5 v TTL pulse counter

Discussion in 'The Projects Forum' started by needhelp2010, Mar 12, 2010.

  1. needhelp2010

    Thread Starter New Member

    Mar 12, 2010
    I have pulses with 700mv pp, and need to input to an 5v TTL pulse counter. Do I require to have a pre-amp? need help
  2. SgtWookie


    Jul 17, 2007
    It would be best to use a comparator, something like this:


    C1 blocks the DC level of the input signal, coupling through the AC portion.
    R1 and R2 set the average input voltage level to roughly 1/2 Vcc, which is 5v. R3 provides a high impedance coupling to this average input voltage level.

    R4 through R6 set the comparator reference level to the same as the R1/R2 divider. When you are first building the circuit, you need to remove R7, apply power and adjust R4 until the difference between the two comparator inputs is as close to zero as possible.

    R7 provides hysteresis feedback of about 50m. Without this hysteresis, the comparator might oscillate.

    R8 sources current to the output of the comparator. An LM339 has open-collector outputs; they can sink current, but not source it.

    Note that you MUST ground the unused INPUTS of the other three comparators.

    If this is for an automotive application, you should use an LM2903 instead, which is a dual comparator that has better temperature ratings.

    You could also use an LM393 dual comparator for non-automotive applications.

    Note that you should use an 0.1uF cap across the comparator's power and ground terminals.