Solenoids and Slingshots

I disassembled the pop bumpers now that I know they work. Installing them later on the new test board should be trivial. Test Board? Yes, I went to home depot and bought some 1/2 cabinet grade plywood and cut a prototype playfield out. 42" x 20.25" x .5". I found an excellent website called Pinball Makers  that has had a lot of useful data on designing and building a pin. I found a table of many different games and the dimensions of their playfields. There is also a page with CAD drawings and 3D files of many components and layouts. Part of me wants to download all the data I find and tuck it away on my server in fear that the source pages may not be available later, but I think that's just paranoia. I am keeping a small, pocket notebook with me so when I am away from my bench and find an important piece of information I can write it down for later.

I purchased an old slingshot assembly and did my best to mount it to a scrap piece of .5". The slingshots are the triangle thingies above the flippers that bounce the ball on an angle back up the playfield. I also purchased, at the same time, a set of Gottlieb replacement rubbers. Because sometimes you should change your rubbers. I assumed the larges of these bands must be for the slingshots. I spent Friday night drawing the lower part of the playfield in pencil on the prototype. I have seen in pictures that the slingshots start above the shaft of the flippers. So, knowing where my flippers were going to be, I measured from above them back to the start of the inlane guide. After using this measurement to place posts on my scrap board I placed the 4" band on the two posts and pulled the side to the inlane line and more or less had where my 3rd post should be. Now mounting the switches and solenoid assy is what started becoming an issue. I downloaded one of the layouts from pinball makers and snapped some dimensions and printed it. After looking at the measurements it was clear that the slingshot I had a drawing of was much smaller than the one I had just outlined. Ignoring this obvious clue, I attempted to mount all the hardware. It seemed to compact. You know, because my slingshot was too damn big. I was debating purchasing a plastic slingshot cover from an old game and using it to help me lay it all out when I noticed my bag of rubber bands had some documentation I didn't bother to read before. Inside was a detailed layout of the playfield these replacements were for, a chart showing the various band sizes, and a key easily mapping what each one was for. And sure enough, the band I was using was the wrong one. I was using a 4" band instead of a 3".  I will have to layout all the measurements on a new board and have faith the 3" band will work. My guess is it will. I'll have a video up as soon as I test it.

After my agonizing defeat with the slingshot I decided to install the flippers to the Protofield™. This was quite simple and gave me no trouble at all. I just mounted the base, inserted the flippers with new rubber, lined up the flipper and tightened down the shaft.

Indeed

I find the flippers to be a really fascinating piece of engineering. Before I get into why I think that, let me first explain what a solenoid actually is and how it works. When a current is applied to a length of wire, a small,, weak electromagnetic field builds around the wire. If you were to take this wire and then put a single loop into it, the small electromagnetic field will become larger as a result of the two fields joining each other. Now if we take that same wire and instead put many, many more loops going the same direction to form a "coil", we will have created a much larger, stronger, magnetic field. Perhaps in school the teacher had you loop wire around a nail and pick up paperclips when the coil had a battery connected to it. You created an electromagnet. What the teacher probably didn't have you do is apply more voltage to your coil thus driving more current through and inducing an even stronger electromagnet. If he/she would have done this without injuring the students, the nail could have been pulled out of the coil by hand, and when released, "sucked" back into the coil with force. The strength the coil depends on the amount of current being applied. The more windings a coil has, the less current it will draw, the weaker the field will be. So if you had a coil with 200 windings and a coil with only 100 windings, assuming the same voltage is applied to both, the coil with only 100 windings will be twice as strong and draw twice the amount of current. Ordinarily, a solenoid will consist of 1 coil, a ferrous shaft(steel), and a spring. Voltage is supplied to draw the shaft in, and the spring pulls the shaft back out when the voltage is cut off.

Normally, a solenoid is only switched on momentarily to perform an action and switched off quickly to avoid burning out the coil. The problem with pinball flippers however, is the player will want to hold the button down to catch the ball and hold it. It is possible to reduce the current of the solenoid to the point where the coil could stay energized indefinitely. And that would work for holding the flipper up, but the trade off would be that the flipper would no longer have the power needed to throw the ball at high speeds up the playfield. The solution to this is elegant and the reason I love the flipper assembly so much. The flipper solenoid has two sets of windings. one short and capable of producing higher power, and one long and capable of holding the flipper without drawing high current. These two coils meet together in the middle to form one long coil with a tap somewhere in between the two.
The negative side of the power source is routed through a normally closed switch and then to the center tap of the coil. The outer side of the longer coil(holding coil) is connected directly to the negative side of the power source. When voltage is applied to the outer tap of the smaller coil, a large current is drawn through the coil giving the flipper the high power it needs to drive the ball. When the flipper reaches it's end of stroke, a bracket on the shaft of the flipper opens the normally closed switch, causing the only path left to negative to be through the secondary, longer coil. This immediately reduces the current draw to be just enough to hold the flipper in the up right position.