555

Do you mean the 555 timer IC? If yes, here’s a quick refresher you can use for exams.

Pinout (common knowledge):

• 1 = GND
• 2 = Trigger
• 3 = Output
• 4 = Reset (active low)
• 5 = Control Voltage (usually a 0.01 µF cap to ground)
• 6 = Threshold
• 7 = Discharge
• 8 = Vcc

Modes and key equations:

1. Astable (oscillator)

• f = 1.44 / [(R1 + 2R2) C]
• TH (high time) = 0.693 (R1 + R2) C
• TL (low time) = 0.693 R2 C
• Duty cycle = TH / (TH + TL) = (R1 + R2) / (R1 + 2R2)

2. Monostable (single pulse)

• Trigger with a negative pulse on pin 2
• Output high for t = 1.1 R C
• After trigger, capacitor C charges toward Vcc through R
• Typical setup: connect DISCH pin to R and C to form the timing network; output goes high for the calculated interval

3. Bistable (flip-flop) mode

• No timing capacitor required
• Output toggles with trigger and reset inputs

Common tips:

• If not using pin 5, you can tie it to 2/3 Vcc with a small capacitor (0.01 µF) to reduce noise.
• Ensure RESET (pin 4) is held high for normal operation.
• Choose R and C values to place the frequency and timing in the desired range.

If you want, tell me:

• Which mode you need (astable/monostable/bistable)
• The component values you have I’ll compute the timings and frequency for you.