Looking at Window Comparator Circuits

by Lewis Loflin

  

  

A window comparator will output a voltage between two selected voltage points and be off outside that range. In Fig. 1 we have used 2 of 4 comparators in a LM339. The outputs of the each comparator in the LM339 is a open collector transistors tied together and connected to a LED and resistor. This open collector arrangement allows one to use a separate power supply form the rest of the circuit.

Note that U1b is connected as an inverting comparator while U1c is connected as a non-inverting comparator.

See Voltage Comparator Circuits

Window Comparator using LM339
Fig. 1 Window Comparator using LM339
click picture for full size.

The rule for open collector comparator is both comparators must be turned OFF at the same time to output a HIGH. in Fig. 1 we use three 10,000 ohm resistors in series that produces 2/3s Vcc at Vref HIGH and 1/3 Vcc at Vref LOW.

On an inverting comparator if the MINUS input is greater than the PLUS input the internal open collector transistor is turned on. On a non-inverting comparator if the PLUS input is greater than the MINUS input the internal open collector transistor is turn on.

Thus the LED in Fig. 1 is on between these two voltages of 4V and 8V.

Window comparator using LM358 dual Op-Amp.
Fig. 2 Window comparator using LM358 dual Op-Amp at 3 volts.

Window comparator using LM358 dual Op-Amp.
Fig. 3 Window comparator using LM358 dual Op-Amp at 6 volts.

Window comparator using LM358 dual Op-Amp.
Fig. 4 Window comparator using LM358 dual Op-Amp at 9 volts.

Because each LM339 comparator is nothing more than an operational amplifier we can use a LM358 and do the same thing as Fig. 1 other than the output is 12-volts fed into two transistors Q1 and Q2. We use both a non-inverting and inverting comparators together. Vin is TP1.

Window comparator using LM358 dual Op-Amp logic table.

Above is the logic table for the window comparator circuit at different voltages at Vin. Output transistor Q3 is turned on by the 10K pull up resistor when both Q1 and Q2 are turned off at the same time.


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