555 Monostable Circuit Electronic Products Objectives Learn what

555 Monostable Circuit Electronic Products

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

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

The Monostable The monostable times using a capacitor (C 1) which charges up through a resistor (R 2)

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

Capacitor Charging ++ ++ Volts ++ ++ t=RC Time constant – the rate at which a capacitor charges through a resistor ++ ++ Time After one time constant, capacitor is at 0. 6 of its full charge, and fully charged after 5 time constants

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

Starting the timer Things start to happen when the voltage on pin 2 (trigger) is < 1/3 of supply 3 V in this case R 1 is a pull up resistor Keeps the voltage at pin 2 at near 9 V when sw 2 isn’t pressed When pin is > 3 V, pin 7 (discharge) is connects to 0 V, discharging C 1

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

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)