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crossing wave form

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kinarfi

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I have a wave form that crosses and I need to detect the crossing + a little on each side. Untitled.png
Tried a comparator, too narrow, even with hysteresis. Any suggestions out there, & I prefer simple discrete component type solutions.
Signal comes from two pots connected internally in a Wheatstone bridge configuration.
Thanks,
Jeff
 

MikeMl

Well-Known Member
Most Helpful Member
You just were not doing the comparitor right...

How much hysteresis do you want?
Should it be adjustable?
What supply voltages to do have?
Which comparitor chip were you using?
What are the resistor values in the bridges?
 
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MikeMl

Well-Known Member
Most Helpful Member
Here is something to get you started:

D9.png
 

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kinarfi

Well-Known Member
Thanks Mike, not quite what I need. Where my signals delay crossing due to hysteresis, I would like a pulse to 0 volts. The pulse is used to turn off a FET and thus a SCR.
Thanks,
JeffCapture.JPG
 

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MikeMl

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I dont understand what U8/U9 is or how it works?
What is V1?
Why are the flat spots on V(whiteP-1) and V(blackP-2)? Those were not part of your original spec?
Why do you have two comparitors?
Why do you need to have the weird AND gate made out of two nFETs?
Why do you ever use "uic" as part of a simulation?
 
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spec

Well-Known Member
Most Helpful Member
Hi kinarfi,

Below is a circuit that I knocked up for you to experiment with.

I tried to use all transistors as you requested, but it got a touch complicated.

The circuit has separate hysterisis and offset controls.

Input 2 needs to be driven for a low-impedance source (less than 50 Ohms say). Otherwise fit an opamp buffer.

spec


2016_12_15_Iss1_ETO_COMPARATOR_WITH_OS&HYSTERISIS_VER1.jpg
 
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MikeMl

Well-Known Member
Most Helpful Member
I think I may have figured it out:
D9a.png
HighZ circuit doesn't load the joystick, or whatever the thing inside the dashed box is...
Pulse width depends on amount of hysteresis.
If you want more hysteresis, make R1 bigger relative to R2.
 

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kinarfi

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I dont understand what U8/U9 is or how it works?
voltage controlled potentiometers, they simulate they simulate the Wheatstone arrangement of the 2 potentiomete sensor on my steering unit, it has 4 leads, red v+, green v-, black wiper, white wiper. On the right side the Quad comparator, the output from U8 and U9 are compared to sawtooth wave to give PWM
What is V1? voltage waveform for U8/U9
Why are the flat spots on V(whiteP-1) and V(blackP-2)? the flat spot the dead spots as you go from left turn to right turn with a pause at the crossing, not really needed, but it was intended to give me my needed pulse width at the crossover point, it doesn't.
Why do you have two comparitors? H-Bridge driver control
Why do you need to have the weird AND gate made out of two nFETs? this is what I need the pulse width for, when the outputs from the left side of the quad comparator are both high, at the crossing, both fets provide a path to ground to take another nfet's gate to ground and stop the current flow through an SCR and turn it off.
Why do you ever use "uic" as part of a simulation? because I'm impatient and I run the same sim several times with very minor changes, just to see what that change creates, for example, with a LM317 in the circuit, non uic sims can take a while and real often, the information I want shows up at the beginning of the plot.
This last project is 4 separate files added to a composite file, so if I'm trying to figure one thing out I don't add the parts not needed so sims are quicker.
Here is the complete file.
Thanks for asking questions, while trying to make sure you understand what I'm doing, I get a better understanding of what I'm doing and I think I found the solution, Hysteresis seems to do the trick, at least on paper, now to go try My new values.
Thanks,
Jeff
 

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kinarfi

Well-Known Member
I think we agree on this, I didn't read your post until I posted mine, Thanks
Jeff
 
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