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sensing circuit w/ out comparator

Discussion in 'Electronic Basics' started by SklettTheNewb, Apr 7, 2005.

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  1. I have created a simple light sensing circuit using 2 voltage dividers
    and a comparator. Basically, high resistance and the output from
    comparator is high.

    I need to explore other options due to the fact the comparator needs
    more power than I have available. I would like to make this circuit
    with 1.5 volts and I've found a couple LED flash circuits that will
    allow me to handle that end w/ 1.5v
    (http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#15flash.gif)

    but the comparator.... I dunno.

    I thought I could use a voltage divider with a transistor, but when I
    sat down to actually make this circuit, I realized I couldn't figure it
    out.

    I've researched op-amps as a comparator but that doesn't seem like a
    good fit either.

    I would be interested in any ideas that you guys might have.

    Sorry to post so many questions here, I've been really active on this
    project lately and I'm learning a lot here so it's hard to not come
    back ;)

    -Steve
     
  2. Bob Monsen

    Bob Monsen Guest

    You can use a simple two-transistor oscillator to pump up the voltage
    sufficiently to light up a white LED from a 1.5V source. Using another NPN
    transistor to kill off oscillations. Here is a try:

    1.5V
    .----------------o--------------------.
    | | o-----.
    | | | |
    | ' | |
    | .-. 100u C| |
    .-. | |1k C| -
    | | | | C| ^ ->
    | | Variable '-' 10k 330p | |
    '-' Resistor | | |
    | | ___ || | |
    | .---------o---|___|--o---||----o-----'
    | | | | || |
    | | o----------)-. |
    | ' | | | |
    | |/ \| | | |/
    '----| |--------' '-----|
    |> <| |>
    | | |
    | | |
    | | |
    '---------o--------------------'
    GND

    NPN = 2n4401
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    Your variable resistor may not be right for this. As the resistance
    increases above about 470k, The thing will start oscillating, causing the
    LED to fire up. If you want it to go when there is light, you'll need to
    use a slightly different scheme. As it gets lighter, your LDR probably
    has less resistance. Thus, replace 'variable resistor' with a resistor
    that approximately equals the resistance of the LDR at the point you want
    it to turn on at. Then, put the LDR from the base of the leftmost
    transistor to ground. When it gets lighter, it'll turn of the leftmost
    transistor, allowing the oscillator to go.
     
  3. Thanks Bob! I will try this out tonight, looks interesting.
    The only component I don't recognize is the one in parallel to the LED
    that says 100u, what is that?

    THanks!
     
  4. Chris

    Chris Guest

    Hi, Steve. Sorry about the delay. Let's recap the bidding. You've
    got a sensor that exhibits a variable resistance between 2K ohms and
    64K ohms. When the sensor resistance is greater than 4K, you want an
    LED to flash about once a second. The catch here is that you've got a
    fresh Duracell AAA battery, and need to be able to have this thing
    flash for 45 days with that battery as its only power source.

    We talked before about your power requirements, and how a standard
    oscillator won't do the job because of excess current which will eat up
    the battery. The National Semiconductor LM3909 was made exactly for
    flashing a standard LED off a single 1.5V cell with very low power
    drain. Unfortunately, they're not making it anymore. However, there
    are some parts still available if you look or scrounge, and there's
    always the possibility of building one out of discrete components, as
    linked in an earlier post.

    National published an app note on the LM3909, called AN-154: "1.3V IC
    Flasher, Oscillator, Trigger or Alarm"

    http://www.fulcrum.ru/Read/CDROMs/NS/AN-154.pdf

    On the second and third pages of the app note, it describes the circuit
    operation for the basic 1.5V flasher circuit shown in Fig. 2. From
    looking at their description and examining the feedback paths, it seems
    that the flasher will not oscillate if only we can keep pin 2 (the +
    end of the 300uF cap) from exceeding 1V. That shouldn't be too hard,
    if we can find a comparator to do the rest of the job. Here's the
    circuit (view in fixed font or M$ Notepad):


    ~~
    ~~
    .--------o-----|<------. .---------.
    | | RED LED | | |
    | | | | |
    | | | | |
    | | | | |
    | .--o------o------o------o--. |
    | | 8 7 6 5 | |
    | | | |
    | | | +|
    | | | ---
    | | LM3909 | -
    | | | 1.5V|
    | | | |
    | | | |
    | | 1 2 3 4 | |
    | '--o------o------o------o--' |
    | | | | |
    | | 330 | | |
    | | uF | | |
    | | ||+ | | |
    '--------o--||--o '---------'
    || |
    |
    Mystery Signal |
    From Control Ckt |
    |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    The only question left is figuring out a way to turn the LM3909 (or the
    discrete equivalent) on and off depending on the sensor resistance.
    National Semiconductor came to your rescue again with the LM10 (which
    is still in production).

