OK, so this project came about because a member from another arcade site was asking about repairing or obtaining a replacement opto board for his WMS Dracula and didn't seem to be having much luck. He had confirmed his board as faulty by substituting it with a known good one from a friends machine.
Part number A15646 and curiously named "24-opto PCB Assembly". I can only guess that the "24" part refers to the DISTANCE that the opto spans in inches and not that the board controls 24 optos, because it is a single circuit??
Anyway I have a BSD so I opened it up and had a look. I had never previously realised that there is an opto that spans the width of the playfield and it 'looks' across the track that the mist-multiball travels along.
I imagine it is there to determine when the mist ball has been knocked from the magnet that hauls it across the PF?
Anyway the circuit is pretty straighforward. It uses a 555 timer IC to generate a square wave that is fed to the IR TX LED on one side of the PF.
The RX LED is connected to a special IC called a Remote Control Amplifier/Detector. This chip basically amplifies and detects the incoming square wave and triggers an output when it senses a valid square wave coming in. The reasoning behind using the square wave (at a certain frequency) is to prevent spurious light signals from upsetting the detection process and that is what makes the circuit a lot more complicated than a simple TX RX pair of sensors that you would normally see used in many places on most DMD pinball machines.
This particular IC is well and truly obsolete and I believe this explains why all of the pinball parts places I looked at had this board marked as "no stock".
So I set about looking at what could be used to replace this IC. Naturally (as with ALL Homepin replacement board designs) any replacement MUST mount using the same holes and connect using the existing wiring harness. I found a "tone decoder" IC that is still current production made by National Semi. and decided, after a quick look at the specs sheet, that this IC would be a perfect candidate for a re-design.
I will need to design a new front end to amplify and condition the incoming signal and then calculate the component values required to sense the correct frequency of the incoming square wave. The exact same values used in the original board will be retained for the 555 generator so the frequency of operation won't change. This should mean there will be no noticable difference between this board and the original.
I have started the board layout and I am now waiting for a few parts to arrive as I don't have them all in stock. I will add an extra LED so that it's easy to fault find the circuit..........more updates as things progress.
The original board:
The replacement initial board layout (yet to be finished and tweaked):
