Ne555 Timer

NE 555 Timer ENGI 251 ELEC 241

Comparator

• • • •

High Gain Op Amp Operated Open Loop Designed to compare an input to a reference voltage Gives output (digital level) to indicate if input is above or below reference – Circuit designed to give VOSAT and –VOSAT only

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Comparator Operation Example

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LM 311 Comparator

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Window Comparator

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Determine the Output

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Schmidt Trigger 7414

• A Schmidt trigger (a comparator with Hysteresis) is a bistable digital (two-state) device • It accepts virtually any analog input and provides a logic 0 or 1 output – A typical use is to take distorted digital signals (due to RC time constant of transmission line) and provide a used to square-wave output – Can be used to eliminate noise near reference point that would cause problems in analog comparators ENGI 251/ELEC 241

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Hysteresis

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Block Diagram of 555

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Notes on 555 Timer/Oscillator IC • • • • • • •

Widely used as a monostable or astable multivibrator. Can operate between 4.5 and 16 V. Output voltage is approximately 2 V < VCC. Output can typically sink or source 200 mA. Max. output frequency is about 10 kHz. fo varies somewhat with VCC. Threshold input (pin 6) and trigger input (pin 2) are normally tied together to external timing RC.

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555 as a Simple Oscillator Duty cycle is:

D=

tch R + R2 = 1 T R1 + 2R2

Given fo and D, 2D −1 1− D R1 = ; R2 = 0.693 f o C1 0.693 f o C1 tch = 0.693(R1 + R2)C1 tdisch = 0.693 R2C1 T = 0.693(R1 + 2R2)C1 ENGI 251/ELEC 241

Note that D must always be > 0.5. To get 50% duty cycle, R1 = 0, which would short out VCC.

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Astable Multivibrator

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Astable Multivibrator

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555 Used as an Astable Multivibrator

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Astable Design Considerations

DC =

t1 R1 + R2 = T R1 + 2R2

DC =

The maximum value of R1 + R2 is 3.3MΩ and the minimum value for R1 or R2 is 1kΩ. The minimum recommended value for C is 500pF but best results are obtained when C is 1000pF or larger. ENGI 251/ELEC 241

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NE555 Astable Schematic

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NE555 Astable Waveforms

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555 Square-Wave Oscillator

D= R1 =

R1 R1 + R2

D 1− D ; R2 = 0.693 f o C1 0.693 f oC1

For 50% duty cycle, tch = 0.693 R1C1 ; tdisch = 0.693 R2C1 fo =

R1 = R2 =

1 0.693( R1 + R2 )C1

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555 as a Timer / Monostable Multivibrator

t = 1.1 R1C1 Time pulses from a few µs to many minutes are possible The limitation for very long time delays is the leakage in the large-value capacitor for C1.

R2 (typically 10 kΩ) is a pull-up resistor. C2 (typically 0.001 µF) is for bypass. Timing starts when trigger input is grounded. ENGI 251/ELEC 241

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Monostable Multivibrator – One Shot

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