PCB design (and creation) help (kit car fuel gauge)

Community PCB Design
1

I’m trying to design and build a PCB based on the LED fuel gauge on http://www.mez.co.uk/ms13.html - but I need some help with the design (and will need more help when it comes to actually creating the board!). I’ve already made a bread board prototype, so know the circuit works (though it could do with some refinement).

The basic circuit is simple enough, using a pair of LM339 comparitors to power eight LEDs. I’d like to modify it slightly to allow adjustment of the LED brightness.

The big design challenge (for me) is that I need three of these circuits (I’ll be using one for fuel, temp and oil pressure) on one board. I’m trying to lay them out in a way that the lines don’t end up like a bowl of spagetthi, but I’m finding it a headache. Furthermore, the suggestion was made elsewhere that I should use an LM3917 as it would be much simpler. I can’t find any documentation online that can tell me what an LM3917 even is!

Here, for reference, is my terrible attempt at designing the PCB for just the single circuit. You can imagine what three of them looked like! :shock:

http://img69.imageshack.us/img69/7609/circuit.jpg

I’d be really grateful for any help.

Tom

2

I think the part you were referred to use was either an LM314, LM315, or LM316. These are dot/bar display drivers - basically your circuit in a single IC. You can get data on them from the Naitonal Semiconductor web site: http://www.national.com/mpf/LM/LM3916.html#Overview. You can purchase them from Jamecohttp://www.Jameco.com for about $2.50.

3

Hey, that’s great. The dot display drivers look simpler, though I’ve got a bit of reading to do on the data sheet before I understand exactly how they work.

Thanks a lot!

4

After some investigation, it looks like I may have to use the first circuit. Although a LM3917 is simpler to use, the fuel level sender I use doesn’t give a linear output. I’m going to need to be to able to define at what point each LED lights up at.

Ignore what I said previously about having three circuits combined in one. I need to simplify things.

Can anyone give me some help refining the PCB circuit in my first post? Specifically, I’m not sure about the widths of the 12V and ground lines.