Hello
I'm after a bit of general advice on using CMOS for pulse generation.
I'm detecting a water meter magnet with a Hall device that has active low output (no magnet, output high)
This feeds into an XBee Series 2 on a battery which normally sleeps until the Hall wakes it up.
The meter can run fast or slow or the magnet can sit in front of the Hall for months at a time, so what I'm after is something to generate a pulse of known (ish) length, say 500-mS, to wake the XBee then let it go back to sleep.
At the moment, I'm using a 74HC14 inverting Schmitt with a capacitor in series with the input and a pullup resistor to Vcc - values of 1-meg and 0.1uF work well on the falling edge. Slow input pulses are not a problem.
The problem is that for very fast pulses, the Schmitt output pulse is also reduced in duration.
Is there something better than the 74HC14 that will give a fixed width pulse regardless of the input width? Realistically, the meter pulse width won't go much less than 50-mS, but I need the output longer to get the XBee up and running and warm up another sensor.
If I wasn't strapped for time, a small MCU would do the job, but I haven't set an AtTiny or similar for sleeping before and I don't have the time to do it right now.
Hope this explains it, no circuit drawing I'm afraid, but the Schmitt edge detector circuit is quite well known.
Thanks
I'm after a bit of general advice on using CMOS for pulse generation.
I'm detecting a water meter magnet with a Hall device that has active low output (no magnet, output high)
This feeds into an XBee Series 2 on a battery which normally sleeps until the Hall wakes it up.
The meter can run fast or slow or the magnet can sit in front of the Hall for months at a time, so what I'm after is something to generate a pulse of known (ish) length, say 500-mS, to wake the XBee then let it go back to sleep.
At the moment, I'm using a 74HC14 inverting Schmitt with a capacitor in series with the input and a pullup resistor to Vcc - values of 1-meg and 0.1uF work well on the falling edge. Slow input pulses are not a problem.
The problem is that for very fast pulses, the Schmitt output pulse is also reduced in duration.
Is there something better than the 74HC14 that will give a fixed width pulse regardless of the input width? Realistically, the meter pulse width won't go much less than 50-mS, but I need the output longer to get the XBee up and running and warm up another sensor.
If I wasn't strapped for time, a small MCU would do the job, but I haven't set an AtTiny or similar for sleeping before and I don't have the time to do it right now.
Hope this explains it, no circuit drawing I'm afraid, but the Schmitt edge detector circuit is quite well known.
Thanks