Digital LAB

Lab 2

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The 555 timer is an 8-Pin D.I.L. Integrated Circuit or ‘chip’

What it looks like

Notch

Pin 1

555 Timer

555

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8

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DIP chip

DualIn-LinePins

top view

555 timer

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Operation:•Contains 25 transistors, 2 diodes and 16 resistors

• Maximum operating voltage 16V

• Maximum output current 200mA

If you input certain signals they will be processed / controlled in a certain manner and will produce a known output.

INPUT PROCESS OUTPUT

• Best treated as a single component with required input and output

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555 timer

What the 555 timer is used for:

•To switch on or off an output after a certain time delay i.e.

Games timer, Porch light, Childs mobile, Exercise timer.

•To continually switch on and off an output i.e.

Hazard warning lights, Soft toy, Bicycle indicators.

•As a pulse generator i.e.

To provide a series of clock pulses for a counter.

555 timer

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Digital Terminology:

To use the 555 timer correctly you have to be aware of the correct terminology:

• Monostable Time

• Astable Time

555 timer

• MONOSTABLE: This is a system which has only one stable state.

It can be made to change but it will always return to its original stable state. A spring operated push button switch is an example of this. Monostable circuits are used as timers and as a single pulse generator.

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Schematic diagram of 555 Monostable timer

• The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

• time period, T = 1.1 × R1 × C1

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Monostable

time period, T = 1.1 × R1 × C1 T = time period in seconds (s)

R1 = resistance in ohms () C1 = capacitance in farads (F) The maximum reliable time period is about 10 minutes

• R1 should be in the range 1k to 1Mohm

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• ASTABLE: This is a system which has no stable state. It

changes from one state to the other all the time. A pendulum swings from one side to the other continuously. Astable circuits are used to flash lights or sound a loudspeaker. It can act as a square wave oscillator

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555 timer

Schematic diagram of 555 Astable timer

• The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second. T = 0.7 × (R1 + 2R2) × C1

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Astable

f = 1.4/ (R1 + 2R2) × C1T = time period in seconds (s) f = frequency in hertz (Hz) R1 = resistance in ohms () R2 = resistance in ohms () C1 = capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts Mark time (output high): Tm = 0.7 × (R1 + R2) × C1 Space time (output low): Ts = 0.7 × R2 × C1

• R1 and R2 should be in the range 1k to 1Mohm

555 Astable frequencies

C1

R2 = 10kohmR1 = 1kohm

R2 = 100kohmR1 = 10kohm

R2 = 1MohmR1 = 100kohm

0.001µF 68kHz 6.8kHz 680Hz0.01µF 6.8kHz 680Hz 68Hz0.1µF 680Hz 68Hz 6.8Hz1µF 68Hz 6.8Hz 0.68Hz10µF 6.8Hz 0.68Hz 0.068Hz

(41 per min.) (4 per min.)

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Theme : A Safe Environment

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Astable:Hazard / warning lights / school

crossingBicycle indicatorsBurglar AlarmSteep Gradiant IndicatorPersonal Alarm

Monostable:Egg Timer : Food / Electrical safetyBath / Oven Timer

Flashing LED Project

555 timer

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Parts Required

• resistors: 470, 1k, 220k • capacitor: 1µF 16V radial • red LED (or orange, yellow or green if you

prefer!) • 555 timer IC • 8-pin IC holder (a 'DIL socket') for the 555 IC • battery clip for 9V PP3 • stripboard: 6 rows × 21 holes

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Instructions

• Solder the 8-pin IC holder in the correct place on the stripboard.

• Break the 4 tracks under the IC holder with a track cutter tool. You can allow extra holes if your piece of stripboard is large enough.

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Instructions

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Instructions

• Use the resistor colour code to identify the resistors which are marked with coloured bands to show their value.

• Insert and solder the resistors in the correct position, they can be put in either way round, but you must line them up correctly with the IC holder.

• Identify the other parts, then solder them in the correct position and the right way round.

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Instructions

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Instructions

• Solder the 2 wire links in place around the IC holder, it is easier to use plastic-coated single-core wire. (The flexibility of stranded wire is not needed for connections like this and the strands can be difficult to push through the small hole).

• Finally insert the 555 timer IC and connect a battery!

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