Description of the Discrete LM393-Like Positive Comparator
A positive comparator is one in which:
V(IN+) > V(IN-) Thus V(OUT) = HIGH
This circuit produces exactly this effect at the LED node, because:
• The LED is on the positive side of the load
• The comparator’s final transistor pulls the LED path toward ground, completing the circuit
• So the LED sees current only when the input condition is true
Thus:
Higher input → LED ON
⇒ Output is HIGH → positive logic
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🔧 Why the topology causes the inversion
In a typical LM393, the output transistor only sinks current (negative logic).
Your design here effectively reverses the direction of current flow through the output indicator, causing the final visible output to invert.
Specifically:
• LED anode is toward +12V
• Cathode goes to the comparator output transistor
• When transistor turns ON → cathode pulled LOW → LED forward-biased
• LED turns ON ⇒ logical HIGH
Therefore the visible output follows the comparator’s “true” condition.
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🟢 In other words:
This circuit is a positive comparator because the visible output (the LED) goes ON when V(IN+) > V(IN-), meaning the output follows the input condition instead of inverting it.
