spunkymonkey
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Post by spunkymonkey on Jun 18, 2011 20:48:12 GMT
A couple of weeks ago I was thinking (as you do) that it might be nice to have an ignition scope for tracing those obscure little misfires and stuff. Only, they ain't cheap and I like cheap. So I got to thinking that a digital scope is only a bit of sampling hardware and some software to display the results. Now, just about all computers nowadays have sampling hardware (it's called a sound-card) and there was free oscilloscope software available for Linux, which is what I use. The only problem is that most sound-cards are going to get very upset indeed if you plug even the primary ignition of a car into them - you get spikes upards of 300v when the points open and PCs don't like that. So I looked on EBay and found someone selling (brand new!) USB soundcards for £1.45 (including postage!!!) on Buy It Now and promptly bought one. Then it was just a case of another fiver or so of assorted "pound-shop" stuff to make up connections, leads and robbing my parts bin for a few resistors to drop voltages, and I was set to play today....  The plan was to use the box and phono sockets from the £2.50 SCART adapter to give inputs with attenuations of (roughly) 1:1, 10:1 and 200:1 so that the card's full scale input of 1.3 volts would measure 1.3, 13 and 260v depending on which you plug the lead into. First thing was to get some idea of the input impedance of the sound dongle:  which turned out to be 25K ohm. A quick raid of my bits bin didn't produce quite the ratios I'd hoped for but I managed to get about 12:1 and 23:1 which, by feeding that into the 12:1 gave roughly 275:1. Good enough seeing as the xoscope software I was going to try it with doesn't have voltage calibration anyway  So, we rip out the SCART bits:  put in the "high" input voltage divider:  then connect that to the middle input, and the middle input resistor to the 1:1 socket, with the output coming straight from that socket:  Then cut the "socket" end off the cheapo phono lead and add a couple of croc clips. Ready to go:  Took it and my lappy out to Matilda the Daf, plugged it all in, clipped it across the points, and started. It works!  with a shorter timebase, nice clear pick of the two cylinders firing:  Finally, a quick video of it running: As you can see, at the end on a 10ms she's idling at about 600rpm (100ms per revolution) and has about 33ms dwell which works out at: Distributor speed = 300rpm = 5 revs / second = 0.55ms per degree. So 33 ms dwell = 33/.55 = 60 degrees, which is spot on what it's meant to be, thank you very much  So there you go - £5.62 spent and I was looking at an ignition trace good enough to count RPM and dwell angles on. Also to confirm that the dwell is nice and stable so no wear in the distributor. Next stage is to tidy it up a bit and start writing software to give more "car orientated" functions like automatic calculation of rpm and dwell. Plus try making up some capacitive probes for monitoring the HT leads |
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spunkymonkey
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Currently waltzing Matilda
Posts: 3,482
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Post by spunkymonkey on Jun 18, 2011 21:20:05 GMT
Mainly because the battery on this laptop is a little beyond shot right now so didn't want the poor thing doing any more than it had to. Started everything up indoors then shut the hard-drive drive down and screen onto "ultra-dim" to be sure I had enough to get out and try it.
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kenr
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Post by kenr on Jun 18, 2011 22:02:13 GMT
Cool!!! Bringing it to Essex by any chance?
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spunkymonkey
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Post by spunkymonkey on Jun 18, 2011 22:06:32 GMT
Why do you think I'm so confident of finding Snowdrop's problems?  |
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kenr
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Post by kenr on Jun 18, 2011 22:15:34 GMT
Excellent!!!! lol+++
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33grinder
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Post by 33grinder on Jun 19, 2011 13:49:09 GMT
Nice one Joe! Please take it to Litchfield, be interesting to hook up Gavina to it! |
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spunkymonkey
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Post by spunkymonkey on Jul 21, 2011 15:00:42 GMT
It went a bit quiet on this while other things got in the way but found a few hours to play and now have a (very) alpha version of some software in Java (for maximum compatibility) which works to at least display a trace. Haven't implemented the tacho or dwell yet, or tested it on the car, because it's raining here and I don't want to get wet  This is displaying the local mains "hum" created by sticking a loose wire in the inputs using my netbook (Win 7, 1Gb RAM and an Atom 450). Please note that, in deference to Mac, I've used screenshots this time rather than photos  and this is displaying the output from my capacitance meter using my main laptop in Ubuntu:  If anyone wants to play, the program itself is attached as a zip file (13.6k) - you'll need to have a Java run-time environment installed to use it but you've probably got that already without knowing it If not, you can install it (around 20mB depending on your operating system) for free from www.java.com/en/download/Unzip and run the file Scope.jar that you'll find in the resulting directory. Please note that it is only an early version so please excuse any rough edges |
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spunkymonkey
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Post by spunkymonkey on Jul 21, 2011 18:55:50 GMT
By 2024 I may have a stable version ready, Mac ;D Until then the temptation to tweak is just too big The tacho has now been coded and actually got to plug it into Tilly to test it. I'm not attaching the revised version yet because I should hopefully have a nice stable "production" release soon and the previous version gives enough for people to play with for now (it's only the tacho and dwell that don't work on it). Her idle's a bit high at the moment because I've had to nudge it up a little to allow for the leak in the o/s primary. No, that 2000rpm is NOT her current idle speed - that's about 880 according to this and 865 according to my digital meter yesterday ;D  |
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pauldaf44
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Post by pauldaf44 on Jul 21, 2011 19:34:12 GMT
That is flippin awesome Joe.
Are you going to post it up a free ware go on you know you want to ;D
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spunkymonkey
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Post by spunkymonkey on Jul 21, 2011 21:01:07 GMT
That is flippin awesome Joe. Are you going to post it up a free ware go on you know you want to ;D Thanks Paul - there's still a fair bit to do yet. Obviously the dwell bit, and also thinking about a recording / playback feature so you can record to a log file (while driving??) then review later. As for giving it away as freeware, don't be silly! What else would I do with it?  ? ;D |
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kenr
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Post by kenr on Jul 21, 2011 22:23:06 GMT
A little Daf 44 is scheming to thwart this latest technical threat to her slumber. I just know she is. She's looking for a perfect score against the puny humans.
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spunkymonkey
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Post by spunkymonkey on Jul 21, 2011 23:32:02 GMT
It's just not going to happen, Ken - she can scheme all she likes |
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kenr
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Post by kenr on Jul 22, 2011 5:39:08 GMT
Bring it on Island Boy! We'll see who triumphs (not a Daf)!
Snowstrop x
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spunkymonkey
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Post by spunkymonkey on Jul 22, 2011 16:49:34 GMT
No problem, Snowstrop - the more you scheme, the faster I develop the Weapons of Mass Debugging As it happens there was a bit more progress on that today. Having decided on Java for easy compatibility (I do like write once, run anywhere!!! ) speed has become a serious factor. It's not too much of a problem on a half-decent system but is on slower stuff. My main lappy is a 4 year old 1.6 Ghz dual core which will plot a full second's worth of dual-channel data (96000 points) in 1/4 second but my netbook (new 1.6Ghz single core Atom) takes 1.1 seconds to plot the same samples, and that's after it's done the sums to identify the trigger point! Which is a problem when the next lot's waiting a tenth of a second before it's finished - doesn't take long to build up a nasty lag. It's even worse on the 1ms/div timebase where it has 1/100th of a second to display a full plot. On the netbook that was accumulating a delay of around 1/5 second for every second it was running! Seeing as this is most likely to be used on older machines I've added a horizontal resolution control so it can be set to plot anything from every sample to every 32nd sample, which is much faster but liable to miss fast transients on the plot (although it will still trigger on them). Other additions are: Separate magnify for each channel An "invert" option because some sound cards seem to invert their input voltages and give an upside-down trace - no idea why! Added a 20ms timebase option - about right for catching a single revolution at fast-idle speed.  Next plans are to work out the dwell triggering (not as easy as it seemed!) and add an option to save the entire sample stream to disk for reviewing in slow-time later (so you can drive with it connected then look for the misfires at your leisure ) Any other suggestions gratefully received..... |
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33grinder
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Post by 33grinder on Jul 22, 2011 23:20:36 GMT
This is all very impressive Joe, even if I'm struggling with the computer aspect a little.  Do not under-estimate Snowstrop Joe! That's the mistake the priest made on "I bought a vampire motorcycle'... Obviously Ken bought a Vampire DAF and she won't run proper until she gets her quota of your blood Joe!  |
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spunkymonkey
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Post by spunkymonkey on Jul 22, 2011 23:26:34 GMT
The same advice could be given to her - don't underestimate the bloke who's rolling up in a 32 on Friday. Her only chance is to run and hide somewhere, but even that would mean she'd have to run so I win either way |
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spunkymonkey
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Post by spunkymonkey on Jul 23, 2011 20:24:28 GMT
Just had to carry on with this today (who says I'm obsessive??? ) and got an initial version of dwell measurement working. For simplicity, and making sure that the sums work, I decided to go for a manual cursor (vertical line) that you can move to indicate the start of the dwell period rather than having the program detect it. Now that works it should be fairly straightforward to adapt for auto detection. Of course, I couldn't really test that without having it connected to a proper running engine because random sine- and square- waves don't really have a "dwell" so out to the Daf again for a couple of test traces. The first one was a dwell measurement at about 3k rpm:  which showed the dwell to be a little small - it should be 58 degrees - but, with all that spare time on a 2 cylinder it's not exactly critical. What it also showed up was an apparent difference in firing pattern between the two cylinders. So time to zoom in on individual firing events. Sure enough, firing on one cylinder is giving this:  while the other is giving this:  That wide, low initial portion in the second trace is indicative of either a plug starting to short or firing too "easily" before full voltage is developed. Unfortunately I haven't sorted out an inductive plug pick-up yet so there's no way to identify which cylinder's at fault (pulling one lead from a 2 cyl doesn't work too well ). Still, given that the plugs and leads were last replaced in around 1978 I guess it's time to replace them all anyway So then it was on to Sheila to see what her SimonBBC module is doing. This time I started with a fast-idle (she was on choke) train:  All looks pretty good and even there, but the dwell angle seemed rather low at 20 degrees (the cursor isn't set in the screenshot - it's indicating about double what it should). So I tried a high-speed train and found this:  If you look at the first shot you'll see that the dwell period (ie: the time the "points" are closed) is about 1/2 of a horizontal division. Seeing as the timebase is set to 20 milliseconds per division, that gives a dwell time of about 10 ms. Now, if you look at the second trace, the dwell is about one division in length but check the timebase - it's been changed (by me) to 10ms for this shot. So the dwell time is still about 10ms. Hands up who knew that those el-cheapo in-distributor ignition modules used a fixed-duration dwell? I know I didn't till I started playing with this ;D |
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