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555 Monostable Circuit for Electronic Projects

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Explore the functionality and applications of a 555 timer as a monostable circuit. Learn the working principles, capacitor characteristics, and timer configurations. Prepare for exam questions related to 555 timers' operation and utilization in various circuits.

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555 Monostable Circuit for Electronic Projects

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  1. 555 Monostable Circuit Electronic Products

  2. Objectives • Learn what a 555 timer is and what it can be used for • Learn how to connect a 555 timer up as a monostable • Understand how a monostable works Note: Every year there is at least one long exam question on 555 timers so make sure you understand this fully

  3. The 555 Timer • An 8 pin Integrated Circuit • Quite old & simple but very useful • Can be used in lots of circuits • We are only interested in two: • Astables • Oscillators, toy organs, light flashing • Monostables • Egg timers, switch debouncing

  4. The Monostable • The monostable times using a capacitor (C1) which charges up through a resistor (R2)

  5. Capacitors • Electronic buckets! • Non electrolytic • Generally < 1uF • Not polarized • Ceramic disc most common • electrolytic • Generally > 1uF • Polarized • Metal can (axial & radial)

  6. Capacitor Charging t=RC Time constant – the rate at which a capacitor charges through a resistor + + Volts + + + + + + + + + + + + + + After one time constant, capacitor is at 0.6 of its full charge, and fully charged after 5 time constants Time

  7. Capacitor Discharging t=RC Time constant – the rate at which a capacitor discharges through a resistor Volts Time

  8. Starting the timer • Things start to happen when the voltage on pin 2 (trigger) is < 1/3 of supply • 3V in this case • R1 is a pull up resistor • Keeps the voltage atpin2 at near 9V when sw2 isn’t pressed • When pin is > 3V, pin 7(discharge) is connectsto 0V, discharging C1

  9. When it ends • When pin6 (threshold) sees a voltage > 2/3 of supply • 6v in this case • When this happens, pin 7 (discharge) connects the capacitor back t0 0v again

  10. The 556 • Two 555’s in one 14 pin package • Smaller and more power efficient • We will use one of these in our next project – a monstable (1 x 555) controlling an astable (1 x 555)

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