This prototype circuit is a good example of how my design process works. The point at which I built this prototype was in the middle of the design process, and not the starting point. The customer already had an electronic design, and wanted to have a Printed Circuit Board made. So, I replicated the electronic design with the circuit design tool I was using at the time (OrCAD). Then generated the Printed Circuit Board layout. Once we had released the Gerber files for fabrication is when I decided to prototype the design while we were waiting on the fabricated Printed Circuit Boards.
My main concern is that we got all the connections that needed to be made hooked up correctly. There was one enable pin that needed to be hooked up in order for the circuit to work properly. I was able to stop the PCB Fabrication House before they printed the PCB, and get the extra connection added. The Printed Circuit Board worked the first time once we had received, and assembled it.
The operation of the circuit works like this. First is the oscillator is a Quad Op Amp, LM324, which is configured as a relaxation oscillator. The LM324 has four Op Amps in it, so I used one for the relaxation oscillator, and the other three as buffer amplifiers. The clock signal is then fed to a digital counter, CD4024, a seven bit binary counter, which takes an incoming pulse train, the clock, and counts the number of pulses. The output is in binary, and there are seven output pins for the seven bits. The high order bits (4, 5, and 6) are fed to the sequencers address bits. The sequencer is a MC14051 Analog Multiplexor. The address input bits select which output channel get connected to the source. So, when you input address bits in binary order they will switch the outputs in numeric order, the sequence. This could also be an arbitrary selection if you were to use a processor to drive the address inputs.
This prototype is typical for small electronic prototypes. I use single sided printed circuit boards with 100 mil hole spacing, and plated with a pad per hole layout. All the connections are made with discrete wires, in this case 26 AWG solid core wire. Not all the components have 100 mil spacing connections, and you wind up bending legs, and modifying the board as needed to accommodate the components. I have made hundreds of prototype bards similar to this to work out circuits, and do proof of concept work. This one is fun because it is visual with all the LEDs.
Ethanol claims another victim. Small engines generally do not like Ethanol. Cheap pump gas usually contains 10% to 15% Ethanol. Ethanol eats the seals inside these small carburetors. So, if you use cheap pump gas, and leave it in your small equipment continuously then eventually you will have a fuel system breakdown, and have to repair your small gas powered equipment.
In this case it is a Stihl MM55C Tiller. It is a good Tiller, but I don't use it very often. Generally I use it a lot in the springtime, then it sits there the rest of the year. Well, I left fuel in it over that time. One springtime when I went to use the Tiller it wouldn't start, and wouldn't crank either. I try to pull the cord, and the motor wouldn't move at all. Well, it had a hydrostatic lock. There was liquid fuel in the combustion chamber, and it wouldn't let the piston move up. So, I pulled the Spark Plug out to relieve the pressure, and it spewed liquid fuel out of the Spark Plug port. Then I realized that a seal had failed in the carburetor, and was letting liquid fuel get into the engine. Big Puppy Sigh...
So, this is pretty typical of small engines. The equipment is kind of old, a few, or more years. I had been thinking that I had been using "good" gas, Super, 92 octane, but it still has Ethanol in it. Small Engine Manufacturers usually will have a compatible fuel available for their equipment. Stihl makes Ethanol Free 2 Cycle Fuel for their equipment. It is expensive, but is it worth the time, and energy needed to repair a damaged fuel system? So, the Carburetor is well gunked up due to use, and years of sitting. Well start pulling the fuel system apart.
Stihl makes small engines very well. They have a clean design, and are simplified as much as possible. They take service well, and I can get parts fairly easily. There were three screws in the back cover, one screw in the fuel tank, one screw in the air cleaner, and two nuts held the carburetor on. All the fuel lines are press fit, and there is some tricky part shapes that make everything fit together when the screws are in. It is refreshingly simple.
The Fuel Tank is pretty icky, as usual. I bought all the fuel system parts, so we'll be replacing the fuel lines, fuel filter, and the carburetor. Generally I like to get a rebuild kit that has all the replacement parts for the carburetor, but in this case the only option was a replacement carburetor.
The Fuel Tank itself needs to be cleaned, not just to get the gunk off the outside, but also to flush out the old gas inside, and any particulates that have got into the Fuel Tank. So the Fuel Tank get a full kitchen dish washing with soap, and hot water. We want to totally flush out the Fuel Tank, and then let it dry out completely before re-assembling the Fuel System.
When re-assembling the Fuel Tank you can use some Silicon Spray to lubricate the press fitted Fuel Lines. A little Silicon Spray will help the Fuel Lines to go on easier, and then the Silicon Spray will dissipate into Silicon Hoses with no problem. It is certainly looking much better at this point.
Installing the new Carburetor is making my Small Engine look a little more complete. I salvaged the exterior gaskets from the old Carburetor. They seemed to be in marginal shape. I used a little more Silicon Spray when I installed the Fuel Lines on the Carburetor.
Something I noticed is this Carburetor doesn't have a Choke Valve in the Carburetor. The Choke Valve is integrated into the Air Cleaner Assembly, and is all plastic. I'm not sure of the reason why it is like this, but I'll assume it is another way to simplify the design, and possibly reduce cost. It is neat, anyway.
Once it was all back together I double check my work, then gassed it up, and gave it a krank. I spent a little time tuning the mixture screws, and my little MM55C is working again. Lets go rip some dirt...
In my FR-S, Ripper, the Cup Holder wound up in an unfortunate location. The Cup Holders are where the Arm Rest should be. They are right behind the shifter, and emergency brake, precisely in the path of my elbow when I am shifting into even gears. Yeah, I know there is limited space in the FR-S, and they had to do what they could to get a couple Cup Holders in there. But, yeah, not the best place. The Cup Holders work a lot better as Coin Cups, or maybe a Cup of Chile Con Queso, LOL!
So, what to do... What to do... Let's fabricate some auxiliary Cup Holders. Did you know that you can order Black PVC from Home Depot? You can get pipe, and sheets. So I wanted these auxiliary Cup Holders to accommodate all my needs, away from the center console. There are pockets in the doors that will hold a bottle, but not upright, so they need sealing lids. I usually carry water, and coffee, plus I need an ashtray. So, we need three holders. The diameters of the things I want to hold are all a little less than three inches. I picked three inch inside diameter black PVC Tube. Then to hold them all together I got a sheet of black PVC quarter inch thick.
The neat thing about PVC is that it can be glued together very effectively. This eliminates a lot of hardware that would have potentially been there. The PVC Tubes were cut to appropriate lengths for the things I wanted to hold, and then glued to part of the Sheet PVC. I had to angle the water bottle a little to clear the Glove Box Door. Then the PVC Sheet is connected to a center console panel with a couple machine screws. The center console panel then gets connected to the center console with snap clips, four of them, and it has a secure fit. This gets all of the things I want to hold out of the way of my elbow, yet still within a reasonable reach. The Cup Holders only protrude into the passenger space by about four inches, so, not too bad, LOL!
