(3) PV One Shot Astable PDF

Preview

Summary

module notes and tutorials for practice...

Description

One-shots A simple one-shot circuit.

One-Shots The one-shot or monostable multivibrator is a device with only one stable state. When triggered, it goes to its unstable state for a predetermined length of time, +V then returns to its stable state. For most one-shots, the length of time in the unstable state (tW) is determined by an external RC circuit.

REXT

CEXT CX

Q

RX/CX

Trigger

Q Trigger

Q tW

One-Shots Nonretriggerable one-shots do not respond to any triggers that occur during the unstable state. Retriggerable one-shots respond to any trigger, even if it occurs in the unstable state. If it occurs during the unstable state, the state is extended by an amount equal to the pulse width. Retriggerable one-shot: Trigger Retriggers

Q tW

One-Shots An application for a retriggerable one-shot is a power failure detection circuit. Triggers are derived from the ac power source, and continue to retrigger the one shot. In the event of a power failure, the one-shot is not triggered and an alarm can be initiated. Missing trigger due to power failure

Triggers derived from ac

Retriggers

Q

Power failure indication

Retriggers tW tW

tW

One-shots

One-shots

One-shots Three ways to set the pulse width of a 74121 nonretriggerable.

One-shots Logic symbol for the 74LS122 retriggerable one-shot.

Tw=0.32RCext(1+0.7/R) Rint=10kΩ

One-shots

One-shots A sequential timing circuit using three 74LS122 one-shots.

The 555 timer The 555 timer can be configured in various ways, including as a one-shot. Internal functional diagram of a 555 timer (pin numbers are in parenthesis).

The 555 timer A basic one shot is shown. The pulse width is determined by R1C1 and is approximately tW = 1.1R1C1. +VCC

(4)

(8)

R1 (7) RESET

VCC

DISCH

The trigger is a negative-going pulse.

(6) (2)

C1

(3)

THRES

OUT

TRIG CONT GND (1)

(5)

tW = 1.1R1C1

The 555 timer One-shot operation of the 555 timer.

The 555 timer The 555 can be configured as a basic astable multivibrator with the circuit shown. In this circuit C1 charges through R1 and R2 and discharges through only R2. The output +V frequency is given by: CC

f =

1.44 ( R1 + 2 R2 ) C1

(4)

R1 (7)

The frequency and duty cycle R 2 are set by these components. C1

(6) (2)

RESET DISCH THRES

(8)

VCC

OUT

TRIG CONT GND (1)

(3) (5)

The 555 timer Operation of the 555 timer in the astable mode.

The 555 timer Given the components, you can read the frequency from the chart. Alternatively, you can use the chart to pick components for a desired frequency. +VCC 100

10

(7)

1.0

1

R2

0.1

(6) (2)

0.01 0.001 0.1

RESET DISCH

(8)

VCC

kΩ

Ω

kΩ

0k

Ω

MΩ

M

10

10

1

10

C1 (μF)

(4)

R1

C1

THRES

OUT

TRIG CONT GND (1)

1.0

10

100

f (Hz)

1.0k

10k

100k

(3) (5)

555 Astable Calculations 1.44 f = ( R1 + 2 R 2)C1 tH = 0.7( R1 + R 2)C1

tL = 0.7 R 2C1 R1 + R 2 DutyCycle = 100% ( R1 + 2 R 2 )

The 555 timer The addition of diode D1 allows the duty cycle of the output to be adjusted to less than 50 percent by making R1 , R2.

555 Astable Calculations (DC < 50%)

1.44 f = ( R1 + R 2)C1 tH = 0.7 R1C1

tL = 0.7 R 2C1 R1 DutyCycle = 100% ( R1 + R 2 )

Selected Key Terms Timer A circuit that can be used as a one-shot or as an oscillator....