Electronic Products

ObjectivesLearn what a 555 timer is and what it can be

used forLearn how to connect a 555 timer up as a

monostableUnderstand 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 TimerAn 8 pin Integrated CircuitQuite old & simple but very

useful

Can be used in lots of circuitsWe are only interested in two:

Astables Oscillators, toy organs, light

flashing Monostables

Egg timers, switch debouncing

The MonostableThe monostable times using a capacitor (C1)

which charges up through a resistor (R2)

CapacitorsElectronic buckets!

•Non electrolytic•Generally < 1uF•Not polarized•Ceramic disc most common

•electrolytic•Generally > 1uF•Polarized•Metal can (axial & radial)

Capacitor Charging++

++

+++

+Volts

Time

++

++

+++

+

t=RCTime 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

Capacitor Discharging

Time

Voltst=RCTime constant – the rate at which a capacitor discharges through a resistor

Starting the timerThings start to happen when the

voltage on pin 2 (trigger) is < 1/3 of supply3V in this case

R1 is a pull up resistorKeeps the voltage at

pin2 at near 9V when sw2 isn’t pressed

When pin is > 3V, pin 7(discharge) is connectsto 0V, discharging C1

When it endsWhen pin6

(threshold) sees a voltage > 2/3 of supply6v in this case

When this happens, pin 7 (discharge) connects the capacitor back t0 0v 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)