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Author Topic: transistor schmitt trigger question  (Read 1877 times)

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steveo

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transistor schmitt trigger question
« on: June 29, 2015, 12:18:08 PM »
Hi All, 2 questions:

1. in the "Transistor Schmitt Trigger Implementation" at:
http://pcbheaven.com/wikipages/The_Schmitt_Trigger/?p=1

it shows
VTHRESHOLD_HIGH = VSUP x R4 / (R1 + R2 + R4) - 0.62
VTHRESHOLD_LOW = VSUP x R4 / (R1 + R2 + R4 + R1 x R4 / R3) + 0.61

but in the calculator it links to:
http://pcbheaven.com/drcalculus/index.php?calc=st_tr

the Calculate() function has them switched.
   ThresLow=(Vs*R4/(R1+R2+R4))-0.61;
   ThresHigh=(Vs*R4/(R1+R2+R4+R1*R4/R3))+0.61;

Which one is right?


2. I'm also looking for a "self powered" schmitt trigger, i.e. vcc IS the input, with similarly spartan design.  But everything just scales with vcc in this version.  I'm looking to turn on a relay when a charging battery reaches a certain voltage and am wondering what the minimal circuit might look like.

Thanks,
Steve.

kam

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Re: transistor schmitt trigger question
« Reply #1 on: June 30, 2015, 16:20:47 PM »
1 if you're talking about the parenthesis, it is the same.
2 so, make this circuit but use a voltage regulator, say 5Volts using the 7805. power your circuit from the 5 volts. this way the circuit will not depend on the power supply as long as the suply is higher than 6.5 volts

steveo

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Re: transistor schmitt trigger question
« Reply #2 on: July 01, 2015, 07:23:48 AM »
Thanks for the reply!

re 1:  the issue is that to compute the high threshold in the page they don't factor in r3, but in the javascript calculator they do.  Vice versa for low threshold.  And I'm not smart enough to know with any certainty which one got it backwards.

re 2:,  thx, yah dome sort of voltage reference is helpful and common, yup.