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Experiment 5 - The THRESHOLD Input

From the internal circuit diagram of the NE555 you can see that the THRESHOLD input Pin 6 (THR) is connected to a comparator. The second input of the comparator is connected to the voltage divider, at which the voltage V = 2/3 Vcc = 6 V applies. The output of the comparator is connected to the RESET input of the RS latch.


Internal circuit diagram THRESHOLD input NE555.

Now consider the circuit shown...

Circuit THRESHOLD input NE555. (Enlarge)

Circuit diagram THRESHOLD input NE555.

As soon as you push the pushbutton switch S2...

  • At the THR input a voltage of Vcc is applied, which is greater than V = 2/3 Vcc = 6 V ,
  • therefore the output of the comparator goes to HIGH
  • thus the RS latch gets a RESET and resets the latch,
  • and the output OUT goes to LOW. LED1 goes back on.

But there is also another way to reset the NE555...
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