Hi all,
I'm attempting to condition an analogue pulse into a nice clean square pulse to trigger an edge-detecting microcontroller interrupt downstream. (see 20160403 cct.pdf)
The analogue signal, from a phototransistor, (label A) looks like a rounded square pulse with Vmax=4.8V and Vmin=0.2V (phototransistor pulse.pdf). The pulses are variable duration in the range 1-10 milliseconds long. The signal ramp rate is approximately 7 Volts per millisecond (ie. taking around 0.5 milliseconds to transition).
I noticed when hooking the collector signal directly to a 74LS02 input (labels A->B) that a significant number of additional edges were generated on the NORout output during rise and fall. These edges occasionally falsely trigger the downstream microcontroller interrupt, which is a problem.
So I instead hooked up the collector signal directly into a 74HC244 Schmitt Trigger (labels A->C), and was surprised to note the Schmitt's output BuffOut also showed additional high frequency oscillation edges for around 100 microseconds after the first edge (see 'Input and Schmitt output.pdf' with input on top, output below at a different scale). I had hoped for a single square pulse output. I also note that somehow the input signal is oscillating now as well.
My questions:
1 Shouldn't the 244's hysteresis prevent the oscillation and give a single output pulse ?
2 I found some online references to custom hysteresis calcs for using a handful of resistors around an LM393, but given I have a large number of phototransistor inputs to deal with, what solution would keep component count and cost low ?
Many thanks.
I'm attempting to condition an analogue pulse into a nice clean square pulse to trigger an edge-detecting microcontroller interrupt downstream. (see 20160403 cct.pdf)
The analogue signal, from a phototransistor, (label A) looks like a rounded square pulse with Vmax=4.8V and Vmin=0.2V (phototransistor pulse.pdf). The pulses are variable duration in the range 1-10 milliseconds long. The signal ramp rate is approximately 7 Volts per millisecond (ie. taking around 0.5 milliseconds to transition).
I noticed when hooking the collector signal directly to a 74LS02 input (labels A->B) that a significant number of additional edges were generated on the NORout output during rise and fall. These edges occasionally falsely trigger the downstream microcontroller interrupt, which is a problem.
So I instead hooked up the collector signal directly into a 74HC244 Schmitt Trigger (labels A->C), and was surprised to note the Schmitt's output BuffOut also showed additional high frequency oscillation edges for around 100 microseconds after the first edge (see 'Input and Schmitt output.pdf' with input on top, output below at a different scale). I had hoped for a single square pulse output. I also note that somehow the input signal is oscillating now as well.
My questions:
1 Shouldn't the 244's hysteresis prevent the oscillation and give a single output pulse ?
2 I found some online references to custom hysteresis calcs for using a handful of resistors around an LM393, but given I have a large number of phototransistor inputs to deal with, what solution would keep component count and cost low ?
Many thanks.
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