08M temperature and I/O interface

[RobertN]

This is a project to limit the infloor heat to a garage.

The object is to interface with the existing pumps and control system to apply heat to the floor only if the return water is < a set value.

The interface is implimented by breaking the normal direct connection between the thermostat and circulating pump control, with an interface that operates the circulating pump periodically while monitoring the return water temperature. If the return temperature remains below a set temperature during the interval, the relay for the boiler is activated and the pump stays on until the return water exceeds the set value by a number of degrees. The interface will operate only if the thermostat is calling for heat, ie, <50F.

This appears to be doable with a 08M, 2 inputs and 2 outputs and some long time intervals. It would be nice to have some advice about interfacing the temperature sensor, 28VAC I/O, and pump relays.

- What is the best temperature sensor to use with the 08M in the 32 to 130F range? This sensor would be clamped or otherwise thermally connected to the return pipe.

- Would optical isolation be needed on a 28 VAC 08M input, or a simple diode, capacitor, resistive divider, and Zener diode circuit?

- Any recomendations for driving low current 28VAC thermostat relay? Would optical isolation be recommended here also? The low current 28V signal, that replaces the thermostat signal, maybe doable with a 5VDC coil relay driven directly from the 08M or with a simple transistor coil driver.

- it maybe best to drive the 110AC 90 watt circulating pump motor with a solid state relay to minimize the interface to the 08M. Or possibly a relay circuit like the 28V circuit above.

I could scratch this out myself, the object is to see how the system works with a minimum of gotchas. Any comments or suggestions appreciated. I don't have a way to, or know how to get a schematic on this page other than manually drawing it.

 

[Dippy]

"I don't have a way to, or know how to get a schematic on this page other than manually drawing it"
- no, it's a real pain that they don't draw themselves isn't it
You will have to create an image then upload it with your post.

"Thermostatic Relay" - sorry can't help, I don't know how yours works.
Why do you need them if sensing using PICAXE?

The DS18B20 is the most popular choice for temperature measurements as there is a nice built-in command (Readtemp) to make life easier for you. SADLY, Manual 3 (Interfacing Circuits) makes no mention of it.... but manual 2 shows how to use it under 'READTEMP'.

PICAXE inputs are maximum 5V DC if your PICAXE is running at 5V DC. For 28V DC I would have thought the easiest and safest would be an AC optocoupler. No doubt you will be told of other options.

For switching mains a mechanical or SSR is fine. Read Manual 3 about controlling mechanical relays and read up the data sheet pn your 'favourite' relays to make sure coil and contact spec are OK. I suggest you avoid cr*p from Ebay. Get a proper one with a Data Sheet. SSRs can be used obv.

I think a Schematic will help others here. I'm afraid you will have to draw it as the Clairvoyancy subroutine on the Forum has failed due to unforeseen circumstances

General Questions:
How much electronic and/or programming experience do you have? Do you know your anode from your cathode?
Are you expecting people to provide a full schematic and 90% of the code?

 

[RobertN]

Thanks Dippy,

I have worked the electronics side of vehicle transmission controls for years, do not expect software or schematics, just don't want to pick components or approches that may not be a good choice for the job.

I'd prefer to use an analog temperature sensor like Infineon KTY or other. I'm not into serial comms even with 'readtemp'. After all Picaxe is the first programing I've done and have enjoyed it so far.

I have Microsim 8 but don't know how to change their file type into jpeg or other, let alone create an 'image'. Recently it has taken to kicking out a fatal fault intead of printing.

"thermostat relay" = relay for providing an on - off signal like a common room thermostat. The object is to replace the function of the common wall thermostat and sense the floor temperature via the return water temperature rather than the room temperature.

It looks like SSR would be best for handling the AC I/O. Will have to look to see if there are isolation modules for converting 28VAC to lodgic levels that won't break the bank.

Comments and suggestions appreciated.

 

[westaust55]

I don't have a way to, or know how to get a schematic on this page other than manually drawing it.

Search for and download one of the free pdf converters. To your PC software they look just like a printer driver but output to t .pdf format file.

For a simple sketch, I often draw up a diagram in Excel then print to the pdf writer/converter. This file can them be uploaded.

