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INFORMATION REGARDING THERMISTOR CIRCUITS

AND DARLINGTON PAIRS

V. Ryan 2007 - 2009

 

PDF FILE - CLICK HERE FOR PRINTABLE VERSION OF EXERCISE SHOWN BELOW

Sensors such as the thermistor based circuit shown below are not always reliable. This is usually due to the circuit relying on one transistor. The circuit below has two transistors, called a ‘Darlington Pair’. This arrangement of transistors amplifies current flowing from the thermistor and ensures that the LED lights.

 

When the temperature is LOW. Resistance of thermistor high and current flows through the transistors allowing the LED to light

   

High temperature. Resistance of thermistor reduces significantly and current flows from positive straight to negative. The LED does not light.

 
   

QUESTION:

1. The LED in the circuit above lights when the temperature is low (icy). However, it would be useful if the LED would light when it is warm/hot. This circuit could be used for a warning device when temperatures are too high.. Draw an updated version of the circuit that would allow this to happen.

     
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