Looking to build a single supply OP Amp one shot

Discussion in 'The Projects Forum' started by bonanzaman, Oct 15, 2010.

  1. bonanzaman

    bonanzaman Thread Starter New Member

    Joined:
    Oct 15, 2010
    Messages:
    6
    Hi all,

    I'm trying to build a single power supply op amp based one shot.

    I'm interfacing to a accelerometer which has a 10mv/g ac wave riding on a 12 dc bias.
    I've used a single op amp from a quad package op amp to amplify the signal and now I would like to use that to light an LED.

    Since the accelerometer requires 27 volts, I'm using three 9v batteries for power for both the accel and the op amp.

    The pulse I'm getting is too narrow to see easily on the LED so I'd like to trigger a pulse from the one shot and use that to keep the LED lit longer.

    The input will be a small ac pulse. i would like to extend the pulse to about a second.

    Any suggestions?

    Thanks

    Dave
  2. wayneh

    wayneh AAC Fanatic!

    Joined:
    Sep 9, 2010
    Messages:
    8,121
    Location:
    Roscoe, IL
    Couldn't you just use a peak detector to rectify the pulse and charge a cap, which would discharge over a second and hold the op-amp "on" during that time?
  3. campeck

    campeck Member

    Joined:
    Sep 5, 2009
    Messages:
    194
    555 monostable
  4. bonanzaman

    bonanzaman Thread Starter New Member

    Joined:
    Oct 15, 2010
    Messages:
    6
    I thought about using a 555 but would like to keep it down to a single IC and I'm currently not using any voltage regulation for the op amp. The package i'm using is a quad op amp.
    Could the 555 trigger from the 10mv signal generated by the accel and then I wouldn't need the op amp?

    Wayne, I'm not sure about using a peak detector. Not sure what it involves. Could you add a little detail?


    Thanks

    Dave
  5. wayneh

    wayneh AAC Fanatic!

    Joined:
    Sep 9, 2010
    Messages:
    8,121
    Location:
    Roscoe, IL
    A brief spike is passed across a diode to the top of an RC tank, a resistor and capacitor in parallel between the op-amp input and ground. The spike takes the voltage high , the diode prevents the voltage dropping, and the cap holds it there, depending on the choice of R and C, with the product of R-times-C being the time constant. For example, 100K times 10µF = 1 second. Output from the op-amp then depends on the voltage on the other pin.
  6. Kermit2

    Kermit2 Well-Known Member

    Joined:
    Feb 5, 2010
    Messages:
    2,497
  7. bonanzaman

    bonanzaman Thread Starter New Member

    Joined:
    Oct 15, 2010
    Messages:
    6
    Hi Wayne,
    What I guessing is that I still use the initial op amp for amplification then put the peak detector at the input of the next amp.

    Sorry If I seem a little slow, but I haven't done a thing with op amps for over 30 years let alone single supply ones.
    Need to put my thinking cap on and fasten it down.

    Dave
  8. bonanzaman

    bonanzaman Thread Starter New Member

    Joined:
    Oct 15, 2010
    Messages:
    6
    Hi Wayne and group,

    I tried to use the detector and no joy. I got the capacitor to charge some, but when I connected it up to the input to the next amp, it appeared to load it down.
    Looks like I may need to use the 555 to generate the pulse.

    Any other suggestions?

    Thanks

    Dave
  9. Audioguru

    Audioguru New Member

    Joined:
    Dec 20, 2007
    Messages:
    9,411
    Location:
    Ontario, Canada
    Post the schematic you used for a peak detector with its power supply voltages and the part number of the opamp you used.
    Any opamp works perfectly from a single supply if its input is biased properly.
  10. gootee

    gootee Active Member

    Joined:
    Apr 24, 2007
    Messages:
    443
    Location:
    Indiana, USA
    Capacitor is too large (and/or resistor too small), probably.

    But it depends on the pulse. What is the peak voltage of the pulse? How long does it last?

