HVAC Controls.

I have been working for a while in some spare time to build my own controller for the use in building HVAC applications. I wanted to prove a few of the circuits so I squeezed as much as I could on a 10cm x 10cm board.

So this is very much just a proof of concept to check that the elements work.

SMD parts were soldered using a mini oven from Argos. I have to confess this is my first time using this method and on my first attempt I heated up the board too quickly which caused what can only be described as electronic popcorn.
The second time result is not perfect but a relative success. I set the temperature manually as required by the SMD paste temperature graph every 30 seconds.

The board has five universal inputs, selectable as follows using the jumper adjacent to each input:

• Digital input.
  
• Current 4-20mA input. Note that loop to be externally powered.
  
• Thermistor Input.
  
• Voltage 0-10vDC (Not PWM).
  
  

Four analogue outputs 0-10vDC (20mA max)

Four digital outputs via relays.

The board has a set of female headers on the back to plug the Espruino into. Note that my Espruino has the pin headers mounted in a downward direction.

I have tested some of the very basic functionality.

• Thermistor reading but not as stable as expected. Further investigation required.
  
• Only two of the analogue outputs are working correctly. Suspect this is down to the tiny resistor network components not soldered quite right.
  
• Only one relay soldered on so far but working.
  
  

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Wow, that looks really nice! How are you doing the analog outputs?

I'm pretty pleased with it too considering its only the second PCB I have designed!

Analog outputs are generally as below.

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Just a quick update.

ALWAYS USE PROTECTION... I should have known better really but I got lazy and there were actual flames, but no injuries!!

Connected the 24VAC GND to the DC GND and it really does not like it when using a bridge rectifier.

After a small moment of panic and further investigation, it appears the only damage was the bridge rectifier, primary side rectifier PCB tracks disintegrated and the two tiny jumper wires I was using connected directly to the 24VAC transformer were the items that caught fire.
Most importantly the Espruino is still okay, its still relatively isolated being powered via USB at the moment.

Next week I will replace the bridge rectifier with half wave rectification, use some protection on the secondary side of the transformer, make sure the wires are suitably sized and try again. In theory this should do the trick.

I have finished adding on all the input connectors and hooked up a thermistor bead on each input. They at least seem to be working just fine now.

Need to track down a 4-20mA sensor to test. I'm sure I had one kicking around!

Argh. Still, glad the damage is still relatively superficial! Hope you get it up and running properly soon, it'll be great to see how it works :)

@Gordon
There has been slow progress with the project.

However new year and its my intention to make further progress with this project. I want to design the next version which is to be compatible with the new Pico.

Would it be possible for you to create an Eagle part for the new Pico so I am able to get a head start for when it arrives?

Yes, someone asked in another thread too. I'm waiting until I get the final ok from the PCB manufacturers... Because the boards will have castellations they may still need a few tweaks to the outline so they can be tiled on a panel nicely.

Once that's done I'll release an eagle library though... It should only be a week or two away.

...flames ...panic

A fuse - on the board - wouldn't hurt - would it?

@StuntMonkeh I just found this. Your doing a lot of what I want from an general industrial board.
I was going to get someone to quote to the design I'm wondering if we could collaborate?

There is an example on this thread for a 4-20ma source generator
http://cr4.globalspec.com/thread/2905/4-to-20-MA-Source-Circuit-Diagram

Lawrence

My Pico arrived through the letter box this morning. Good work Gordon with the delivery of the Kickstarter project.

This little project will now be alive again to roll into its second iteration after a great deal of learning from the first!

@LawrenceG I have sent you a PM.