For the temperature range 32°F (0°C) to 130°F (54°C), this is well within the capability of the DS18B20 temperature sensor (-55°C to 125°C).
As Dippy says, it is the most popular for PICAXE users as the READTEMP commend is design to work specifically with the DS18B20.

I'd prefer to use an analog temperature sensor like Infineon KTY or other. I'm not into serial comms even with 'readtemp'. After all Picaxe is the first programing I've done and have enjoyed it so far.

Why? the DS18B20 is extremely easy to use.
The DS18B20 only involves 3 wires and a resistor. Look at PICAXE Manual 1 page 60 for the wiring diagram

 

[Dippy]

Have I got this right;

You currently have a thermostat which operates a mains electric pump?
You want to insert a PICAXE controlled circuit which does some other temperature processing between thermostat and pump?

The thermostat is powered by 28V AC?
The output from the thermostat is 28VAC? (Or has it a built-in mains relay to switch the pump?).
If the output from the thermostat is 28VAC there must be another relay somewhere.

This is where a schematic of the current and proposed system is so useful - even if it's just hand-drawn (neatly) and scanned.... indicating the voltage levels too.

People could spend hours thinking and suggesting something and you might say "Na, its not like that" - the more help you can provide then the more help and speed others can provide.

 

[stocky]

hope this helps with the the 28vac input....

look up HCPL3700 opto - allows you to sense any voltage up to about 250vac with a single resistor to set the threshold - output is LOGIC LOW - perfect to interface to picaxe!

This opto has options for AC and DC inputs although i use just the AC pins as its not polarity sensitive then.

I used to interface 24vac solenoid outputs from domestic irrigation controllers to logic level inputs on MCU's, RTU's etc etc - very handy

I use a resistor of 4k7 for the 24vac application - gives a threshold of 18v from memory.

Output - I too would suggest a SSR - well within picaxe pin drive capabilities

Stocky

 

[Michael 2727]

An 08M using a DS18B20 for the Temp Sensor and an 08M output driving the LED in a SSR
would be the simplest way to go. ( bypass the 28VAC all together, messy stuff 28VAC )

The 08M only needs a single I/O pin to communicate with the DS18B20 (pin 1, 2 or 4) plus Pos +5V & Neg - 0V, and 1 x 4K7 resistor.
The Readtemp command does all the work for you, the value gets put where you assign it,(b ? or w ? ) simple.
Note: Use a 5A to 10A SSR with zero voltage detection/switching for the motor.

2 x I/O pins left over, you could even include an alarm using "Tune" and still have
room for a 7 Seg Display.

Use a 5V DC regulated plugpack/wallwart for the picaxe supply.

 

[RobertN]

stocky,
Thats the kind of information I was looking for, a proven recommended part. The HCPL3700 looks like it will do well with most any high voltage input into a Picaxe. What SSR do you recommend for interfacing with a 110 VAC 50watt circulating pump motor, there is a ton of them out there?

Dippy,
This circuit is inserted between the original 28VAC (powered and output)thermostat and the pump control box. The control box has relays and some lodgic chips that turn on the boiler and circulating pump anytime there is a call for heat from any one of 6 thermostats. Would like to minimize the operation of the boiler (propane = $2.00/gallon) while maintaining the garage floor above freezing. The 08M would do this by controling the garage circulating pump separate from the boiler control box, measuring return water temperature, and turning on the boiler if needed via the thermostat input.

westaust55,
I'm not adept at moving, downloading, etc. The majority of my computer experience was on a company system that strictly limited downloading, file manipulation, etc. Having to come up to speed at home. Just got a printout of a MicoSim file (that caused a fatal error when trying to print) by exporting it as a dfx to AutocAD and printing from there. Need to make a schematic of this project and see about displaying it here.
The KPY sensor requires only 2 wires, a resistor, and calibration. May try the DS18B20 to see how the slow conversion affects things.

Thanks guys for the input.

 

[westaust55]

pdf file creation

For a pdf creator program use this link:
http://www.pdfforge.org/products/pdfcreator/download

and click on the top (of two) link with the title "Download PDFCreator 0.9.5"


Install the program and then you can convert almost any file to a .pdf type which can be uploaded here.

If MicoSim is your drawing package you just select the pdfcreator instead of your normal printer when you want a pdf file.