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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
CapacitorsElectronic buckets!
•Non electrolytic•Generally < 1uF•Not polarized•Ceramic disc most common
•electrolytic•Generally > 1uF•Polarized•Metal can (axial & radial)
Capacitor Charging++
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+Volts
Time
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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