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The 555 Timer Connections

The 555 Timer Connections

The accompanying illustration (Figure A) shows the connection diagram for the 555 timer IC. Although Small Outline and Surface Mount Packages are available, the illustration shows the DIP version for the sake of simplicity.

The 555 DIP terminal designations are:

  • Pin 1: Ground – connects the 555 timer to the negative (0 V) power supply rail.
  • Pin 2: Trigger – also the negative input to the first internal comparator; a negative level or pulse on this terminal sets the internal flip-flop when the voltage drops below 1/3 of VCC causing the Output to switch from low to high.
  • Pin 3: Output – the digital Output can drive any TTL circuit; it is capable of sourcing or sinking up to 200 mA of current at an output voltage 1.5 V less than VCC.
  • Pin 4: Reset – resets the internal flip-flop, which controls the Output state; it is an active-low input; when not used, this terminal is typically connected to a high or logic “1” level to prevent false resets.
  • Pin 5: Control Voltage – controls the timing of the 555 by overriding the 2/3 of VCC level; the Output signal can be varied independently of the RC timing network by applying a signal to this terminal; it can be used to alter the timing of the 555 in PWM or frequency-modulation applications. When not used, Pin 5 should be connected to ground via a 0.01μF capacitor to eliminate noise.
  • Pin 6: Threshold – also the negative input to the second internal comparator; it resets the internal flip-flop when the voltage applied to it exceeds 2/3 of VCC, causing the Output to switch from high to low; it also connects directly to the RC timing circuit.
  • Pin 7: Discharge – connects directly to the collector of an internal NPN transistor, which is used to discharge the external timing capacitor to ground when the Output switches to low.
  • Pin 8: +VCC – connects the 555 timer to the positive power supply rail, which can be any voltage between 4.5 V and 15 V for general-purpose TTL 555 timers; it is typically 5 V when working with digital ICs.