I would appreciate any advice with my project!

I'm trying to add some additional control to a central air conditioning system in a workshop. I'm going to use a series of 555 timers to control the amount of time the compressor stays on and off: like 15 minutes on and 10 minutes off, and keep the fan circulating the inside air for 5 minutes after the compressor turns off.

I need to take the 24 VAC signal that turns on the compressor and translate it into a TTL (0 or 5 VDC) signal to trigger the 555 timer to begin a new on/off cycle. Then I need to use the TTL output of the 555 to switch the 24 VAC signals to the compressor and the fan on and off. I figured a simple relay for the switch--any suggestions for a low current 24 VAC relay are welcomed! What I don't know how to do is the 24 VAC to TTL conversion.

The three choices I can see are: 1) a dedicated IC that does the job for me; 2) half/full bridge rectify the AC and use an RC filter to keep the signal high or low, then use an optoisolator to convert the rectified signal to TTL; 3) use an optoisolator to convert AC to TTL--on/off 60 Hz--and then use an RC filter to keep the TTL high or low.

Any suggestions? Thanks in advance!

The three things I want to accomplish are:

1) Adjustable duty cycle to turn the compressor on/off when the 24VAC signal from the thermostat is active . I can do this with one 555 timer, if I can translate the 24VAC to TTL.

2) The inside fan will come on when the compressor does and when the compressor turns off, keep the inside fan blowing for 5 minutes. When the output of the compressor 555 timer goes low, trigger another 555 to countdown 5 minutes. I figured using an OR gate to have either the compressor 555 or the fan 555 keep the fan relay open.

3) Insure that the compressor and fan run at least 5 minutes out of every 60 minutes to control the humidity. Another 555 timer with a 5 minute on and 55 minute off cycle time. The output of this 555 timer would be OR'd with the output of the thermostat signal, and this 5 minute 555 timer would be reset when the compressor 555 timer turns off, starting a new 60 minute period.

Any feedback is welcomed! I might try to do this the digital way, but I think I can do this--the analog way--quicker. I haven't done any work recently with microcontrollers, and have looked a little at the "pic", "stamp", and the "Z8 Encore" controllers. If anyone has links to "developement kits" for these controllers, please post.

Thanks for your attention!!!

farrier
Posted on 2004-07-10 16:11:55 by farrier
Fujitsu has the relays you are looking for. I just constructed a project like that. The Fujitsu FTR-K1CK048W is a 24VDC at 250VAC/16Amps. I used an opto-isolator coupled to a driver transistor through a half wave rectifier as a second DC source. I was very constrained in dimention of this PCB. You can use a PIC processor for your timing and TTL level triggers. The TMR0 overflow interrupt is very accurate and will reduce you part count dramatically.

P.S. Fujistsu of California's phone number is 800 380-0059
Posted on 2004-07-13 07:23:16 by mrgone
Since you are a fan of the 555, I would say, use a 555 to tell you when the 24VAC is present. Turn the signal into a square wave (use a resistor + 5V zener combination, for example). Use that square-wave to trigger a 555, configured as a one-shot, with a period longer than 16.66 msec, say 25 msec. That way, at each falling edge of the square-wave you trigger the one-shot, but you trigger it faster than it can recover, so the output stays H as long as the 24VAC is present. I hope this is what you meant.

As for the PIC's, those are a great option, probably the best by far. You could probably solve your problem with a single PIC12x508. That can take care of the 24VAC sensing and all other timing. Since the timing is not critical in your case I would use the internal RC oscillator. That will free up two pins, so you have maximum six I/O's available (5 I/O's, 1 input only, if you use the internal reset). The internal RC oscillator is within 5% at room temp. That is an acceptable error for your application.

The development tools are free from Microchip. You need a programmer, though. However, you can build a simple one. See links below. The programming commands/ algorithms are similar, so the same programmer would work with most devices.

Check out these links:

http://ww1.microchip.com/downloads/en/AppNotes/00589a.pdf
http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdf
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en019469&part=SW007002
Posted on 2004-07-13 11:47:05 by VVV
Trying to post a schematic. "It worked! Ureka!" Anyway the input to 330 ohm resistor is directly connected to PIC I/O pin to control the 24V being switched through the transistor.
Posted on 2004-07-13 12:35:39 by mrgone
I went through all that only to discover I miss understood your question. Yeah if you run the 24VAC though a single rectifier diode you will end up with approx. 12V pulsating DC. Run that through say a 1K resistor and to an electrolytic capacitor of atleast 100uF from resistor to ground. Then use a LM7805 (5V voltage regulator) for your TTL voltage.
Digi-key has all kinds of low current relays. Phone: 1-800-Digikey
Posted on 2004-07-13 13:04:11 by mrgone
From PCB layout pointer, have a TTL side and a AC side with the optoisolators/relays in the middle/DMZ zone. This cut down on the fry them up accidents, that happens from time to time.

