adjustable schmitt trigger with positive outputs

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danjel

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I want to make a schmitt trigger circuit that will be fed voltages in the range of 0-10V but have adjustable threshold (initial voltage trigger) and hysteris. The output should be swing between 0V and a positive value (anything greater than 5v is good). The key is that this output should be noniverter and never go negative.

What is the least amount of parts to achieve this (but using separate pots for the threshold and hysteresis adjustments)
 
Add a couple of nand gates

Hi. I did a simple schematic a time ago since I had the same question.

On this particular drawing, the trigger levels is set to 4 and 5 volts.
 

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wow that is a pretty cool idea! So I would just use two pots to set the voltages feeding the two voltage ref points. So if the voltage ref on the top OP Amp is lower than the bottom one will the output be inverted?
 
They can be comparators with Hysteresis added in. Give this link a read and note the similarity of the Window comparator. You will see a strong similarity to Grossel's circuit. Read the portion about Hysteresis also. The supply would be single ended. The Vref for each comparator in the window comparator can be individually set using a few pots. The outputs can be modified to drive 1/2 of a quad NOR gate like a 4001 using Grousel's configuration.

Ron
 
Is there a problem if the pot setting Vref goes to 0V or +12V?

i.e. do I need to put a series resistor to make sure there is some bounds to the range?
 
danjel said:
Is there a problem if the pot setting Vref goes to 0V or +12V?

i.e. do I need to put a series resistor to make sure there is some bounds to the range?
Click to expand...

If the pots were for example 10 K you could place a 1 K resistor above and below each pot. With a 12 volt VCC that would give each ref pot a range of 1 to 11 volts. Depends on what you want the window to be. The upper ref also needs to be greater than the lower ref.

Ron
 
So what about the attached circuit?

I chained the two pots that created the ref voltage so the top one is always higher than the bottom and spaced them with 1K so the voltages never go to the rail values or overlap.
Since these are 339 I added pullups to the outputs.


I added 1M positive feedback resistors to add more hysteresis to each comparator.
In the past I have had issues lm393/339 false triggering due to noise (from pcb layout issues) and I want to make sure these are pretty solid.
 

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I would say not use the feedback resistor because I think the circuit R29 because you make some vey odd input impedance to the circuit.

Personally I wouldn't bother using any feedback resistor because once the SR flipflop sets active, it stay there, no matter what voltage you get from pin1 on your opamp.
 
Do you want to use one pot for trip level and a different pot to set the amount of hysterisis (+-Δ from trip point), or do you want one pot the set the positive going trip point, and a different pot to set the negative going trip point?

What is the widest delta between the trip points?
 
danjel said:
Is there a problem if the pot setting Vref goes to 0V or +12V?

i.e. do I need to put a series resistor to make sure there is some bounds to the range?
Click to expand...

If you decide to use a LM393, or similar comparator, then take a look at page 5 of the data sheet that Reloadron linked to, in post#5.

Note: 6 will give you the limits for the common mode range for 'Vref'.
Never closer that 1.5V to the postive supply rail, and NEVER negative by more than 0.3V (at least, not without a current limiting input protection resistor!)

With a 12V supply rail, Vref higher than 10.5V can give unpredicable output results.

Letting an input go negative will probably destroy the device.

There may be similar limitations with other comparator ICs. Worth checking out, although, as you can see, the details can be buryed down in the small print!!
 

I played around with a simulation of this in Multisim and I think the best solution was to just have a pot for each of the ref points (one for hi and one for lo).

I connected the second pot so that its source voltage comes from the wiper of the pot that sets the hi level. This way it can never be greater or equal to the Hi point (no overlap).

I used series resistors to make sure the hi pot could never go to within 1.5V of the rails and the low pot has resistor for offset to. The signal being fed to the comparator will be rectified so there is no negative voltage.

Attached is the new configuration.
 

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danjel said:
That is exactly what I want to have happen (variable hysteresis).
Click to expand...

That's not hysteresis, it is change in the size of the window. As drawn, your circuit will trip within a window based on the two reference inputs to the comparator. There is no hysteresis.

Ron
 
You should look at a 555. It has two comparators and a set-reset flip-flop. Sound familiar?

The trip points are hardwired to 1/3 and 2/3 of Vdd, but there are tricks to change that.
 
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