 at 12:26 PM.

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    I would be concerned that using a simple quad nand might require you to step down the input voltage though.,,or some various creative calibration.

    using a gate is high impedence so its more safe than branch low resistive circuits. the addition of small calibration resistors ot the A/B inputs will be fine now. The reasons for calibrating each input is to keep the this operating within normal design specs. usually gates expect 1-3 v as a considered "HI" input. what you have to figure out is how to lower or close in that gap between your reference and monitored input so when the circuit you are monitoring drops below exactly X potential, you end up with an led triggered.

    I would place a small pot dialed before hand to max on the inputs then feed both pots with a 9v battery. I would then wire an LED on the output back to battery - (polarity sensitive). Next I would dial the pot until i had 2v on my ref A input. (the light should now be on) Next i would dial my monitor input B until the the light went out. (ie- two high inputs = no output or light out). Now when the volage level on B goes is < A, you have a Y output to the led triggered.



    I looked at various comparator circuits and there are and a 1458 dual op amp might be another simple option.

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    Quote Originally Posted by Kitz Kat View Post
    Just t off a resister with the voltage you want to light the light. how do you know your electric fp is accurate?
    not sure how sensitive the circuit is but A parallel branch may divide the current too much for the oem circuit being monitored to operate properly.

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