Schmitt Trigger Question

Discussion in 'The Projects Forum' started by Dammi, Feb 22, 2011.

  1. Dammi

    Thread Starter New Member

    Feb 15, 2011
    Hi guys.

    I have a problem with an IR sensing photo-diode. I have to amplify the signal using a schmitt trigger. I think the output from the photo-diode will not be a strong one, around 0.02V (I have yet to measure it, in the meantime I just want to know how the Schmitt trigger works)

    On this site I've found a calculator to help me with the resistance:

    Low Threshold: 0V
    High Thershold: 0.02
    Output Votlage, Low: 0V
    Output Voltage, High: 3V (My microcontroller will interpert this as 1)

    But my question is, what happens to Vcc in these calculations? Does it matter what Vcc I use or does it only depend on the Op Amp I'll use?

    Thanks in advance :)
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    The supply voltage Vcc will affect the high output voltage. A rail-to-rail output op-amp would require +3V Vcc to deliver +3V high output, other types would require somewhat more: see the op-amp data sheet .

    Note however that the very small threshold voltages you require are likely to lead to a small input resistance for your Schmitt, probably smaller than the output load resistance of your photo-diode. This may prevent correct operation: to overcome this, why not use an op-amp to amplify the photodiode signal to at least a few hundred millivolts. The Schmitt will then be working with better levels from a low impedance.

    One final point. Your upper threshold for the Schmitt should be somewhat less than its maximum input level, and the lower threshold must be somewhat more than its minimum input. If you try to set the thresholds to exactly the maximum and minimum levels, the circuit is unlikely to work reliably - can you see why?
    Dammi likes this.
  3. Dammi

    Thread Starter New Member

    Feb 15, 2011
    Yeah, the signal could land beteween the thresholds and there for be undetermined :)

    Thanks for the reply, it helped alot !