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1. Astable Mode:
   • In this mode, the 555 timer operates as an oscillator generating a continuous square wave output. It's used in applications like pulse generation, LED or lamp flashers, and clock pulses.
   • Key external components typically include two resistors and a capacitor to set the timing interval.
2. Monostable Mode:
   • Monostable mode enables the 555 timer to work as a one-shot pulse generator. When triggered, it produces a single output pulse whose width is determined by an external resistor and capacitor.
   • Common uses include timers, missing pulse detection, bounce-free switches, touch switches, frequency division, and more.
3. Bistable Mode:
   • In bistable mode, the 555 works as a flip-flop. The output can be toggled between high and low states based on two separate trigger inputs.
   • Applications include basic memory circuits.

1. GND (Pin 1): Ground reference voltage, low level (0V).
2. TR (Pin 2): Trigger, an external trigger input, usually a negative pulse ($< \frac{1}{3} V_{CC}$) to start the timing cycle.
3. Q (Pin 3): Output, the output signal of the timer.
4. R (Pin 4): Reset, resets the timing interval when connected to ground. Active low.
5. CV (Pin 5): Control Voltage, allows overriding of the internal voltage divider. By default, it is $\frac{2}{3} V_{CC}$.
6. THR (Pin 6): Threshold, the input for the high-level threshold comparator.
7. DIS (Pin 7): Discharge, connected to the external timing capacitor for discharge.
8. VCC (Pin 8): Supply voltage, typically between 4.5V and 15V.

• Connect Pin 8 (VCC) to the supply voltage.
• Connect Pin 1 (GND) to Ground.
• Connect Pin 4 (R) to VCC (to disable Reset).
• Connect Pin 5 (CV) to Ground via a 10nF capacitor (optional, for noise reduction).
• Connect Pin 7 (DIS) to the junction of two resistors that set the timing interval.
• Connect Pin 6 (THR) and Pin 2 (TR) to the junction between a timing capacitor and one of the resistors.
• Pin 3 (Q) will provide the square wave output.