My good friends Joe and Lou left for Morocco in a modded Transit van in early July. Since then, they’ve been performing copious amounts of windsurfing and kite surfing and probably some other water-sports that I’m unaware of. They made it to Morocco a while back and are now on their way back. This week they had a guest appearance on their blog.
In the month running up to their departure, Joe worked really hard at modding the interior of his van so that they could sleep and cook in it and store all the equipment they’d need — including windsurf boards, sails, bikes, kites and many more things. It was an impressive feat, and it was finished days before they were due to leave.
Earlier this year, in February I think, I subscribed to fitting a dimmed LED lighting system into the back of the van for Joe. I didn’t actually get around to doing this until about a week before Lou and Joe were due to leave. At this time I was working for ECS and was due to leave for Iceland in a few weeks. Unfortunately, the electronics that we needed to take to Iceland really needed a few more months until I’d certify it as shippable — so I was already working reasonably long hours (the extremely long hours didn’t start until the week before we left for Iceland, but I’ll leave this story to another time) and only had a few hours each evening to work on it. Thus I had to bodge the dimmer together fairly quickly.
The dimmer was an MSP430F2002 connected to two FETs and two pots. The MSP430 used PWM to control the lights. Joe and I put the circuit into wall-mounted dual-knob dimmer switch casing and recycled its pots. I bunged the circuit onto stripboard in an ad-hoc manner and then Joe and I gave it a test in the van. There were two major problems:
- The lights would flicker. Due to the hardware limitations of the chip I was using, the PWM outputs had to be switched on and off by the software from a timer interrupt. I realised that I’d been suffering from a similar problem Alexis and I had before on the SR PWM board, and moved all other functionality out of the interrupt routines so that nothing could add noise to the PWM timing. This mostly solved the problem, but not completely. I think there was quite a lot of noise in the readings from the pots, but I didn’t get to the bottom of this in time. In the end, I added two filters — a moving average and an IIR filter that both acted to low-pass the readings from the pots.
- The MSP430 would brown-out sometimes when the board was initially switched on. This meant that the electricity supply to the back of the van needed power cycling every time this happened (there is a switch on the dashboard). This was really annoying and could have been solved by a reset controller. Unfortunately, I had none.
The pots turned out to be non-linear. In my initial investigation of the pots, I’d put my multi-meter across it’s terminals and seen a linear change in resistance. I obviously hadn’t investigated this thoroughly enough, as when I came to fit the pots into the circuit I found that the thing acted non-linearly. On closer examination, the resistance changed linearly between two terminals of the pot and the other two were linked by a fixed resistance. If you do your sums, you’ll find that this results in a non-linear change in output voltage if you just use this thing as a voltage divider.
I went mad. Then Jeff took apart one of the pots. We looked at it. We cried. We scratched off some of the resistive track that formed the fixed resistance. If we could reconnect the now free terminal to one end of the track then we’d be done. After a rather unsuccessful attempt at doing this with a rivet, we gave up on that approach.
If we could get our hands on the innards of a linear pot that was the right size, then maybe everything would turn out alright. We ferreted around in the pots we had lying around. It turned out that a pot that I had from a project we’d been doing about four years ago was exactly the right size. And I had two of them. Perfect. Jeff spent a few of the early hours in the morning cutting away bits of the pot casing to fit the new ones in. Success. The first pot was now linear. Then came the second pot. I was dead at this point, so I went to bed. Something went wrong with the second pot mod. Luckily, Maplin still sell a pot that’s of a compatible size! Joe went there the next day. Problem solved.
Jeff also spent the last few days in the run-up to Lou and Joe’s departure building a dashboard-mounted battery monitor for them. I’m sure he’ll blog about it soon.
Then they left. I heard very little about my precious LED controller for two months. Jeff heard nothing of his battery monitor either. We were worried. Was the electronics working? Lou and Joe were blogging, but about the wrong things! They were talking about wind surfing, kite surfing and camp sites. Were they avoiding blogging about the electronics because it had stopped working? It was great to hear about their progress along the way, but come on guys! Where were the electronics posts?! All Jeff and I could think about whilst we were walking around on an Icelandic glacier was “is the van electronics still working?”*.
So. September began. Jeff decided that he was going to fly out and stay with Joe and Lou for a week. Obviously he chose to do this to find out how the electronics was doing. And pretty soon after his arrival he provided me with an update. Now he’s managed to get the required information into their blog.
And the moral of the story is: if you need to find out about the status of a remote system you’ve put together, send an Engineer. Wait, that’s not entirely connect. Perhaps this is better: If you need to find out about the status of a remote system you’ve put together, sending an Engineer works.
* OK. So that’s not entirely true.
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