    http://cache.national.com/ds/LM/LM10.pdf

    It's a low power combination op amp and 200mV reference that will work
    well on power supplies down to 1.1V. Life is good. Here's the mystery
    control circuit:

    ___
    .----|___|---o----------------------------|<------->
    | 330K | .-----------------. 1N4001 To
    | | | | Flasher
    | | | |
    | .--------o------o------------. |
    | | |6 |8 7| |
    | |OpAmp.--|------|-----o------o----|--------.
    | | | | | | | | |
    | |2 |\| | | |\| Ref | | |
    .--|---o---|-\ | '---|-\ Amp 1| |200mV +|
    | | | | >-' LM10 | >-----o----o ---
    | o---o---|+/ .---|+/ | | -
    | | |3 |/| +| |/| | | |1.5V
    | | | | --- | | | |
    | | | | 200mV - | | | |
    | | | | Ref | | | | |
    | | | '---------o-----o------o----|--------'
    | | | 4| o----.
    | | '----------------------------' | |
    | | .-. |
    | | 10K| | |
    | | | | |
    | | '-' |20K
    | | | .-.
    | '-------------------------------------o | |<---.
    | | | | |
    | .-. '-' |
    | Sensor| | | |
    | 2K to 64K| | | |
    | '-' | |
    | | | |
    | GND GND |
    '---------------------------------------------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    And here's how it works. The 200mV reference part of the LM10 provides
    a reference voltage for a voltage divider between the 10K resistor and
    your sensor. When your sensor goes above 4K, the VDVR goes above about
    57mV. That's the value you've set with the 20K pot. Both of these
    voltages are compared by the op amp section of the LM10. The output of
    the LM10 goes high when that happens, which means it isn't interfering
    with the charging of the 300uF cap, and the flasher does its thing.
    When the sensor gets below 4K, the output of the op amp goes low. With
    the series diode drop, it will keep the voltage at pin 2 of the LM3909
    at around 0.8V, which will prevent oscillation/flashing. The 330K
    resistor is added as positive feedback at the non-inverting input to
    provide hysteresis, which prevents the in-betweens" from messing things
    up.

    I guess you're about set here. Even though both ICs are "micropower"
    (at least by early 80s standards), you're going to be cutting it a
    little close with the 45 day part on one AAA battery. I suppose it
    could be tweaked a bit, but I think it will probably be good enough.
    As long as you use a Duracell alkaline, you should make it.

    Good luck with your project, Steve.
    Chris
     
  5. I'm speechless, that is the most awesome experience I've had on the web
    w/ maps.google.com following in a close second! :)

    Thanks a million, Chris, I will go off this weekend and get everything
    I need to prototype this guy.

    I'm thinking I will make the LM3909 w/ discreet parts on a separate
    breadboard, then make the sensing on another. That way I can get each
    one working separatly and then tie them together for the grand finale!

    Thanks again, I really appreciate the time you spent on this.

    Have a great weekend,
    Steve
     
  6. Bob Monsen

    Bob Monsen Guest

    It is an inductor, 100uH.

    Regards,
    Bob Monsen
     
  7. Chris

    Chris Guest

    Slight error -- you need a series resistor with the diode to avoid
    overcurrent at the LM10 output:

    ___ ___
    .----|___|---o-|___|----------------------|<------->
    | 330K | 100 .-----------------. 1N4001 To
    | | | | Flasher
    | | | |
    | .--------o------o------------. |
    | | |6 |8 7| |
    | |OpAmp.--|------|-----o------o----|--------.
    | | | | | | | | |
    | |2 |\| | | |\| Ref | | |
    .--|---o---|-\ | '---|-\ Amp 1| |200mV +|
    | | | | >-' LM10 | >-----o----o ---
    | o---o---|+/ .---|+/ | | -
    | | |3 |/| +| |/| | | |1.5V
    | | | | --- | | | |
    | | | | 200mV - | | | |
    | | | | Ref | | | | |
    | | | '---------o-----o------o----|--------'
    | | | 4| o----.
    | | '----------------------------' | |
    | | .-. |
    | | 10K| | |
    | | | | |
    | | '-' |20K
    | | | .-.
    | '-------------------------------------o | |<---.
    | | | | |
    | .-. '-' |
    | Sensor| | | |
    | 2K to 64K| | | |
    | '-' | |
    | | | |
    | GND GND |
    '---------------------------------------------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Good luck
    Chris
     
  8. I just noticed this schematic when looking for other information:

    http://circuitos.tripod.cl/schem/r15.gif

    It looks as though it may be interesting to your application, if I
    understand it to be a "blinking LED" that also "senses light" and
    works off of a 1.5V AAA battery.

    Jon
     
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