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.
Specifications
Supply voltage 4.5-18V
Supply current 3-6 mA
Output current 225mA (max)
Rise/Fall time 100 ns
Operation
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
Modes
one shot (monostable)
oscillator (astable)
One-shot
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
Oscillator
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
metronome
power alarm
