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### Author Topic: transistor schmitt trigger question  (Read 1805 times)

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#### steveo

• Newbie
• Posts: 2
##### 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

• Hero Member
• Posts: 1849
##### 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

• Newbie
• Posts: 2
##### Re: transistor schmitt trigger question
« Reply #2 on: July 01, 2015, 07:23:48 AM »