Newsgroups: comp.robotics
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From: cs911225@ariel.cs.yorku.ca (KEN E WILLMOTT)
Subject: A better 555 (was Re: IR reciever)
Message-ID: <CJFvG6.8oF@ariel.cs.yorku.ca>
Sender: news@ariel.cs.yorku.ca
Organization: York University, Dept. of Computer Science
Date: Tue, 11 Jan 1994 00:16:53 GMT
Lines: 108

>In article <CJ83IL.Isn@ariel.cs.yorku.ca> (KEN E WILLMOTT) responds:
>>Why, oh, why are people still using this 70's component?
>[to my use of the 555]

>I noticed you didn't suggest a substitute. I did have a good suggestion
>to use schmidt inverter with a resistor from front to back and a cap to
>ground fed into a flip flop. (this would be even easier if there was a 
>version of the 7400 with schmidt inputs.) I could also build an astable
>multivibrator out of transistors which I did for the hell of it using
>a design in Ray Marston's excellent series on transistors in Electronics
>Now (Sept 93 - Jan 94 issues.) The Dec '93 issue had oscillators built
>out of transistors and I built the one in figure 12. (Note there is a bug
>in the schematic, C1 and C2 connect to the collectors as well as the 
>bases as shown) Anyway, I'm certain there are more people besides myself
>who would be interested in knowing about a "90's" compoent or even an
>80's component that supercedes the 555.

If you need a pin for pin replacement, how about the Intersil
part ICM75551PA? Paraphrasing the catalogue:

"Low power CMOS equivalent of industry standard 555... does not
have the large power supply transients of the 555, and thus does
not need large bypassing capacitor... wider range of RC values are
allowable because of higher impedance inputs".


>>I assume you mean phase jitter. I don't think that was your problem,
>>though. You never actually observed any, you just made an assumption
>>that that was the problem. I'll bet that if there was phase jitter
>>in a given signal, it would degrade performance, though.

>Well I do have an oscilloscope and I observed the waveform pretty closely.
>In my experience phase problems are pretty clearly visible as "ghosts"
>in the displayed waveform or smearing of the leading or trailing edges.
>None of these were visible. However using the delayed sweep function to
>expand the edges I could see the period changing by small amounts
>concomitant with variations in the value reported by the frequency
>counter.

Well, you originally used the word "jitter", which does not usually
denote a low frequency phenomenon. I guess that what you're trying
to describe is frequency "drift".


>It was these observations, coupled with the lack of frequency
>variation in a known good circuit (the remote) that lead me to conclude
>that these devices were quite sensitive to small (< .5%) variations
>in the frequency.

I would be theorizing rather than concluding, at this point in
the experiment. After all, how did you know that other problems
did not exist in your prototype? Also, the accuracy of the transmitter
could be a red herring. For example, the frequency might be derived
from a crystal purely as a matter of convenience, and not as
necessary to meet the frequency stability needs of the reciever.


>Given the way that a typical tone decoder is designed
>I suspect that such variations would give a phase locked loop fits.

There is no PLL in the reciever, and it does not have to
differentiate multiple frequencies, as a tone decoder does.


>>This is right on. However, the GP1U52X (and others like it) operate
>>at 41.3 Khz, not 40 Khz. That figure comes from the fact that some
>>transmitters used a 455Khz crystal resonator as a timebase, which
>>when divided by 11, gives 41.364 Khz.

>You may be right, however the documentation from Sharp is pretty
>explicit on the center frequencies that these receivers expect and
>the numbers they give are 40Khz and 38Khz. They do have band pass
>filters whose 3db points are +/- 4Khz, so they may in fact respond
>to 41.364 khz signal. I've not had a spare one that I could pull
>the can off and look inside of.
>
>--Chuck
>-- 
>--Chuck McManis			     All opinions in this message/article are
>FirstPerson Inc.                     those of the author, who may or may not
>Internet: cmcmanis@firstperson.COM   be who you think it is.


The documentation you mentioned; was it from Tandy? I had read the
sheet packed with the part, and noted at the time that it seemed
abridged (mangled?). I don't trust Radio Shack.

If it is to be believed (at least in regards to bandwidth), then
the reciever should accept signals much farther off frequency than
the 0.5% you mentioned earlier (4kHz/40Khz = +/-10%).

In my case, I'm transmitting exactly 41.4 kHz, produced by a PLL
frequency multiplier, locked to a lower frequency timebase generated
by an HC11 output compare pin. Nice thing about that is, if I need
to change the frequency, I can do it easily by changing one software
variable.

Another way to lock the frequency to a crystal, is to divide a
2.0 MHz crystal by 48, i.e. by 16 and by 3. That gives you
41.7 kHz, which is supposably workable.

I have dissected a Sharp reciever, and there are no hints there as
to the exact center frequency.

	-Ken



