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 Comparitors
Essential information given in red
A comparitor as the name suggests compares 2 voltages and rapidly changes from either fully saturated (positive) or fully saturated (negative).

Most experiments  made using a comparitor use a 741 op amp, this is a very common chip that is used in electronics, whilst not a specialist comparitor chip, the very high gain makes it suitable for this application.

Not part of GCSE electronics syllabus it is useful to know how to wire up such a device, any further studies will definitely require this.

The power supply going to the 741 op amp(comparitor) is provided using a dual volt power supply, both the negative 9 volt and positive 9 volt can be used to provide the supply to the chip.

You could if necessary wire 2 batteries together, wiring the + and - terminals together (see below), the remaining output terminals being used to supply the 741 chip.

Also be aware that the output will be signal will swing between + and - 9 volts unlike a more conventional output of 9 volts and ground.

Temperature controlled comparitor

The circuit below shows a 741 op amp connected to sense the the temperature (using a thermister) in a green house.
When the temperature reaches a certain temperature then the comparitors output will change.
An LED will light up but it could also trigger a motor connected to a window to allow the greenhouse to be ventilated.

You will need to understand the layout of a comparitor ciruit and the effects of the voltages on the inverting and non inverting inputs.

When the non-inverting input (V+) is at a higher voltage than the inverting input (V-)
The output saturates at the highest positive

When the non-inverting input (V+) drops below the inverting input (V-)
the output saturates at the most negative voltage it can output

Try filling in the following form
Input A
Input B
Output Vout
4.5
2.3
?
4.5
4.8
?

You should also be able to label the inverting, non inverting and output of the comparitor
Hint - The inverting is marked (-) input A, the non inverting (+) input B, the output marked Vout

 


 
 
 
 
 
 
   
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