555 Timer: Pins and Internal Blocks

What is inside the classic timer chip.

The 555 is an 8-pin timer. Its pins are ground, trigger, output, reset, control voltage, threshold, discharge and supply. Inside, three equal resistors form a divider that sets reference levels at one third and two thirds of the supply.

Two comparators watch the trigger and threshold pins against those levels and set or reset a flip-flop. The flip-flop drives the output and a transistor that discharges the timing capacitor. Pulling reset low forces the output low.

• Pin 1 ground, pin 8 supply.
• Trigger acts below 1/3 of the supply.
• Threshold acts above 2/3 of the supply.

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