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

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

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 at pin2 at near 9V when sw2 isn’t pressed When pin is > 3V, pin 7 (discharge) is connects to 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)