Experiment 8 - The DISCHARGE Output II

But how do you use the DISCHARGE output? Find out how you produce a time-dependent signal, as only the DISCHARGE output makes the NE555 a timer.

Consider to the circuit shown. You replace the key switch, that controls the THRESHOLD input, by the capacitor C1. Simultaneously, the capacitor is also connected to the DISCHARGE output.

Circuit diagram connecting the output of a DISCHARGE NE555 with a capacitor.

Once you push the pushbutton switch S1, the LED2 lights up. After approximately one second the NE555 switches back independently. The LED1 lights up again. How does this happen?

Circuit for connecting the DISCHARGE output of the NE555 with a capacitor. (Enlarge)

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

• OUT goes to HIGH.
• DISCHARGE becomes high impedance,  C1 charges up via the resistor R2. 
• When across C1 the voltage reaches V = ²/₃ V_cc, the THRESHOLD switches OUT to LOW.
• DISCHARGE becomes low impedance, C1 discharges. 

This is the basic circuit of a monostable multivibrator or monoflop using a NE555. More on that later...

But there is another way to use the DISCHARGE output.