buffering low variable ac source as input

**nomad** · - 1st December 2007, 19:50

well i ditched that idea as counting low freq pulses is far less accurate than timing the pulse. I went similar to your idea, actually using a 339 comparator. While the data sheet stated the differential input voltage can exceed supply voltage with no damage, it failed to state that over supply voltage it's output is VERY erratic. it worked well up to 240 hz then went wacko. i used two led's in parallel to the input as an ac clamp. the max output of the sensor doesnt even light the led at full brightness. this solved the higher freq but introduced erratic output when there was no input. i then used two 10k pullups to stop that, and it almost work, it's good at a stop or over 20 hz, but still flakey at low input voltages. ( less than .5v ) so i'm going to remove all that and supply the comparator with 13.8v. With the 339's open collector output i think this may work out. my backup is use opto's instead of leds to signal between stop or ok to read. who needs to read below 3mph anyways. lol thanks Mackrackit.