Need help with interfacing Schmitt Trigger Circuit with uC

[arunbhaskar]

I am using dual supply op-amp as Schmitt Trigger circuit for my project. I need to count the number of pulses at the output of Trigger. Since AT89S51 is not dual compatible, I'm using a zener regulator as shown. What value of Zener voltage and series resistor would be the best for my circuit as shown.
THanks.

 

[IanP]

Re: Need help with interfacing Schmitt Trigger Circuit with

R can be between 4.7k and 10k, depending on the opamps supply voltages ..
For example, supply:
+/-12V, R=>10k
+/-5V, R=>4.7k

Zener -> 4.7V [for microcontroller's Vcc=5Vdc] ..

IanP

 

[arunbhaskar]

Re: Need help with interfacing Schmitt Trigger Circuit with

Thanks IanP.
I'm using +/-12V supply
Would 10k resitor work. Would there be enough current with 10k resistor for the counter
or should I decrease the resistor value?

 

[betwixt]

Re: Need help with interfacing Schmitt Trigger Circuit with

Microcontroller in puts usually draw just a few µA so in theory you can make the resistor much larger, possibly into the MΩ region but other effects will start to show. Very high values will slow the rise and fall of the waveform so you might get timing problems. You shouldn't have any problem with 10K though.

Brian.

 

[arunbhaskar]

Re: Need help with interfacing Schmitt Trigger Circuit with

Thank you. I have one more doubt. In the negative cycle, there will be a small negative voltage applied to the counter because of the diode drop in forward bias. Is that acceptable?

 

[IanP]

Re: Need help with interfacing Schmitt Trigger Circuit with

The absolute minimum voltage on any pin is -0.5V, so I think you should consider Schottky diode, like BAT-85, configured as follows:
The cathode of the diode is connected to the pin; Its anode is connected to GND .. see attached picture ..

IanP
:|

 

[arunbhaskar]

Re: Need help with interfacing Schmitt Trigger Circuit with

Thank you very much.
The schottky has a forward voltage drop of < 0.5 volts which doesn't affect the microcontroller right?
I'm curious to know what might happen if negative voltage appears across any port of the uC.

 

[IanP]

Re: Need help with interfacing Schmitt Trigger Circuit with

Negative voltages, specially Out-of-Tolerance Voltage Spikes, can cause irreparable damage to the CMOS devices ..
CMOS design guidelines require that no pin be taken above VCC or below VSS and violation of this guideline can result in a hard failure (damage to the silicon inside the device) or a soft failure (unintentional modification of memory contents) ..

The schottky has a forward voltage drop of < 0.5 volts which doesn't affect the microcontroller right?

Right ..

IanP
:|

 

[arunbhaskar]

Thanks for having cleared all my doubts.

 

[IanP]

Re: Need help with interfacing Schmitt Trigger Circuit with

You're welcome mate ..

IanP