Home > Tech geeks > Pump Skid Design Musings

Pump Skid Design Musings

My batch of single-sided presensitized copper-clad boards arrived today from Allied Electronics and I am ready to etch my boards for the pump skid control box. These things are 9″x6″, so I will cut them down to smaller sizes as needed. I have found that both a jig saw and mitre saw do decent jobs, although both require a little grinding / filing after the fact to get the rough edges smoothed out. A couple passes with the Dremel takes care of that, no problem.

The pump skid circuit designs have gone through a few changes since I originally started working on them, and although most of you couldn’t care less and probably read this stuff as technobabble, I figure I’d go ahead and document where the design is and how it got there.

First off, I simplified my 3-board approach (sensor, control panel, and main board) into two boards, a main board w/ sensors and an auxiliary control panel board. This allowed me to reduce alot of jumper wires on the main board and simplify the overall design. I kept the control panel board seperate for a few reasons – If I find I want to change around the control panel I won’t need to re-etch the main board, and if the main board must change, I can at least (maybe) keep the control panel board the same. It also allows me to do a test etching / drilling / soldering on a much smaller scale than trying to tackle the main board first, which allows me to develop confidence and skills prior to the “main event”.

So here are some of the design thoughts that went into the current iteration of the main control board:

Power – The main board houses the power supply (Cosel VAA505) which can step down 120V AC to 5V DC in a single step. The board is protected by a 2A slow-blow fuse (as recommended by the manufacturer) which may not save my ICs in the event of a big problem, but it should at least protect the power supply. Power is fed into the board via a 3-pin .156″ header connection (much bigger and beefier than other header connections on the board).

Serial Communication – The standard 3.5mm (1/8″) stereo jack connection used to upload programs to the Picaxe chip (and receive feedback via the SERTXD command) has been replaced with 3-pin .100″ header connection that will connect a panel-mount stereo jack on the back of the control box. This will allow me to plug into the box without having to open it up and potentially expose myself to live electricity. This will be used infrequently to tweak the PICAXE code and will not be the primary mode of receiving data – for that I’ve included a place for a 315Mhz 2400 BPS wireless transmitter from SparkFun Electronics. Being wireless means the pump skid can sit close to the main brew sculpture and transmit data back to it without having a physical wire running between the two – one less tripping hazard to deal with.

Integrated Circuits (ICs) – There are 4 main ones on the board; The Picaxe 28X1 that functions as the heart of the automated functions, a CMOS 4001 quad nor gate to combine the “automatic” control signals generated by the Picaxe with the manual switch signals from the control panel, a ULN2803A Darlington driver to somewhat decouple the solid-state relay (SSR) signals from the rest of the circuit, and a DS1307 real time clock that is used to count elapse seconds between the filling & pumping operations. There is also an on-board LM34 digital temperature sensor that will report the internal control box temperature, although at this point it is more for information than anything else, and will show me if my cooling fan is undersized.

Headers – Good god, there are alot of them. There is 10-pin header that connects the main board to the control panel board, a couple 2-pin connectors for the SSR signals, loads of 4-pin headers for the sensor connections, and a few others thrown in. This board is gonna look like a nightmare medusa once everything is all connected up.

Resistors – There are really only 4 resistor values being used: a single 330 for the on-board power LED that mostly just provides the circuit with a means to de-energize when turned off, 4k7 for switch and sensor signals, and the obligatory 10k and 22k resistors for the Picaxe download circuit.

Misc. Empty Pads – there are a few places where I put pads in for “just in case” circumstances. There is a place for a decoupling capacitor in case my Picaxe gets fritzy once everything is running (I’ve never tested the whole design together, and weird things might happen). There are several tie-ins to the 5V and Ground rails in case I need to add something else (like more smoothing or decoupling capacitors), and pads for the unused input and output legs of the Picaxe. I also put in a place for a crystal resonator, although I expect it will be unused since the 28X1 has an internal resonator – again, it is a just-in-case. The wireless transmitter has a pad next to it for an auxiliary antenna in case the signal strength sucks at 10 feet. I sure hope not, but you never know.

Some things I would like to change (but not in this version) are:

(1) Improved plugs for the level and temperature sensor switches. While phone jacks work and are readily available from my Home Depot, they are not very robust. Since I order so many components online anyways, I should really have spent more time identifying the correct plugs to use.

(2) Use of a MCP23008 or 016 I2C I/O expansion IC to localize the control panel functions and reduce Picaxe pins used to a couple I2C connections (instead of a 10-pin header). This would also free up more digital I/Os on the Picaxe, and I could actually use 2 more analog channels to take temperature readings (maybe some LM34s mounted directly on the SSRs).

(3) Make the main board double-sided to eliminate the jumper wires and reduce the overall size requirements. I know the layout is currently sub-optimized, but given my level of inexperience in this whole process, I think I did pretty well this time around.

(4) Use a backlit 20×4 LCD screen on the control panel to display temperatures, elapsed time, estimated sparge rates, etc… I have the component, but since this totally gratuitous it can wait for the second iteration to come around.

(5) Make the signals from the grant vessel to the main control box wireless – this would eliminate 3 holes and a ton of jumpers & wiring. It would also require me to rethink the grant design a bit because it would need onboard (battery) power and its own PCB, but how friggin cool would that be. Not only could it transmit its data to the pump skid control box, but it could also send it to the main brew sculpture. The only problem with that approach is the control box would still need to send its signal as well to indicate the pump statuses, internal box temps, and manual switch statuses… So I would need to figure out how to prevent data collisions… and it wouldn’t be possible using the wireless gear I linked to above from Sparkfun. Again, a longer-term development item.

I’ll try to take some pictures as I prototype the boards so I can give you all a better idea of what the PCB process is (for those that haven’t seen it). That post probably won’t come until later in the month when the Dremel drill press has arrived…

Categories: Tech geeks
  1. July 15, 2008 at 7:37 pm

    … and I’m having trouble just balancing a kegerator šŸ˜‰

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