    You might also need to condition the input pulse for the detector, i.e. you might need to remove the DC, and might need to amplify it to be above the diode's threshold (or use an "ideal diode" opamp circuit that rectifies down to zero volts), and might even need to attenuate it with a voltage divider. It all depends on the pulse's size and shape.
  11. bonanzaman

    bonanzaman Thread Starter New Member

    Joined:
    Oct 15, 2010
    Messages:
    6
    LM148
    power is three 9v batteries for 27 volts.

    Opamp 1 appears to work ad expected. I get a large signal at the output (10V+). I put the cap at the output to remove the dc and the diode to rectify the signal for the charging of the RC circuit.

    I can get the circuit by itself to charge up to 1-200 mv or more but when I hook up the second stage it seems to have a great effect on the charging.
    I've played with both the cap and resister.

    I've played around with the resistance values on #2 as to not load the rc circuit.

    The pulses from the first stage around 300 us and the amplitude is to the top rail or 12 volts peak with lesser peaks after initial stimulus.

    Attached Files:

  12. Kermit2

    Kermit2 Well-Known Member

    Joined:
    Feb 5, 2010
    Messages:
    2,497
    try a different arrangement of your 1st opamp output to the second opamp input. Pull out the series cap. or try the diode as a parallel to ground component instead of series. Something is preventing transfer of charge and the second op amp is supplying the problem current by keeping its inputs balanced.

    Also the wave shape may need to be cleaned up as suggested earlier. A schmitt trigger buffer might be called for to clean up the 'pulse' you are looking to amplify.
  13. Audioguru

    Audioguru New Member

    Joined:
    Dec 20, 2007
    Messages:
    9,411
    Location:
    Ontario, Canada
    I removed many useless resistors and removed the coupling capacitor that feeds (and is charged by) the rectifier.

    I don't know why the second opamp had a 20k attenuator at its input that cut the signal in half then had two resistors as feedback to amplify the reduced signal back up to the the original signal level.

    Attached Files:

    Last edited: Oct 20, 2010
  14. gootee

    gootee Active Member

    Joined:
    Apr 24, 2007
    Messages:
    443
    Location:
    Indiana, USA
    Using Vcc to derive the reference, to "subtract out" the 12 volts so you can amplify only the tiny pulse will probably cause difficulty.

    It's a lot more robust if you create a low-pass filtered version of the sensor output and take the difference between that and the original sensor output. That way the absolute DC levels won't matter. Otherwise, tiny differences in the DC levels of the inputs will swamp the output with DC.

    My circuit is probably a lot more complicated than it needs to be. But the most important parts are the difference amplifier (U1 and R2 through R4) and the low-pass filter for the sensor (R5, C7, and the U5 buffer). Note that the V3 voltage source is your accelerometer output voltage.

    D2 and C3 plus R5, R12, and R9 make up the detector. And I added an adjustable output voltage clamp (U2, D1, and R7, R8) and an output buffer, U3.

    You might want to vary some of the component values, to better-suit your signal levels and desired LED on-time. Almost any opamp that can handle the supply voltage should work.

    The LTspice circuit simulation file is at http://www.fullnet.com/~tomg/accel.asc.txt . After downloading it, remove the ".txt" from the file name.

    [​IMG]

    http://www.fullnet.com/~tomg/accel.jpg

    Edit: My ISP must be having some sort of problems. My link and picture aren't working.
    Last edited: Oct 23, 2010
Similar Threads
Forum Title Date
The Projects Forum newb looking to build automotive led driver Jan 22, 2014
The Projects Forum New to electronics, looking for help building the best temp sensor possible Dec 14, 2013
The Projects Forum Looking for someone who knows WindowBuilder Oct 20, 2013
The Projects Forum Looking for Advice on Building Test Bed for Experimenting and Learning Feb 28, 2013
The Projects Forum Looking for an advice to build an isolation transformer. Sep 14, 2012

Share This Page