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low. There are two
classical circuits used
with the 555. The first in
Figure 2 is an astable
multivibrator or free-running oscillator.
External capacitor C
charges through RA and
RB and discharges
through RB. The
charging and
discharging of the
capacitor triggers the
comparators alternately
setting and resetting the
flip-flop; this produces a
rectangular output
waveform. The frequency
(f) is given by:
■ FIGURE 3.
A 555
one-shot.
f = 1.46/(RA + 2RB)C
■ FIGURE 5. Semtech’s SX8122
555-like IC.
The duty cycle (DC) is given by:
DC = RB /(RA + 2RB)
With the older 555s, the upper frequency limit was
a few hundred kilohertz (kHz) at best. The duty cycle
was not 50% as is usually desirable, but more like 45%.
Yet many circuits used this configuration. With large
external resistors and an electrolytic capacitor, the
output frequency could be as low as a few Hz — great
for flashing LEDs. The other popular circuit is a one-shot
or monostable as shown in Figure 3. A single input
trigger voltage initiates a single output pulse whose
duration (t) is given by:
T = 1.1RC
■ FIGURE 4. Advanced Linear Devices’ ALD7555/7556 ICs.
This circuit was good for generating a delay or a
sequence of timed pulses. The 555 can also be used in a
variety of circuits; way too many to list here. Some
examples are a pulse width modulator (PWM), variable
frequency oscillator (VCO), and a DC-DC converter. Just
search ‘555 timer’ with your browser to find a jillion or
more application circuits. Nuts & Volts has published
thousands of 555 circuits over the years, as well.
ADVANCED LINEAR DEVICES’ 555
Advance Linear Devices recently announced their
improved version of the 555 called the ALD7555 and its
companion dual timer version the ALD7556. It is made
from CMOS, unlike the original bipolar versions of the
555 (see Figure 4). It features 200K ohm voltage divider
resistors instead of the original 5K parts. This gives the
device lower power consumption and higher input
impedances. The device also includes an internal MOSFET
driver transistor (see Figure 1 again) that was usually an
external transistor in some 555 designs.
