Follow the Bouncy Needle

Since I'm on a roll with the electrical wizardry, I thought I'd tackle another little project: smoothing out the needle on my fuel gauge.

The fuel gauge is pretty simple.  A sender with a float on an arm fits in the tank.  As it moves it changes the resistance of a coil inside the sender.  The gauge is powered to 12v and is grounded.  It picks up the difference in voltage based on the position of the sender's varistor and moves a needle, where no resistance is 'full' and max resistance is 'empty'.

 

This works very well... too well, in fact.  The gauge is so responsive to the sender's position that the needle bounces a lot while you drive, as the sender moves up and down while going over bumps or around corners.  It 'twitches'.

The solution is to add a capacitor in parallel with the sender that can act as a voltage buffer.  The capacitor charges when the sender moves to full and discharges when it moves towards empty.  The drag during the charge state (because the capacitor is taking some of the volts from the sender until it's full) and the extra volts the capacitor supplies during the discharge state smooths out the voltage changes enough to let the sender bounce up and down a little without affecting the voltage at the gauge.

Now, the needle still moves but it doesn't jump around if I jiggle the sender.

I tried a few different combinations, but found a 220uF, 16v non-polarized capacitor worked best.  It was easy enough to solder in place and it fits between the gauge terminals, so I'll still be able to install the gauge in the dashboard.  And, I made sure my sender worked well, too.

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Frequency Over Amplitude

One of the things I really want to do is to listen to my tunes while I'm driving the Midget.

Long ago, I had installed a CD player with 200 watts of amplifiers and dual 8" subs.  It sounded great, but I didn't like the look very well (and it sucked all the juice out of my little dynamo).  I went back to the original AM radio.  It looks cool but even the news is on FM now... and come on, I'm a nerd!  I have to do something cool.

About a week ago, I ran across an article on Spridget Guru about how to plumb a line-in jack into an AM radio.  Bingo!  I'm no electrical engineer, but I can follow instructions and I solder pretty well.  So I picked up some supplies:

• 1 3.5mm stereo phono jack
• 2 1k ohm resistors 1/4 watt
• 1 stereo audio cable 3ft

First things first; I took the case off to see what I had to work with.

 

The key element here is the volume pot (top left).  That accepts a line level input from the tuner and sends it to the amp.  It's accessible and I identified the positive/negative (red and black, go figure).  This means I can do this.

I cut the plugs off the audio cable and cut it in half, then stripped the ends.  Since this is a monaural radio, I combined the left and right channels on each end.  Then, I soldered the resistors to each leg of the phono jack, trimmed to length and tied the ends together.  Finally, I soldered the mono lead on the cable to the resistors and the ground to the ground leg on the jack.  The result: a stereo-to-mono input jack.  The resistors drop the input level a bit (I probably didn't need them, but was recommended).  I encased it in shrink tubing to keep the smoke inside.

Then I ran the other end of the cable into the radio through a convenient hole in the side where the speaker wire comes out.  I grabbed my iPod Shuffle and another 3.5mm audio cable and plugged it into the jack, and turned it on.  I powered up the radio (I have a portable charger that outputs 12v) and turned it on, too.  Then I touched the ends of the cable to the positive and negative leads on the pot, and incredibly I heard Paul McCartney singing "Wonderful Christmas Time".  (I use the Shuffle for Christmas music in my cabinet radio).

Wow!  It works!  So I soldered the cable ends to the pot, and voila!  I plugged in my phone and I had Van Halen singing "Beautiful Girls".  It sounds really good--good volume and tone without clipping.

I picked the iPod Shuffle because it also has an FM tuner.  So I switched to FM and it came in crystal clear.  Now, the title of this post makes sense, right?

I have volume control of the line input and still have AM reception.  All I have to do is tune to an empty band to listen to music through my new aux jack.  If I don't I get both the aux input and the tuner input, which is pretty funny.  I may have to put a switch on the antenna input, because I might not be able to find a dead band when the radio's installed.

Here's a demo:

Now that it was all working, I repainted the case with wrinkle finish paint.  It's pretty cool to watch this stuff dry.

  

Once dry, I'll reassemble and it's all done. I'll also add a USB charge port, in a little box that will hold both the aux jack and the USB port (and not ground out the radio--remember, it's positive ground).  The USB port needs a 12v-to-5v converter, which are only about $10 on Amazon.

Update: Here's the finished product, mostly installed.  I used this adjustable voltage converter for the charge port, and put the USB and aux ports in a plastic housing so they're ground-isolated.  It works great with my iPhone (with the Lightning-to-headphone converter you need for an iPhone 8 or newer).  There is a brown 12v lead available on the harness, though it's unswitched... which I prefer, since I can charge my phone and play the radio without running the fuel pump.

 

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Dashboard III

I've decided to go with a custom paint job on the dash...

That's right... Hunter Green.  The rest will be black wrinkle finish, and I think it will look really cool.

It came out OK for a first attempt.  I might sand it down and try again because there are some defects, and I'm too detail-oriented to let them be.

EDIT: I did in fact remove the paint and repainted it, and I'm really happy with the look.  No defects.

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Vent Windows

It's time to clean up those vent windows.

The window assemblies are in good shape, but the rubber sealing gaskets need to be replaced as they're not really sealing anything any more.  The window channel felt needs to be replaced, too.

Here's one fully disassembled, before and after cleaning.

 

That white stuff is the soap they used to make the gasket fit during original assembly.  It took a while to clean all that stuff out and then to polish with some MAAS metal cleaner.  I used a very little bit of soap during assembly and the gasket fit nice and neatly.  I also had to tighten the rivets on the driver's side window that hold the little swivel pin, because I bet that one got more use over 50 years.

The result:

 

Oh, and the other one, too:

It appears I bought the wrong window channel felt, so I'll have to re-order it.  It's cheap ($3/item) so I'm not too busted up about it.  I need the felt for the other channel too (that fits inside the door), so I'll be out about $20.  Not too bad, and they're done.

Boy, I think I just have to finish up the dash and I'm officially out of things to clean up...

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Dashboard II

There's a hole in my dashboard, dear Liza, dear Liza...

It appears that long ago, someone cut an extra hole for a fog light switch or some such.  It's not supposed to be there, so I closed it up.  If I want a hole, I'll make it myself, dammit.

 

 

Now I'm all ready for paint.

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