Connect with us

Comparator for sensor

Discussion in 'Electronic Basics' started by Manimozhi Baskaran, Jan 31, 2006.

Scroll to continue with content
  1. Hi,

    I am building an application that needs to beep when the voltage output
    from the sensor device is less than 0.8 Volts. I am not sure what would
    be the right design that I can use for the buzzer to beep. The voltage
    for the sensor is 5 V DC supply. Should I consider using 741/LM series?

    Regards,
    Mani
     
  2. The dual comparator, LM393 has inputs that work all the way to the
    negative supply rail, so it should detect .8 volts just fine. These
    comparators have an open collector output that turns on a connection
    to th negative rail when the - input is more positive than the + input.
    http://www.fairchildsemi.com/ds/LM/LM2903I.pdf
     
  3. John Fields

    John Fields Guest

    ---
    No. Consider using a real comparator, like John Popelish suggested.

    However, an LM393 may not be able to give you enough output current
    to run your beeper, so if you could post what its voltage and
    current requirements are we can suggest an appropriate means of
    driving it. Also, if you could tell us what you're using for a
    sensor we'll be able to suggest how to set up a proper "front end"
    and provide any hysteresis the circuit may require.
     
  4. My Infra Red sensor will detect the temperature range of -40 degree to
    +60 degree centigrade variation with:
    -40 degree to output a typical voltage of < 0.21 V
    -20 degree to output a typical voltage of 0.21V,
    -15 degree to a typical voltage of 0.3 V etc.,

    I like my sensor to detect only the temperature range -40 degree to -5
    degree range which would typically vary between 0. 2 V to 0.8 Volts. The
    typical supply voltage to the sensor will be 5V and its supply current
    will be 2 mA. The output of the sensor will be anywhere between 0.25
    Volts to 4.75 Volts. The output current will be less than 2mA. What type
    of circuit design should I use in order to detect the range so that it
    can beep when the voltage is less than 0.8 V which I believe is what I
    should use as my reference voltage.

    Any assistance will be appreciated.

    Thanks
    Mani
     
  5. John Fields

    John Fields Guest

    ---
    Please bottom-post.

    I'm confused.

    In your first post you said that you wanted the circuit to detect
    when the sensor's output voltage was less than 0.8V, but now you say
    you want the circuit to detect only the range between -40C to -5C,
    which corresponds to the voltage range of 0.2V to 0.8V.

    I take that to mean that if the sensor's output voltage is less than
    0.2V or greater than 0.8V you don't want the beeper to beep.

    You also say that the sensor's output will vary from 0.25V to 4.75V,
    which means that it will never be able to get to 0.2V.

    If, since you can't get to 0.2V, all you need is something which can
    detect an output voltage of less than 0.8V from the sensor, you
    could do something like this:


    +5V>---------+---------------+--------+----------+
    | | | |
    [1K2] | [10K] |
    | | | |
    | +---|--[1M]--+ |
    Rin | | | Rfb | |
    VIN>--[10K]--------------+--|+\ | E
    | | >------+--[1K]--B 2N4403
    +--[1690]---+--|-/ LM393 C
    |K | | |+
    [LM385-2.5] [806] | [BEEPER]
    | | | |
    GND>---------+-----------+---+-------------------+

    Rfb is used to provide positive feedback ("hysteresis", in this
    case) around the switching point, which will keep the output of the
    comparator from chattering if the input rise and fall times are
    slow. With the values given for Rin and Rfb, that hysteresis will
    correspond to about 1% of 0.8V, or 8mv, which should be enough if
    there's not a lot of noise on the input signal.

    If there is a lot of noise on the input signal, try bypassing VIN to
    GND with an "appropriately" sized capacitor.
     
  6. Jamie

    Jamie Guest

    at that voltage, a simple drive to the
    base of an NPN via a resistor would work to
    clamp down on a PNP base driven circuit.
     
  7. Jasen Betts

    Jasen Betts Guest

    741's won't run off a single sided 5V supply, but you could do it using
    1/2 a LM324 qaud op-amp to drive a piezo speaker.

