So you want to build your own bar monkey? Here's a brief overview of how we built ours.

We didn't want to spend a fortune making the bar monkey, so we spent a long while searching for parts that would work on Ebay. After a month or two of watching and careful bidding we were able to collect all of the main components. These include:

• Soda fountain beverage pumps, such as the Flojet N5000 Series or similar Shurflo models. These are pumps which operate using compressed air.
• A small air compressor, or if you'd rather a quieter monkey, a cylinder of compressed air or CO2.
• To go with the air compressor or air tank, you'll need a pressure regulator. These are fairly common and if you buy a CO2 tank, you can get them at the same place. If you're using a CO2 tank you'll need two pressure regulators, one to step down the extremely high pressure of the CO2 tank to ~100PSI and then a second to get the pressure down even further to ~30PSI to drive the pumps.
• Some kind of electrically controlled valve. We used ice-maker values that are normally found in household freezers.
• Depending on what pumps and valves you choose to use, you'll need all of the appropriate fittings and some high pressure tubing to plumb it all together.
• You're going to need lots of pipe clamps to keep the pipes attached to the fittings. The system operates under roughly 30PSI pressure, so this is critical.
• The brain of the monkey - we'll get to this later.

All of these components are generally not cheap when bought new, but you can occasionally find really good deals on Ebay for sets of beverage pumps. You'll need to spend a fair amount of time running the pumps through with some cleaning supplies to get all of the syrup out. The plumbing and fittings is easiest to find at your local hardware store, although if you're building a large bar monkey you may want to look for volume discounts on some of the fittings. We were able to find a small air compressor at a local hardware store for about $50. The benefit of using an air compressor is that its much cheaper than getting a CO2 cylinder and the repeating cost of getting that filled. Additionally, the air compressor is more forgiving about air leaks. If you can't quite get your system air tight, the air compressor will come on more often, but you won't have to run out and get your air tank refilled.

One of the hardest parts about building the bar monkey is figuring out how you're going to mount all of these components until you do this, its hard to judge how much tubing you're going to need and what the best way to connect everything together is. For our setup we actually connected each pump and valve together and then used a short length of wire to hang each set on a rod across the top of the enclosure. This left the pumps suspended two feet above where the bottles would be located making for relatively short lengths of tube. This also has the advantage of making everything somewhat flexible and able to move as you access the bottles to refill, etc.

Alright, so now you've got all of these parts, and you want to put them together. We need to know how this system is going to work, so lets start with the pumps. Each of our beverage pumps is responsible for controlling one ingredient. There are four openings on each of the pumps. Two for compressed air, and two for the beverage. These are usually clearly labeled on the pumps themselves. We need to connect tubing up to three of the four connections.

The beverage input simply needs a short length of tubing from the pump to the bottle. There is no reason to use reinforced tubing here as the liquid will not be under any significant pressure. Use a length of reinforced tubing to connect the beverage output side of the pump to the input of one of the valves. Using reinforced tubing here is essential because of the way these pumps operate. These pumps convert air pressure into liquid pressure. Essentially the pumps build up an equivalent pressure on the beverage output side of the pump as the air pressure on the input side. What this means for us is, the pump will run until we have a significant amount of pressure being held back by the valve.

The final connection to the pump is to our air supply. For this you'll want some way of splitting the output from your air compressor to each of the pumps. We chose to use T-joints. Since we couldn't find (or weren't willing to pay for) tees that were the same diameter as the pump fittings, we improvised. We found some tiny T-joints used for individual plant irrigation systems, and several lengths of increasing diameter tubing. The tubing fits snugly inside the next size up, so we were able to build a 1/8" tube to 1/2" tube converter by slipping the pipes inside each other and securing with a clamp. There are some very slow air leaks, but they don't effect the ability of the bar monkey to operate. In any case, make sure you're using reinforced tubing here as we are working with 30-40PSI of air pressure. Once you have all of your pumps hooked up to an air tube connect that to the output of the pressure regulator, and connect that to the air compressor. Make sure the regulator is set to give you only the bare minimum to run the pumps. This is generally in the 25-40PSI pressure range. We use the minimum to limit the stress transfered to the valves.

Once you've got all that plumbed up, you've almost got a working bar monkey. The only thing missing is some way of automatically opening and closing the valves. If you've used ice maker valves, these are generally operated by applying 120V to the terminals. This activates an electro-magnet and draws a small pin inside the valve up, allowing liquid to flow through to the output side of the valve. Once power is removed the pin falls back into place, cutting off the flow. There are various ways of controlling these valves electronically, but the simplest is to find a relay controller board. This is a small device that connects to a computer and allows you to switch on and off high voltage/high current circuits. The problem here is that they aren't cheap. This is the best way to go unless you're into building your own circuits and low level programming.

Of course, we weren't about to shell out a lot of money when there was a chance to build our own electronics. We had a small demonstration board for a networking microprocessor around so we decided to use that. The board has a spot for attaching an IO expander which we had done previously and this allowed us a number of IO pins to wire up relays to. The problem is the relays need much more current than the IO expander can supply to operate reliably, so a second IC is required to interface the relays and processor correctly. The microprocessor is programmed to listen for connections from the network, which simply tell it which pins to toggle and for how many cycles to keep the pin activated. This turns on the connected relay, which then allows the 120V supply to reach the ice maker valve. Once the valve is the built up pressure causes the liquid to dispense out the end of the tube and into your cup. The pump will automatically turn on as soon as the pressure drops too far.

The user interface is all written in C#, against a SQL Server 2005 database. The client application connects to the processor via a network connection to dispense drinks when requested.