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lowest possible current for logic level voltage for PIC micro

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papunblg

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In the given schematic, the sensor’s Vout terminal voltage is 4.4Volts. When it senses, the sensor’s Vout terminal voltage drops to 2.61 Volts. As both the values are more than logic low value of the PIC micro, a transistor is used to act as a switch to provide high and low logic for the PIC16F84A. It is suppose to work in saturation. With a BC547B(Min Hfe 200) R1=1K and R2=10K, the circuit is working as expected. Although, my calculated value shows for R2=10K, current through R2 is 0.5mA.So for min Hfe200, R1 should be 1.48K for required base current of 0.0025mA.
My question is, in this case where no big loads (like relay or leds or other components) are present, what is the minimum collector current possible so that PIC can sense it? Or what is the maximum value of R2? Am I missing something?.

pic1.gif
 

SherpaDoug

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There are a couple of things to watch out for:
1) Static - What is the worst case leakage current of the transistor, the PIC pin, the PCB, and anything else?
2) Dynamic - How fast does this have to work? What is the capacitance of the transistor, PIC, PCB, etc.?

If you don't have to go very fast you can use 1M easily, or 10M with a bit more care.
 

keith1200rs

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It isn't a very good circuit for voltage detection. While you have calculated for one value of hfe, what about other extremes? And Vbe variation? And temperature variations?

A couple of possible improvements would be to add a resistor from the base to ground. You should make the base current a lot lower than the current through the potential divider. An additional option to consider would be adding a diode or two in series with the base, or even a zener. Some combination should be possible which gives a reliable, simple circuit even with component and temperature variations.

Keith
 

papunblg

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Thanks Mr. Sherpadoug and thanks Mr. Are.

Special Thanks to Super Moderator Sir, Mr.Keith for his kind suggestions.
 

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