Nuts and Volts - April 2014

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Bias on Production Run

Regarding Ryan Clarke’s January 2014 article on building a PIC-based remote temperature sensor, I checked the calculation of the base bias resistor for Q1. Clarke apparently used the minimum battery voltage ( 2.7V), the saturation voltage of the transistor (0,75V), the Beta value of 10 from the sidebar, and the measured minimum Ic value of 3.87 mA for the RF module.

The resulting 5,039 ohms will indeed keep the 2N3904 in saturation for all battery voltages ... for that particular RF module.

However, if you were designing this circuit for production, you'd have to plan for some RF modules to draw the specified minimum Ic current ( 11 mA). That would yield a bias resistor value of 1,772 ohms, or

1.6K ohms as the closest E24 resistance value. This would admittedly reduce the battery life a little, but that's better than having some circuits fail due to variations in the RF module Ic current.

David Naegle

Absolutely, and my first "go around" with the design was built that way. However, this is not a production design, but one tailored to what was on hand. So, I got more specific on the resistor as you see. Thank you for your feedback. I appreciate it.