I shall smite yee till kingdom come — Arrrrrr!


In my previous post I described how and why I’ve temporarily wrecked my beloved power supply. Thankfully these 390mΩ 5W resistors are available. Right now I could only find 5% types, but ” bettar tan nottin’ ” as The Voodoo Lady might say. I’ve been playing ‘Tales of Monkey Island lately’. A great game. So I’ll be able to fix the shunt resistor and my power supply should be good again. And I will never use it on a lead acid battery. It should have worked just fine, as it’s got voltage and current regulation, but only IF the control logic is powered = fuses not blown. Now why did the fuses blow… I shall never now. Nor will I try it again.

Back to the beginning: my car’s dead battery. The charging procedure did not pump enough energy into the battery to make the starter work, it just did “whoop…” once. But there was enough juice left to power the ignition system and fortuitously my car was parked on a slope. It was finally started by gravitational pull, after I pushed and shoved it quite a bit. The brakes were jammed by rust a bit. Three weeks in the rain and not being moved a single mm is not good.

I checked the idle current that my car was drawing from the battery: 0.33A !! To make sure nothing funny goes on, I pulled every single relay one after another and rechecked, but still 0.33A was drawn. Then I started pulling fuses. Pretty much everything was controlled by switches anyway, and I made sure these circuits were shut off. The remaining suspects were: motor control, internal lighting (trunk, reading lights and so on) and my radio. I had pulled out the radio the day before.

End of story: THE FRIGGIN’ reading lights were on AGAIN.

I don’t know when or how I could have touched the switch, but the sucker was on. Although my car is quite old now, there’s a buzzer for almost everything. Leave the headlights on: BEEP. Leave the blinker on: BEEP. The radio powers off automatically. Pretty much the only two things that are beyond ANY control are the reading lights and the cigarette lighter. The latter is used for heating the driver’s seat, which has killed a few batteries as well. But the reading lights… they have tortured me many times, but not anymore!

Behold: Mr. Blinky LED

This one had a wrong capacitor, 220pF instead of 220nF, and was therefore blinking at about 3kHz. I’m not that fast. Having a frequency counter in my digital multimeter is quite nice. The displayed RMS value was 5V, a bit low I think. For a square wave the average and RMS should be the same, no? The circuit was powered with 12V, so I expected 6V. The green LED was way too weak as well.

Version 2 comes with a more suitable capacitor of 1µF. Together with 1MΩ it blinks at about 1Hz, clearly visible to humans. The circuit can be found in the datasheet. Component count is minimal ;-)

The camera doesn’t properly show the contrast between the LED and the light bulb. In reality it’s much more visible. Placement of the LED isn’t ideal as well, but I don’t have access to drills and other tools right now.

And now with sound, if you can call it that way. Better turn down the volume!

I wish the engineers at VW had included something like this when they were designing this car. Even better, they should have added a buzzer, or as in more modern cars: auto power off. Oh well. I was also thinking about adding a shunt resistor to my car’s battery, but I’m not sure how much I would have to shell out for one capable of say 500A or something. Its resistance would have to be of extremely low value and I’d definitely need a decent opamp for it. For e.g. a burden voltage of 250mV @ 500A that would be 0.5mΩ To get a resolution of say 50mA I’d have to reliably measure voltage steps of 25µV… in a noisy car. Nope. Somebody else can have a go at that.

This entry was posted in Electronics., Fix me. and tagged , , . Bookmark the permalink.

2 Responses to I shall smite yee till kingdom come — Arrrrrr!

  1. DavidCary says:

    Have you looked at magnetic current sensors?
    They can attach around the battery cable — magnetic current sensors don’t need a shunt resistor.

  2. robert says:

    Ah, I didn’t think of that. Silly me. I’ll go and have a look at clamps.

    The whole current monitoring business needs a bit too much effort for just ‘taming’ a rogue reading light though. I don’t want to think about how I’d get the signal wires to the lamp either. Besides, this problem doesn’t exist in modern cars anymore. Everything powers off automatically.

    But now my problem is solved by a horrible noisy buzzer. NE555 to the rescue :-)

Comments are closed.