Cross posted from EF
Hey Sep, I think your right, the transistor/scr arrangement amplifies the signal much like a darlington pair would. The H11d1 opto isolator is there so there is no direct link to the 555, thus protecting it. You may notice some similarities to the 555 circuitry on this page here:http://tacashi.tripod.com/elctrncs/555sstc/555sstc.htm
Ok, to start with, I have attached a drawing of Johns schematic with a couple of extras which arent necessary, but can help beginners such as myself. If moderators find this a breach of copyright then please remove with my apologies, I know of two places on the net where this schematic is posted in full so I saw no problem with it. Once again, it is Johns schematic not mine, I have only added a couple of things.
Get yourself a little PCB prototype board, preferably like the one in the picture below. It spaces out the pins and makes soldering and joining them SO much easier! Remember each pin on the 555 and the opto must be isolated from each other, important to note if you can only get a PCB prototype board with rows and not separate pin holes!
Also, in the interest of simplicity get a 555 cradle and a 6 pin optoisolator cradle (machined pins if possible) this will save you soldering directly to the components themselves, most likely destroying them if you arent careful. This way you can swap them in and out if you damage them too. Take note of the pin references of each component carefully, especially the opto isolator. There are heaps of places on the net if you want diagrams for specific components.
Pins 8 and 4 on the 555 are joined together, so are 2 and 6. I put an extra cap between 8 & 4 (+) and 1 (-) to smooth out the voltage going to the 555. I was told JB said this is a good idea. 10 uF cap should do fine. The 1 uF cap is specified at 600volts I think but mine worked with a 63 volt 1uF cap. Resistor between 8/4 and 7 and second resistor between 7 and 2/6. I found you could vary the dump speed by varying this resistor. Maybe use a 10k and a 150k pot. Pin 3 is the output and it loops through the H11d1 before returning to ground through another resistor. I put a small 12v LED between 3 and pin 1 of the opto to show the pulse frequency. It is also a good indicator of whether it is working and whether you have left it connected. It will pulse away as soon as your primary battery is connected and all switches are on, EVEN IF THE WHEEL ISNT ROTATING. Only draws about 40ma or so, but it will flatten the battery if left connected.
Pin 3 and pin 6 on the opto arent used. Take note of the way you install the cradle and make sure your opto goes in the right way, same goes for the 555. There is a little notch cut out of both which signifies the top. 1 and 2 on the opto are pretty self explanatory, same with 4 and 5. 5 goes directly to the negative of the charge battery, or to make things easier, just connect it to the anode of the SCR. The Collector of the transistor is also connected to the SCR's anode. The cathode of the SCR goes to the negative terminal of the cap.
For the triggering transistor I couldnt find the one specified so I just tried to match basic charateristics. The 2n3440 or the MJE340 have both worked for me, they are both 300v rated. You may even find a 2n3055 will work ok, if you use a larger capacitor, say in the 1000's of uF's. I believe Aaron has used others here successfully too.The collector of your transistor connects to the anode and the emitter connects to the SCR's gate. The base of the transistor is triggered by the opto, pin 4. It makes sense when you trace the lines on the diagram. Please note if you use the MJE 340 the pins are reversed. What ever transistor you use, google it for a spec sheet to make sure you got everything right.
Take your time, especially with the 555 and opto. The pins can be a little trickey but if you have a cradle for them both you can afford to be a little reckless when soldering Just make sure there are no shorts between the pins.
And as always, I must state that I am no pro, and if anyone else has info or deems some of the above unecessary or incorrect please dont hesitate to post. Enjoy
PS oh and by the way, the 555 is limited to 18 volts input, so if you want to run your front end higher than that then it needs a separate source. For the 24 volt cap pulser I posted above I used a 3 pin voltage regulator to automatically drop input voltage to 12 volts. The voltage regulator is good for 35 volts on the front I think, with a 1 amp output max. Thats heaps as the 555 only needs 40 ma or so.