Sunday, April 10, 2016

Watchdog Timer

So after I fixed all my lamp matrix problems, I kind of ran into another, luckily I was prepared for this one.  See as I use a lamp matrix with 8 columns, any given lamp is only energized 1/8th of the time.  For this reason I decided to use 18v to power the lights, to keep them nice and bright.  This isn't too bad for the lights - I believe 'real' pinball machines do the same.  Problem is, if I stop the program, whatever lights were previously lit will stay lit.  This hammers the lights with a full 18v which isn't very good for a 12v globe and will likely burn it out before long.
I'm also expecting to have a similar issue with my coils.  If I stop the program or it crashes for whatever reason, a coil may stay stuck on.  For a flipper this isn't much of an issue, but for anything else this is a very bad thing, as they can burn out in a short amount of time.  This is why i need a watchdog timer.  There are a bunch of good resources about this around the place but basically its a circuit that keeps it's output high if it receives a pulse every so often.  If the pulses stop (i.e. the program stops or crashes), then the output goes low after a short time (milliseconds).  

There are a bunch of circuits around 555 chips that can are mainly used for rebooting an arduino if a program crashes.  I tried this but the output actually pulses when the program stops, which is fine if you're rebooting a thing, but not so much if you are using it to drive a relay for high voltages.  I found a great example of one from Ben Heck's spooky pinball which I adapted for my own needs.  You can check it out from here:  It's based on a 74HC123 chip and works a treat.  There's even a pot on it to adjust the time.  I hooked it up to one of my column drives on my lamp matrix and bammo - all good.

After some tidying up of my wiring my boards so far look like this:
The watchdog is the yello perfboard thing.  I'm using automotive relays to turn the high voltages on and off.

Lamp matrix v1.1

Lamp matrix issues have been solved!

As it happens I was failing to properly calculate my resistor values on my TIP107 transistors.  By failing to properly calculate I of course mean 'took a wild guess'.  I was using a 10k resistor between the base and ground.  Using 18v this only allows for a total of 0.0018 amps via the base.  The hfe of this transistor is 200, so the current was being limited at .36amps.  Far less than the 2ish amps needed to drive all the lights.  I think the transistors were heating up, due to trying to limit that much current which was causing their properties to change.  Replace those 10k's with a 1k (which allows me 3.6a through the transistor, and all is well with the world.

My dodgy rework job as i cbf getting my boards reprinted.  Notice the pre-burned trace (my fault) and the sticky uppy pin on that there uln2803.

So here's the thing.  I drive my lamps with 18v, instead of their rated 12.  This is because they are only on 1/8th of the time due to the matrix.  Problem is, if my program stops while the lamps are on, i'll get 18v pumping straight through my lamps which will likely lead to a premature death.  Similar to what I intend on doing with coils, my intention now is to have a power distribution board with a watchdog timer on it.  The watchdog timer will go to a couple of automotive relays which control the 18 and 24v drives (lamps and coils).  I'll hook into one of the column drives for the watchdog.  More on that in a later post!

The Lamp Matrix v1

So I got my lamp matrix PCB design manufactured from Dirt Cheap Boards:
After some messing around with the programming, it actually seemed to work:

I couldn't really get the light bright enough when they were strobing as fast as they could.  But then I noticed some oddities.  My TIP107's were heating up a lot.  Still not exactly sure why.  Then this happened:

Also notice the slight modification I had to make to that ULN2803 chip - had to bend the com leg out.  Poor design on my part.   You may not have realised yet, but I don't actually know what i'm doing.  A lot of trial and error goes into this.

So trying to pump 5+ amps through a tiny little trace like that turns out not to be such a good idea.  I needed to re-engineer.  Since i didn't want to wait for new boards to be manufactured and didn't want to have to re-solder all those components.

This seems to work a lot better.  The puzzling thing is that my TIP107's still heat up significantly.  The current draw also steadily increases as they heat.  Still haven't worked that one out.  Since each globe is 0.25a and there are max 8 ever on at once, surely the max draw of this would only be 2a.  This is the case when permanently on, but when it strobes fast (say 74khz), the draw goes up.  I have a feeling i may not be using the tip107's properly, so I need to go back to the drawing board for that.  Still, progress has been made!

Switch Matrix

How do you drive 64 lamps and read 64 switches from a microcontroller (in my case a raspberry Pi) that only has a limited amount of io pins?
Easy answer *might* be to use a beaglebone black, one of which I own, but where would be the fun in that?

One of my requirements when rebuilding the Gameshow machine was to minimise the amount of cabinet rewiring needed.  The idea would be to figure out where the lamps/switches/coils/whatever connect to the circuit boards, and build my boards to accept those connectors.  Now, pinball lamps and switches are usually controlled using a matrix.  The basic overview looks a little something like this:

The idea is that the cpu energises the rows in sequence (1P10 - the white/something wires) and then once a row has been energised, it has a look to see if there is any activity on the columns (1P8 - the green/something wires).  An excellent article on switch matrix theory can be found at

So I'm building hardware, making circuit boards and testing things.  It becomes a massive hassle to hook my prototypes and breadboards up to the machine each time I want to test something so my solution was to make my very own mini switch matrix tester.

It basically is a 3x3 switch matrix wired up in the same way the actual machine is.  This makes my life much easier when figuring out my logic and circuits.

I use a 74238N 3-to-8 decoder to drive the rows, and each column is read directly, via a buffer chip - mainly to protect my pi from anything that might go wrong.

I'm taking a modular approach to my circuit board design.  Dirty PCB's make awesome cheap boards up to 10x10cm.  I cant fit everything i need onto one so each board will have a 40 pin IDE connector on it for linking through.

My switch matrix board is fairly sparse as not much circuitry is needed, apart from the large connectors.

After some faffing about with pins and things, we have a working switch matrix!

The Bally Gameshow

You likely saw a bunch of posts on this very blog about making circuit boards for a pinball machine.  I realise that there isn't much context for this so here goes:

About 2 years ago now - June 2014 I won a Bally Gameshow Project pinball machine on ebay.  IPDB:

My intention for this machine was to take out all of the existing circuit boards and replace it with my own hardware and software.  I know I wouldn't be able to make it work the same as original, but that's cool - I wanted to make my own game out of it.

This is what it looked like when I got it:
A bit dirty and missing some boards.  The cabinet was in OK condition, and the back-glass was in good nick.
On closer inspection I discovered the remaining boards had actually been butchered and were missing a bunch of chips and some things had been desoldered too.

Future posts (and a couple of previous) detail my work in getting this machine up and running into a playable state.