BJT Schmitt Trigger R values

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juz_ad

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I'm trying to reproduce this op-amp Schmitt Trigger with the standard 2x BJT design (but with a Hi/Lo swing between GND and +12V). The input is a triangle wave between 20Hz and 20KHz at approx. +/- 5-6V. The High VRef is +5V and the Low VRef is -5V.

I can set/adjust the Low VRef - but I can't seem to adjust the High VRef which doesn't want to shift off 0V.

Any help on R values or things I can try would be appreciated.

 

Why don't you try an input voltage divider respectively level shift by adding a DC voltage to the input?
 

FvM said:
Why don't you try an input voltage divider respectively level shift by adding a DC voltage to the input?
Click to expand...

Is (part of) the problem that the circuit I posted won't take a negative Low VRef?
 

I guess you want to make the output go into the negative?

Increase R22.
Reduce R18.

Eliminate R27. The bias to Q4 should only be via R26.

R18 value may need to be adjusted within a close range of values, for proper operation.
 

BradtheRad said:
I guess you want to make the output go into the negative?
Click to expand...

The output of the trigger is fine going between GND (Low) and 12V (High) - the problem I'm having is that it's going Low at an input of -5V (correct) but is going High at an input of approx. +1V - which is wrong.

That's the bit I need to sort - so it goes Low at an input -5V and High at an input of +5V.

Thanks.
 

FvM said:
Why don't you try an input voltage divider respectively level shift by adding a DC voltage to the input?
Click to expand...

Tried a few variations on this and ended up in the same spot - but with lesser results.

Tried some other variations - but still seem to be in the same loop. Can set one VRef but not the other.

I'd appreciate any further input on getting this running.

Ignore the lopsided Triangle wave - I broke that doing something else.

 

juz_ad said:
That's the bit I need to sort - so it goes Low at an input -5V and High at an input of +5V.

Thanks.
Click to expand...

You are asking for a very wide range of hysteresis. However this circuit can provide only a narrow range of hysteresis.

Even if you make the schmitt trigger from an op amp, you will have to do some careful adjusting of a potentiometer, in order to get several volts of hysteresis action.
 

FvM said:
Apparently you didn't yet try an input voltage divider.
Click to expand...

I did, with a DC offset as you suggested.

Tried again today without the DC offset and again the same problem - I can shift the lower threshold round but I can't move the high threshold without affecting the lower threshold.

I've also tried reproducing the values I've seen in similar circuits - still no luck.



What am I getting wrong?

Thanks.
 

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