LM555 Timer Used As A Voltage Comparator Or Schmitt Trigger
The next section shows how the LM555 timer could be used as a voltage comparator or a wide offset Scmitt Trigger. Applications for which it is not particularly well suited but one that is in wide use with model railroaders.
Shown on the schematic is a secondary output that uses the open collector at the DISCHARGE terminal (Pin 7) of the timer. This output can sink up to 200 milliamps and would be ideal for driving relays.
The main disadvantage to using this circuit is the the large dead-band (1/3Vcc) between upper and lower threshold voltages. An optional resistor, R5, can be added to the circuit to lower and compress the detection voltage range but this only partially alleviates the problem. [...]
Read more source: http://home.cogeco.ca/~rpaisley4/LM555.html
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