The Craftmans Shop > New from Old
JCB 803 Saga
awemawson:
So time for a little more investigation. The wiring diagram shows the gauge is in series with the transducer and the pair are across the 12v supply from the ignition switch - so no surprises there. Pulling it out of the dash and testing it on the bench across the connections purporting to be the only ones going to the indicator it measured 150 ohms - so reasonable. Applying 12v from a current limited power supply moved the needle to full scale - whoohay :ddb: But absolutely nothing short of poking the needle with a finger would move it back to zero from when the 12v supply would move it back :scratch: :scratch:
Doing a bit of :coffee: :coffee: and :scratch: :scratch: Google gave me interesting facts about fuel gauges - seems that there are several variants - I'd presumed that they were basically a crude ammeter - but this style has a 'low' coil, and a 'full' coil as well as a third the purpose of which is a bit confusing :bang:
About this time I thought it best to measure the tank transducer - or 'sender' as they seem to be called - basically a variable resistor operated by a float. In this case there is a vertical rod fitted from the base of the tank, surrounded by a float that I assume to be magnetic, that rises and falls with fuel level. All very well, but surely it shouldn't be INFINITE ohms :bang: :bang: Yes the transducer is open circuit. I strongly suspect that the indicator has failed putting too much current through the 'sender'
So a parts hunt ensued. The 'sender' is available so I ordered one, but the gauge is made from unobtainium, I traced there to be ONE spare in the world, and the dealer somewhere in the US wanted $7K so obviously wants to keep it for ever !
The actual gauge module unplugs as per these pictures
awemawson:
Try as I might I could not track down any vehicle gauge or even normal ammeter, small enough to fit in the available space - so what to do :scratch:
Well it's not rocket science - we need a bit of electronics that basically shows a reading varying with the variable resistance of the sender - in this case I measured 9 ohms with the tank dry, and 180 ohms with the float all the way up.
After a few false starts playing with miniature digital volt meters I thought it best to design something myself using the LM3914 Bargraph Driver. This chip accepts a DC voltage, and drives it's ten LED driver outputs in an increasing bargraph representing the voltage coming in. It also has provision for scaling, zero offset, and brightness, all in a single 18 pin chip for about a pound sterling !
So I set off pulling out bits of Veroboard to wire up my simple circuit and then thought - I bet someones done this before - go west young man and GOOGLE :ddb:
Sure enough, a nice man in Canada had used the self same chip, in his case to make a VU audio meter, and was selling the very nicely made PCBs on a well know auction site for absolute peanuts including postage, so I placed an order
awemawson:
So by leaving out his op amp pre-amplifier, and putting links where he had capacitors for AC coupling I ended up with precisely the circuit that I had intended to glue together on strip board but in a much neater format :ddb:
Firstly I built it to run directly off the 12v vehicle voltage, but testing showed that as battery voltage varied from 11.5 to 14.5 there was an unacceptable variation in indication. As the PCB allows for insertion of a regulator I tried regulating down to 8 volts and running off that. Excellent results that are stable over the range of voltage that I'd expect in the machine.
Only down side of this is that the regulator dissipates a watt or two and runs hot, and will need a heat sink when mounted. I tried to reduce the dissipation by putting the LM3914 chip into 'single bar mode' so that a single bar 'walks' the indication, but there is an oddity with this integrated circuit that when in this mode, the tenth bar never illuminates :bang:
At the moment I've set it up so that only 24 mA passes through the 'sender' - a nice low level but it might prove noise sensitive. Easy to alter as it means changing a single resistor and re-tweaking the twiddle pots on the PCB.
Here is my prototype running on the bench and using my wire strippers as a heat sink :lol:
(The LEDs are green asnd although bright enough in real life don't show up very well in the pictures)
awemawson:
I have some red bargraph LEDs and might try those.
All that remains is to physically mount it in the instrument cluster - there is enough room but will need a bit of cogitation to work out a best way - then make a mask to put in front to conceal all but the LEDs.
Then of course I need to drain the diesel tank and install the new transducer, but it's wetter than a wet thing with wet bits on it outside so that might have to wait until the Ark is built and all the pairs of animals are safely aboard :ddb:
awemawson:
So how to mount it? I decided to copy the way that the hours meter is mounted, using 'posts' of thick wire soldered to the board, with tubular spacers separating the two pcb's.
Drilling holes matching those already in the display pcb placed 3 conveniently within the ground plane on the main pcb, but the fourth emerged right between a 12v trace and ground, so some careful soldering required so as not to short the supply out :bugeye:
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