Oscillators & Timers

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. Fig. 4-2, Equivalent Circuit

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
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 -V_cc for single rail

2: Trigger

a voltage level of < .3V_cc (or, .5 V_{pin
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 V_cc 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: +V_cc

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 (output high when input low)

often used to clean up noisy switching signals

Metronome

Power Alarm

10-Min. Timer