Experiment 9 - The DISCHARGE Output III

There is yet another way to use the DISCHARGE output. However, this changes the behavior of the circuit.

Consider to the circuit shown.

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

When you take the circuit into operation, the LEDs light up alternately. Thereby LED2 lights up about twice as long as LED1.

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

How does this happen? As soon as you connect the battery...

• OUT goes to HIGH. 
• DISCHARGE becomes high impedance,  C1 charges via the resistors R1 and R2. 
• When across C1 the voltage reaches V = ²/₃ V_cc, the THRESHOLD switches OUT to LOW.
• DISCHARGE becomes low impedance,  C1 discharges. 
• When across C1 the voltage reaches V = ¹/₃ V_cc the TRIGGER switches OUT to HIGH.
• The cycle begins again.

This is the basic circuit of an astable multivibrator using a NE555. More on that later.