Regards, P1 :cool:
Posted on 2004-07-13 14:02:47 by Pone
Thanks for all your suggestions!:)

Right now I'm going to stick with three 555 timers to do the counting/timing, but I've ordered a Z8 Encore developer kit for $40 US to play with and I'm going to start playing with the pic's, once I figure out which one to use and how to program it. I found a number of posts here about programming pic's but I'm still not clear. Thanks for your links, VVV, I'll look at them tonight.

For the 24 VAC sensing, I found the MID400. A little gem of a chip that does what I want and works in my test bed. It will take an AC or DC input, full bridge rectify it and tranlates this signal thru an optoisolator to a TTL output (low when the voltage is present, high when the voltage disappears, for use as an interrupt signal)

For the relays, I found a Fujitsu double pole relay and it does exactly what I want: Intercepts the signal from the thermostat and lets me create my own, as signals to the fan and compressor.

The only problem I see is: If I start playing with pic's and Z8's for control projects, something's going to spark or smoke, that never sparcked or smoked before! I hope not, but I'm having fun.

Any more sugesstions on which pic chip to use and/or how to easily program them will be appreciated!!

farrier
Posted on 2004-07-13 14:20:15 by farrier
What your looking to do is more or less my day-job ;) . I would spec something like the following product (if i didnt have PLC control).

http://www.findernet.com/en/products/profiles.php?serie=86&lang=en&gruppo=gruppo1

You should get the job done with a combination of them:

AE model should get you going with the 5min run after all has been off for 60 minutes.

BE model should hande the run fan with compressor, and extend for 5 min. When fan is off, the AE model is kicked into gear.

LI model should handle your duty cycling on/off of the compressor.

They all come with 24VAC options, but your looking at about 100$ for relay parts tho (in Canada anyways).

Regards,
:NaN:
Posted on 2004-07-13 16:57:08 by NaN
But NAN you should know that cost is the number one consideration in any design...lol. But Programmable Logic Devices are a cheap alternative in themselves. I need to reveiw my boolean's algebra with them...lol
Anyway if you want a good PIC I would suggest the PIC16F84 which has been discussed here much. I have complete software for highly accurate clock or time peice complete with LCD display driver(software) for microchips own AY0438 LCD driver chip.
Posted on 2004-07-13 20:05:24 by mrgone
NaN,

Relays that do the logic and all the thinking for you, WoW. But I think one relay would cost more and be larger than my entire project so far! I've bookmarked that link for future reference though, for a future dream project;) .


mrgone,

I'm going to order some pic packages tomorrow. Not sure if I'm going to try to build or buy a programmer though. Not enough data in the cabeza yet.

Still trying to get some problems with the signal synchronization, almost there.

Thanks again,

farrier
Posted on 2004-07-13 22:44:59 by farrier
I would just get the PICstart. The programmer is cheap enough and you can focus on the PIC it's self. It sounds like timing is a critical issue with your project so exploring the PIC's timer mechanism sounds ideal for you. You could have the fan turn on after 33hrs,22mins and 11 seconds and 4/256 secs. if you want. Basically your timing issues will be resolved for here on..... Hey we're here to help so if you don't ask it's your own dern fault :grin:
Posted on 2004-07-14 09:14:50 by mrgone
finder is a bit pricey. There is many other manufactures of such things. The one I initially looked for online (at home) was for Pheonix Controls' relays. They have cheaper and more compact control relays of similar quality. Practically everybody has some form of smarter relay with some neato type of logic these days. I realy like the alternating relays, where every time a signal is applied the output contacts that holds closed alternates between two contact sets. This is fantastic for pumping stations, as it ensures that both the lead and lag pump both get roughly the same amount of operating duty (basically alternating which pump should start when the low level float is tripped). No PLC needed, and very robust. Enought logic to ensure one pump doesnt rust.
Posted on 2004-07-14 22:55:29 by NaN
Sorry to be comming in here a little late but...

My choice for a (seems to me) one time use project like this wouldn't necessairly be a PIC

(OK, *MY* choice it would be, but I have a ton of expierence and development tools for em).

Try a Basic Stamp, you can get the educator kit for 80 bucks right now, that gives you a programmable unit with plug in breadboard space to experiment on. Once you have your design you can solder it down permanent.

The unit is programmed in Basic over a serial cable. Even has debug windows available if you need.

I keep one around for sanity checks for debugging hardware projects (currently using it to keep tabs on some I2C parts)
Posted on 2004-07-24 06:53:27 by Ernie
I'll give an update:

Thanks for your input Ernie!

