On 21 Sep 2012 at 13:54, Dwayne Reid wrote: > Good day to all. >=20 > I have just finished the redesign of one of our older products for a=20 > long-time (and valuable) customer - and now I'm being asked to make=20 > more changes. The version just finished will go into production but=20 > I need to start on the new version. >=20 > This is a control board in a HVAC application and runs from=20 > 24Vac. There is 120 / 240 Vac switching on one end of the board and=20 > all of the low-voltage electronics is at the other end of the card. >=20 > The card includes a sensitive differential pressure sensor which is=20 > used to control the blower speed. The plenum pressure changes=20 > according to which mode the system is currently operating in:=20 > fan-only, heating, cooling. Pressure adjustments are made using=20 > fairly expensive 3-turn trim pots (3 pieces, one each for cool, heat, fan= ). >=20 > We want to get rid of those expensive trim pots and instead use=20 > up-down programming buttons in conjunction with an on-board=20 > display. I need an absolute minimum of 3 numeric digits but would be=20 > really happy if I can find a low-cost alpha-numeric display of at=20 > least 8 characters. A one-line display would work but a two-line=20 > display would allow for some eye candy (a simple menu system, for example= ). >=20 > The current power supply is a simple half-wave rectifier (fed from=20 > 24Vac) with series resistor feeding a reservoir cap (sits at about=20 > 24Vdc) followed by another series resistor feeding a 5V zener diode=20 > regulator. Because I want to keep the wasted heat at a minimum, the=20 > current 5V supply is fairly limited as to how much current that it=20 > can supply. That pretty much rules out any form of 5V LED-based=20 > display, unless I change that 5V supply to a buck SMPS of some sort. >=20 > I see a few options - but they all sound to be more expensive than my=20 > customer wants to spend. But I'll toss them out here in hopes that=20 > someone can suggest alternatives. >=20 > 1) 3- 7 segment displays running from 24V rail. Means that I have to=20 > use something like 1 or 3 TPIC6C595 shift registers (which can handle=20 > the 24V rail). One SR if I multiplex the displays, 3 if I don't. >=20 > 2) standard intelligent LCD display module. Quite frankly, this=20 > would be my preferred method but I just can't seem to find any=20 > low-cost but tiny modules. Suggestions would be greatly appreciated. >=20 > 3) one of those incredibly nice Organic LED graphic modules as seen=20 > on the Gabotronics Xprotolab modules. His BOM gives it a part number=20 > of UG-2864HSWEG01 and he sells them for $10 each. The downside with=20 > using one of these are: need more PSU current than I have available=20 > and needs a processor with much larger pin count than I am using=20 > now. Neither of these is a killer but I'm looking for=20 > alternatives. I'd also be interested in finding a reliable low-cost=20 > source of these. >=20 > 4) bare LCD glass. Needs a different, much larger PIC than I am=20 > currently using (or a small PIC and lots of shift registers). >=20 > Any suggestions? >=20 > Many thanks! >=20 > dwayne On the small side but not truly tiny, 58x32mm overall, Winstar WH0802A 2 x = 8=20 character LCD. LED backlight could be the power killer, unless you don't us= e it then=20 it isn't. Around USD5.00 or so in low quantities. http://www.winstar.com.tw/products_detail_ov.php?ProID=3D13 --=20 Brent Brown, Electronic Design Solutions 9 Titoki Place, Pukete, Hamilton 3200, New Zealand Ph: +64 7 849 0069 Fax: +64 7 849 0071 Cell: +64 27 433 4069 eMail: brent.brown@clear.net.nz --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .