digital potentiometeroperational-amplifierschmitt-trigger
I am trying to build a Schmitt trigger that would allow adjusting High and Low threshold values with a microcontroller.
simulate this circuit – Schematic created using CircuitLab
I could replace R1-R2 with a DAC, but how do I adjust the feedback resistor R3? Is the trigger going to work properly if I replace R3 with a digital potentiometer?
Design will generate dual frequency signal, but due to asymmetric nature it will change duty cycle more and it would be hard to obtain high hi/lo frequency ratio (precise parts needed).
There are other ways.
simulate this circuit – Schematic created using CircuitLab 2. Use alternating oscillator topology.
Here on schematic second 'slave' oscillator work in two modes: when 'master' is high it is two invertor oscillator (NAND is working as second invertor); when 'master' is low NAND gate is blocked and is is acting as single schmitt oscillator.
Second schematic was discovered while playing with CD4093 quad NAND schmitt. Drawback of it that its frequency ratio hardcoded by schmitt thresholds.
Just plug those resistor values into the calculator you linked in your question and look what happens: -
The thresholds are switching between about 4.2 volts and 4.6 volts - that's the first thing - your input needs to be able to cross those boundaries for your circuit to switch.
Next, take a look at the LM358 data sheet and you'll see that input voltages are only valid between 0V and 3.5V (on a 5V supply) - don't expect this to do what you want when the input is above 3.5 volts.
Next take a look on the d-sheet to see what the output can do on a 5V rail, The d-sheet implies the highest output voltage that it can muster is typically 2V below the positive rail i.e. 3V.
My estimation is that your circuit is kind of working but maybe with the invalid 5V input in scenario 1, you get a weird inversion thing going on (it happens a lot on op-amps).
Welcome to the imperfect world of real op-amps!!
Try lowering thresholds to around 2V and 2.5V and see what happens when the input rises above 2.6V - I'd expect the output to swing down to 0V. When the input drops below 1.9V, I'd expect to see the output swing above 3V.
Best Answer
Completely replacing R1-R2 with a DAC may give you mixed results because the circuit saves a part by relying on the voltage divider's parallel impedance R1||R2. You'll need to ensure that the DAC has the same output impedance as seen from R3.
simulate this circuit – Schematic created using CircuitLab
That will adjust the threshold while keeping the hysteresis constant. To adjust the hysteresis, you can now replace R1||R2 and R3 with a digipot.