    There's a dual version of the 324 too, but I forget it's number,
    and anyway, 324's are dirt cheap.

    possibly something like this:

    ---+-------+----------+----------- +5
    | | |
    | | 100K
    | | |
    | | +-----[22K]-------+
    560K | | 1/4 |
    | |\| | |\ 324 |
    (A)+-----|+\ +------|+\ |
    in | | >--. | | >-----+-+--||
    -------|-----|-/ `-|<--|---+--|-/ | | piezo speaker element
    | |/| 1N914 | | |/ | +|
    | | 100K +----[100K]-' |
    | 1/4| | | |
    110K 324| | === 0.2uF (B) |
    | | | | |
    -----+-------+----------+---+----------------+

    the voltage at point (A) controls when the tone will start
    it should come out pretty close to 0.8V but a trimpot (eg 50K) could be
    insertted here if some fine-adjustment is needed.

    the components near point (B) control the frequency increasing their value
    will reduuce the frequency with the componnents shown it shopuld be a few
    kilohertz which is about right for the plenty of noise from a piezo element.

    Bye.
    Jasen
     
  8. John Fields

    John Fields Guest

     
  9. John Fields

    John Fields Guest

    ---
    Using the 5% (carbon film?) resistors you've chosen for the
    reference divider will result in an uncertainty of the switching
    threshold with the low end being at 0.755V and the high end being at
    0.892V.

    Switching the 560K to 4220 +/- 1% and the 110K to 806 +/- 1% , both
    metal film, will tighten up the error band by bringing up the lower
    end to 0.788V, the high end to 0.815V, and lowering the tempco.

    In actuality, a small pot would be a better choice for tempco, since
    with a resistive element more likely to be isothermal than two
    resistors, the drifts would track (in the pot) and the ratio between
    the top and bottom resistors woulod stay the same, causing the
    reference voltage to remain the same as long as the supply stayed
    constant.
     
  10. Hi John,

    I believe that I missed your original msg on the thread. I am unable to
    determine which is the right comparator chip to use for my specifications. I started
    with 741 then look at LM393 suggested by John P. Also investigated on to LM324 (as Jasen had suggested). But I believe that I missed John F. idea on the thread? What would be the right option to go?

    Thanks,

     
  11. John Fields

    John Fields Guest

    ---
    The circuit you should use depends on how accurately you want to
    detect the -5C point (0.8V) and how much current your beeper needs.
    Can you supply that data?


    Here's a copy of my earlier post:


    "Please bottom-post.

    I'm confused.

    In your first post you said that you wanted the circuit to detect
    when the sensor's output voltage was less than 0.8V, but now you say
    you want the circuit to detect only the range between -40C to -5C,
    which corresponds to the voltage range of 0.2V to 0.8V.

    I take that to mean that if the sensor's output voltage is less than
    0.2V or greater than 0.8V you don't want the beeper to beep.

    You also say that the sensor's output will vary from 0.25V to 4.75V,
    which means that it will never be able to get to 0.2V.

    If, since you can't get to 0.2V, all you need is something which can
    detect an output voltage of less than 0.8V from the sensor, you
    could do something like this:


    +5V>---------+---------------+--------+----------+
    | | | |
    [1K2] | [10K] |
    | | | |
    | +---|--[1M]--+ |
    Rin | | | Rfb | |
    VIN>--[10K]--------------+--|+\ | E
    | | >------+--[1K]--B 2N4403
    +--[1690]---+--|-/ LM393 C
    |K | | |+
    [LM385-2.5] [806] | [BEEPER]
    | | | |
    GND>---------+-----------+---+-------------------+

    Rfb is used to provide positive feedback ("hysteresis", in this
    case) around the switching point, which will keep the output of the
    comparator from chattering if the input rise and fall times are
    slow. With the values given for Rin and Rfb, that hysteresis will
    correspond to about 1% of 0.8V, or 8mv, which should be enough if
    there's not a lot of noise on the input signal.

    If there is a lot of noise on the input signal, try bypassing VIN to
    GND with an "appropriately" sized capacitor."
     
  12. Thanks John for reposting this.

    Can you please clarify this?

    1690 - is this 1690 ohms across [LM385-2.5]? Is LM385 a diode to keep
    the noise/power more stable?
    806 - is that 806 ohms across the - ve and ground on LM393?

    Regards,
    Mani

     
  13. John Fields

    John Fields Guest

    ---
    I keep asking you to bottom post, as is the custom here, but you
    persist in top posting, and I keep asking for the accuracy you want
    when detecting the -5C point and the current required for your
    beeper to work but, for some reason, you refuse to supply that data.

    If you will reformulate your post with your questions at the bottom
    of the post you're replying to, and answer my questions, then I will
    answer yours.

    Otherwise, perhaps someone with more patience than I have at this
    point will do you the courtesy.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-