Argh! The Thing that Wouldn't Die. With full respect for H. Camenzind and
the others involved, and for its obvious commercial success, and for RKovach
who posed a reasonable question here, the enduring popularity of this
product (including among hobbyists) mystifies me, basis in a moment.
"RKovach" in news:
[email protected]...
I am trying to get an astable output of 4ms on, and 20ms
off from a 555 timer. Is this possible?
I believe it is, others have offered details already. For the record though
this may not have the frequency stability or other properties needed by the
specific application, a decent astable oscillator with near-minimum
component count can always be made be putting a resistor from output to
input of a CMOS logic inverter or inverting gate of Schmitt, or hysteresis,
type, and then a capacitor from the logic input to ground (or to a power
supply). Asymmetrical duty cycles are available with additional components;
one way is to split the feedback resistor into two parallel paths, each with
a resistor and a diode, one in each direction; each resistor then controls
one charging ramp at the capacitor, and can be adjusted independently.
A negative feedback loop with a time-integrating element in series with a
hysteresis element is the basis of the classical first-order relaxation
oscillator, and is for instance an upshot in Henry M. Paynter's influential
article "Positive / Negative Feedback in Amplification and Control" which
appeared in multiple places in the 1960s including the publication _The
Lightning Empiricist_ of the George A. Philbrick company of Boston
(popularizers of the op-amp concept). This publication had an important
role in introducing some of the genuinely "analog" concepts now taken for
granted in electronics (and perhaps described today by young professors as
having origins lost in the mists of time).
Having also needed to make, like RKovach, RC relaxation oscillators over the
years I saw the 555 appear in (?) 1971 and first thought it was a great
idea, and in some ways still think so; the design had temperature stability,
conveniences, output driver. What kept me from using it much in practice
were very high DC power demand (from the point of view of applications not
needing high output drive) and a frustrating tendency, at least in early
versions, to parasitic oscillation of the unsought kind, which in turn
demanded careful wiring, sometimes extra capacitors, etc. These days, much
more recent monolithic timers or "silicon oscillators" are available, and
listed online, at low prices with much lower power, higher frequencies,
fewer external components, internally trimmed frequency accuracy, digital
control, etc. On the other hand, if one just needs a casual oscillator
there are still simpler and cheaper solutions, such as the one I outlined
above. It may be that (like Fairchild's 741 op-amp product whose popularity
Jim Roberge analyzed critically in his book _Operational Amplifiers_), the
perceived perfection or convenience of the 555 contributes to its endurance.
-- MH
-- MH
