Experiment 4 - The TRIGGER Input

How do you connect the NE555? Find out how you control the state of the output with the TRIGGER input.

From the internal circuit diagram of the NE555 you can see that the TRIGGER input Pin 2 (TRIG) is connected to a comparator. The second input of the comparator is connected to the voltage divider, at which the voltage V = ¹/₃ V_cc = 3 V applies. The output of the comparator is connected to the SET input of the RS latch.

Internal circuit diagram TRIGGER input NE555.

Now consider the circuit shown...

Circuit TRIGGER input NE555. (Enlarge)

Circuit diagram TRIGGER input NE555.

As soon as you push the pushbutton switch S1...

• At the TRIG input the voltage is pulled down to GND, which is less than V = ¹/₃ V_cc = 3 V , 
• therefore the output of the comparator goes to HIGH
• thus the RS latch gets a SET and switches, 
• and the output OUT goes to HIGH. LED2 goes on.

But how can you switch the NE555 back again?