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Vref > Vin on comparator

Discussion in 'Electronic Basics' started by Manimozhi Baskaran, Feb 10, 2006.

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  1. Doing the ciruit suggested below, I was able to set 0.8 volts on the Vref. Currently my output is about 4.8 volts when the Vin > Vref. How can I get the HIGH output when Vref > Vin instead?



    +----------+---+5V
    | |
    [100k] |
    | |
    +---------|+\
    | | >--[<1N914]...
    Vin>--|---------|-/
    | |
    [50k]<-+ |
    | | |
    +----+-----+---GND

    which will get you your 0.8V point with the pot at adjusted to
    about 19k ohms
     
  2. You already have HIGH when VRef > Vin. If you want the inverse VRef < Vin,
    then swap + and - pins of the circuit. I hope this helps.

    Bill
     
  3. John Fields

    John Fields Guest

    ---
    You're doing something wrong.

    With the signal on the - input more positive than the signal on the
    + input, the output should be close to GND.

    Conversely, with the + input more positive than the - input, the
    output should be close to +5V.

    BTW, if you want to see the circuit working, simulate it.

    Go to:

    http://www.linear.com/designtools/index.jsp

    Download their wonderful free simulator and run this:

    Version 4
    SHEET 1 880 680
    WIRE -416 384 -416 224
    WIRE -416 544 -416 464
    WIRE -304 384 -304 -32
    WIRE -304 544 -416 544
    WIRE -304 544 -304 464
    WIRE -144 -32 -304 -32
    WIRE -144 0 -144 -32
    WIRE -144 256 -144 80
    WIRE -144 384 -144 256
    WIRE -144 544 -304 544
    WIRE -144 544 -144 464
    WIRE -80 224 -416 224
    WIRE -80 256 -144 256
    WIRE -48 -32 -144 -32
    WIRE -48 208 -48 -32
    WIRE -48 544 -144 544
    WIRE -48 544 -48 272
    WIRE 32 240 -16 240
    WIRE 112 240 96 240
    WIRE 112 240 112 224
    WIRE 112 352 112 240
    WIRE 112 416 112 352
    WIRE 112 544 -48 544
    WIRE 112 544 112 480
    WIRE 240 -32 -48 -32
    WIRE 240 0 240 -32
    WIRE 240 128 240 80
    WIRE 240 256 240 128
    WIRE 240 400 240 256
    WIRE 240 544 112 544
    WIRE 240 544 240 480
    WIRE 288 128 240 128
    WIRE 288 352 112 352
    WIRE 400 224 112 224
    WIRE 400 256 240 256
    WIRE 432 -32 240 -32
    WIRE 432 208 432 -32
    WIRE 432 544 240 544
    WIRE 432 544 432 272
    WIRE 528 128 368 128
    WIRE 528 240 464 240
    WIRE 528 240 528 128
    WIRE 528 352 368 352
    WIRE 528 352 528 240
    WIRE 528 416 528 352
    WIRE 528 544 432 544
    WIRE 528 544 528 496
    WIRE 528 608 528 544
    FLAG 528 608 0
    SYMBOL res -160 -16 R0
    SYMATTR InstName R1
    SYMATTR Value 100k
    SYMBOL res -160 368 R0
    SYMATTR InstName R2
    SYMATTR Value 19k
    SYMBOL voltage -304 368 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 5
    SYMBOL voltage -416 368 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    WINDOW 3 24 104 Invisible 0
    SYMATTR InstName V2
    SYMATTR Value PULSE(+5 0 0 1 0 1 1 1)
    SYMBOL res 224 -16 R0
    WINDOW 0 -52 37 Left 0
    WINDOW 3 -48 67 Left 0
    SYMATTR InstName R3
    SYMATTR Value 100k
    SYMBOL res 224 384 R0
    WINDOW 0 58 40 Left 0
    WINDOW 3 64 69 Left 0
    SYMATTR InstName R4
    SYMATTR Value 100k
    SYMBOL diode 96 224 R90
    WINDOW 0 -51 32 VBottom 0
    WINDOW 3 -41 34 VTop 0
    SYMATTR InstName D1
    SYMATTR Value 1N914
    SYMBOL res 384 112 R90
    WINDOW 0 -43 57 VBottom 0
    WINDOW 3 -32 60 VTop 0
    SYMATTR InstName R5
    SYMATTR Value 22k
    SYMBOL res 384 336 R90
    WINDOW 0 -36 61 VBottom 0
    WINDOW 3 -32 67 VTop 0
    SYMATTR InstName R6
    SYMATTR Value 100k
    SYMBOL cap 96 416 R0
    SYMATTR InstName C1
    SYMATTR Value .2e-6
    SYMBOL res 512 400 R0
    SYMATTR InstName R7
    SYMATTR Value 10k
    SYMBOL Opamps\\LT1013 -48 176 R0
    SYMATTR InstName U1
    SYMBOL Opamps\\LT1013 432 176 R0
    SYMATTR InstName U2
    TEXT -450 554 Left 0 !.tran 1


    I've substituted LT1013's for the LM324's, but that shouldn't make
    any real difference in whether the thing runs or not.
     
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