Tuesday, February 2, 2010

555 timer oscillator

555 timer oscillator

The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was also the very first and only commercial timer ic available. It is after 30 years still very popular and used in many schematics. Although these days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular type is still available, however there have been many improvements and variations in the circuitry. But all types are pin-for-pin plug compatible.


Supply voltage 4.5-18V

Supply current 3-6 mA

Output current 225mA (max)

Rise/Fall time 100 ns


external capacitor determines the off-on time intervals of the output pulses

time taken by capacitor to charge to 63.7% of the applied voltage = time constant, t = RC


one shot (monostable)

oscillator (astable)


generates a single pulse of a fixed time duration each time it receives and input trigger pulse

used for turning some circuit or external component on or off for a specific length of time

also used to generate time delays

duration of pulse dependent on RC


generates a continuous stream of rectangular off-on pulses that switch between two voltage levels

frequency and duty cycle (ratio of time-on to time-off) are dependent upon RC

frequency can be calculated with the formula:

f = 1/(.693 x C x (R1 + 2 x R2))

pulse is on for t1 seconds, then off for t2 seconds, total period (t) is t1 + t2.

duty-cycles can be approximately 55 to 95%:

D = t1/t = (R1 + R2) / (R1 + 2R2)

t1 = .693(R1+R2)C

t2 = .693 x R2 x C

Pin functions

1: Ground (if split-rail power supply is being used) or -Vcc for single rail

2: Trigger

a voltage level of < .3Vcc (or, .5 Vpin 5) for at least 1us is sufficient to trigger

trigger input is momentarily taken from a higher to a lower level (output is inverse of trigger)

trigger pulse must be of shorter duration than the time interval determined by RC

if this pin is held low longer than that, the output will remain high until the trigger input is driven high again

must not remain lower than 1/3Vcc for a period of time longer than the timing cycle

if this is allowed to happen, timer will retrigger itself upon termination of the first output pulse

minimum monostable output pulse width should be in the order of 10uS to prevent possible double triggering

a dc current of typically 500nA, the trigger current, must also flow from this terminal into the external circuit

so R must be < 3 MW

3: Output

provides a high-state output voltage about 1.7 volts less than the V+ supply level used

rise and fall times of the output waveform are quite fast - switching times being about 100nS

output pin is inverse of the input trigger

output can also be made to go low by taking the reset to a low state near ground [see "Pin 4 - Reset"]

4: Output Reset

voltage threshold level is 0.7 volt with sink current of 0.1mA for 0.5 µS required

reset is an overriding function

it will force the output to a low state regardless of the state of either of the other inputs

may thus be used to terminate an output pulse prematurely

to gate oscillations from "on" to "off", etc.

delay time from reset to output is typically on the order of 0.5 µS

when not used, it is recommended that the reset input be tied to V+ to avoid any possibility of false resetting.

5: Reference

allows direct access to the 2/3 V+ voltage-divider point

use is optional

by applying a voltage to this pin, it is possible to vary the timing of the device independently of the RC network

control voltage may be varied from 45 to 90% of Vcc in monostable mode

possible to control the width of the ouput pulse independently of RC

in astable mode, the control voltage can be varied from 1.7V to the full Vcc

produces a frequency modulated (FM) output

if not used, ground via a capacitor of about 0.01uF (10nF) to eliminate false triggering.

6: Reset latch

causes the output to go low

a dc current, the threshold current of 0.1µA must also flow into this terminal from the external circuit

R < 16 Mega-ohm

7: Timing capacitor

"on" (low resistance to ground) when the output is low and "off" (high resistance to ground) when the output is high

8: +Vcc

positive supply voltage +4.5 volts (minimum) to +16 volts (maximum)

chip generates a big (about 150mA) supply current during each output transition

therefore recommended to use a large decoupling capacitor as near to this pin as possible

even so, the 555 may have a tendency to generate double output transitions
More Examples
schmitt trigger
10 minute timer


power alarm

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