Specialized Schmitt Trigger.

Discussion in 'The Projects Forum' started by MikeML, Nov 29, 2014.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
    I am interfacing a radio receiver to an Arduino. There is a node in the receiver squelch circuit that switches when the receiver detects a RF carrier. No-signal voltage is 7.77V, and it switches (rather slowly) to 4.44V when a RF signal is received. It is a high-Z node, so I cant load it without buffering, likely not more than a few uA.

    I want to convert this to a logic level for an Arduino input port pin, using a Schmitt Trigger level-shifter with about 100mV of hysteresis. The trip points should be ~ 7.0V and 6.9V. The voltage levels at the Arduino pin should be near 0V and near 5V, not over 5.3V.

    Arduino is powered with 5.0V. The radio is powered with 9.0V, so either or both of these supplies are available to power the Schmitt Trigger.

    I have some ideas, but I'd like some suggestions from the forum for a stable circuit with few parts ...
    Last edited: Nov 29, 2014
  2. crutschow


    Mar 14, 2008
    Does your Arduino have a high impedance A/D input? If so perhaps you could just use a high resistance voltage divider to reduce the voltage from the receiver, and then detect the voltage level and do the hysteresis in software, for a minimum of external hardware implementation..
  3. bertus


    Apr 5, 2008

    The following circuit might work for you:


    You can adapt the level with the 100K and the hysteresis with the 820K.


    PS I have attached the datasheets for you
  4. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
    Zapper: Unfortunately, the ADC is heavily committed to another use, so I would prefer not to have switch the processor's analog multiplexor back and forth to two different analog channels...

    Bertus: That is pretty much what I was thinking about. The open-collector output from the comparator being pulled up to 5V is what makes a clean interface to the Arduino pin. Thanks for that...