I ordered a Z8 Encore Developement Kit for $40 US and I'm having all sorts of fun with it and I may use it as the basis of my controller.

http://www.zilog.com/products/partdetails.asp?id=Z8F64200100KIT

The IDE allows you to write in C or Z8 asm. The board has 2 60 pin headers and a 26 pin header (with 12 10bit A/D connections), 4 timers, I2C AND SPI interfaces, IRDA built in, 60 I/O pins, ...

I got some PIC chips and I fully believe I can whip something together to program the PIC chips with the Z8 Encore kit!

But for now I've got 3 555 timers running the system with most of what I wanted to do, and I'm going to re-learn C and learn Z8 asm and transfer everything to the Z8 board. This will allow me to eliminare the sizable capacitors I had to use to achieve the 20 and 60 minute delays I need.

Thanks for all your input. The only thing I haven't explored is the Basic Stamp options; I'll look at them in the future. I'm coming up with all sorts of ideas for new projects.

thanks again!!!

farrier
Posted on 2004-07-24 12:19:12 by farrier

I'll give an update:

Thanks for your input Ernie!

I ordered a Z8 Encore Developement Kit for US and I'm having all sorts of fun with it and I may use it as the basis of my controller.

http://www.zilog.com/products/partdetails.asp?id=Z8F64200100KIT

The IDE allows you to write in C or Z8 asm. The board has 2 60 pin headers and a 26 pin header (with 12 10bit A/D connections), 4 timers, I2C AND SPI interfaces, IRDA built in, 60 I/O pins, ...

I got some PIC chips and I fully believe I can whip something together to program the PIC chips with the Z8 Encore kit!

But for now I've got 3 555 timers running the system with most of what I wanted to do, and I'm going to re-learn C and learn Z8 asm and transfer everything to the Z8 board. This will allow me to eliminare the sizable capacitors I had to use to achieve the 20 and 60 minute delays I need.

Thanks for all your input. The only thing I haven't explored is the Basic Stamp options; I'll look at them in the future. I'm coming up with all sorts of ideas for new projects.

thanks again!!!

farrier


I was thinking of buying one of those Z8 Encore starters as well, but I have a homebuilt PIC16F628 programmer, lol I can program the chips all I did was make an LED flash at a 500ms interval, but I have no idea what else to do with them, I have like 8 chips just sitting there I can do something with them lol. How is the Z8 Encore kit? $40 US is quite steep for a Canadian :D but I think it is worth it.
Posted on 2004-07-24 17:15:08 by x86asm
"Minutes" timer, use 555 is not good, because "minutes" duty cycle is very long time.

Try this ..

CMOS 4541 (Used in many low cost timer devices).
http://wwww.ges.cz/sheet/m/mc14541.pdf
Posted on 2004-08-28 02:05:12 by Kestrel
Kestrel,

Thanks for your reply. Actually the only problem I experienced was the large size of the capacitors required for 60 minute cycles. The actual performance was acceptable--plus or minus 1 minute over 60 minutes.

I changed everything from analog to digital, and I can only say one thing: I'm digital and I ain't goin' back! The Z8 developement board was a little overkill, but has worked great. Very flexible and easily programmable/reprogrammable.

My next step is to do the pic version. I only have to decide how to program them and how to include them in a circuit. Mere details!!

farrier
Posted on 2004-08-28 10:25:27 by farrier
My next step is to do the pic version.

at PIC version, maybe you need try PIC12C508A:
http://ww1.microchip.com/downloads/en/DeviceDoc/40139e.pdf

in MicroChip's MicroControllers, PIC12C508A is lowest cost, and smallest size, 8 pin only. So, many Electronics LTDs used it made many low cost products.
Posted on 2004-08-28 14:05:19 by Kestrel
PICs are alot cheaper. The pic will calcutate your timer beautifully. If you don't want a controller than the AC plug is a pretty accurate souce for timing seconds. If you are in USA than you have sixty cyles per second that can be counted with counters. I like the 74hc191's because they are 4 bit binary counters (count to 16) and can be jammed loaded to be anything less than 16 like a decade counter. Just couple the AC to the clock input of first counter with a small capacitance of 1000pf or less then place a 10K resistor in shunt with clock input to give reference.
Posted on 2004-09-01 16:28:54 by mrgone
I went back and looked at a design I did a while back and the shunt resistor was 470K. Quite a difference from 10K. LOL the point is you need a DC reference. Also you may include some series resistance in between coupling cap. My design required I use 470 ohms because it had two purposes.
Posted on 2004-09-04 00:13:03 by mrgone