From: karol+@pitt.edu (Filip M. Gieszczykiewicz (fmgst+@pitt.edu))
Newsgroups: rec.aviation.homebuilt
Subject: making altimiters (was it this group?)
Summary: got a lead... a bit long...
Keywords: yi-ha!
Date: 19 Nov 92 14:18:08 GMT
Followup-To: rec.aviation.homebuilt
Organization: University of Pittsburgh


	PLEASE REPLY TO "fmgst+@pitt.edu" NOT THIS ACCOUNT. THANKS.

	Greetings. I seem to remember that there was a thread here
	on building altimiters (maybe it was sci.electronics?) Anyways,
	this might interest some:

	I came across a 1977 National Semiconductor Pressure 
	Transducer HANDBOOK that - you guessed it - has a few sections
	dedicated to a solid-state altimiter. Here's the first
	few words:

	"An inexpensive, all solid state altimiter, which operates
	from -1000 feet to 50,000 feet is described. It is intended
	as an example of digital conditioning and not as a method for
	manufacture of alitimiters to meet FAA requiraments"

	[Above is called "save-ass" statement :-) ...continuing...]

	"... So, we can summarize what we must do to be able to use
	the [transducer] in an altimiter. First, we must be willing 
	to trim each altimiter we build. And second, we must use
	auto-referencing if we are to approach accepted altimetric
	standards."

	[Alright!! ...continuing...]

	"...we are interested in 510 increments of 100 feet each over
	the range of -1000 feet to +50,000 feet of altitude..."

	[These same folks talk about an approach not being a good solution
	because "[it] requires a very large ROM... 22,528 bits" :-) (Gee, 
	I have 1MB of EPROM now :-)  We're talking 1977! I bet a
	typical 68HC11 would make a killer altimiter with a LOT better
	resolution than 100 feet - since the transducer is capable of
	it... AND you'd have cycles to spare for a wind-speed indicator...
	continuing...]

	"... Figure 4 shows the actual circutry employed in the
	altimiter to do the analog portion of the dual slope A-D."

	[Yup, they're using a LM108 and a LM111 with glue TTL being
	the "counter and control logic". All that this thing needs
	is +15V (and +5V obviously)... continuing...]

	"Even with the excellent function shaping afforded by dual
	slope integration and accurate improvements resulting from
	use of the syringe-auto-referencing technique... it is
	doubtful that standard commercial grade pressure transducers
	can provide system accuracy and reliability required by FAA.

	However, less stringent altimiter requirements for mountain
	climbers, drones, gliders, balloons, altitude rate indicators
	and meteorological mensuration (sic)... are all good candidates
	for these techniques."

	[End of quotes]

	What I want to know is how does the military get the digital
	readouts in their new planes? It has to be a variation of
	the above? How about having 3 transducers and averaging
	them for the altitude? Also, I'm sure a micro would do wonders.

	If I have time - and can fine the required transducer - I'll
	try hacking one of these... (*JUST* as a toy, Mr. FAA :-)

	By the way, the part # is LX1702AN and I just hope that they
	still make them or at least have comparable parts... Also,
	if anyone wants a copy of the...4 pages... just send me a SASE.
	(but first, e-mail to "fmgst+@pitt.edu" for my address :-)

	Also, this book is soooooo cool. They have practical chapters
	on: making a microphone from a transducer, flow-metering, 
	electronic saxophone, flow velocity, audiometer, load cells,
	accelerometers, theory, acoustics, measuring pressure underwater
	by using a rubber kitchen glove :-), mounting, wind speed,
	fuel management, cooling systems, barometer, engine diag...

	[I'm drooling...sigh]

	Anyways, if I can get my hands on a few of these, I can see
	some fun ahead ;-)

	Take care.
-- 
-------------------------------------------------------------------------------
DO NOT REPLY HERE. REPLY TO "fmgst+@pitt.edu". THANK YOU. If that doesn't work,
send that e-mail to "fmg@alpha.smi.med.pitt.edu". Proud co-owner of a Jeanie's
Teenie serial# 1: all-Al, 285lb, low-wing, 53 hp, 120 knts, experimental plane!!
Newsgroups: rec.aviation.homebuilt