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Eagle PCB design opinions (non-audio related)

post #1 of 14
Thread Starter 

This was my first time using Eagle, and I created this circuit/board.  The circuit is used in a vehicle's emissions system.  When power is NOT applied to the relay, the O2 sensor signal comes in and then out to the DME.  However when power IS applied, a 200 mV signal (through the potentiometer) is generated and sent to the DME (instead of the O2 signal).  Trying to keep it small, currently fits on a 2x2" board and will be fitted to a plastic project box.

 

Opinions are welcome...

 

V.1

BMWE36SAPSIMSchematic.png

 

V.1

BMWE36SAPSIMBoard.png

 

Maybe a little more background into the circuit... There is a "Secondary Air Pump" (SAP) on the vehicle, which the "Digital Motor Electronics" (DME) signals ON during a cold start. The SAP pumps air into the catalytic convertors to speed up their heating. Thus reducing emissions by making the CATs effective quicker.

When the DME signals the SAP ON it expects to see a "lean" condition from the O2 sensors due to the extra air. If it doesn't see a "lean" condition, then it assumes the SAP is faulty and you'll get a "Check Engine Light" (CEL).

The point of the 200mV signal is to temporality fool the DME into thinking there is a "lean" condition. Now it doesn’t need to be exactly 200mV, just in the ballpark.

 

V.2

BMW E36 SAP SIM Schematic_v2.png

 

V.2

BMW E36 SAP SIM Board_v2.png


Edited by bmwpowere36m3 - 10/8/10 at 4:29pm
post #2 of 14
Thread Starter 

bump

post #3 of 14

Do you want this to be a single-sided PCB? Regardless, I would use a ground plane. To do a ground plane, draw a polygon on the board side that you want it on, then name it the same as the GND signal name. You can increase or decrease the amount of spacing of the ground plane around pads and traces using the ISOLATE command. I assume that you intend for the diode across the relay coil to be mounted on the bottom? Also, I would beef up the Vdd trace.


Edited by Pars - 10/8/10 at 5:07pm
post #4 of 14

On V2, move C2, and you can re-draw the traces such that everything is single-sided, or you can have all the signal traces on one side and a ground plane on the other. Move C1 and the power connector too, and you can shave 10-20% off the size of the board. If not, at least get rid of the vias, which are very unnecessary. I might also rotate things around so that the trimpots are on the end of the relay, minimizing trace lengths and generally making everything neater.

 

As an added exercise in efficient PCB design, you can also probably lay everything out such that the TO-220 voltage regulator fits on the reverse of the PCB, in the gap between the pins of the DIP relay, and save even more PCB space. And once you do *that*, if you're doing a double-sided board, you might as well switch to the cheaper SMD package, and use a power plane as a heatsink.

post #5 of 14

I think the trace he is jumping on the topside is GND, so that will go away with a ground plane and no traces would need to be on top. I think a single layer board would work, but I guess I'd need to see the ground pour. I would rotate the 2 pots 90 degrees clockwise though.


Edited by Pars - 10/8/10 at 6:45pm
post #6 of 14

Good point. Well,it'll go away so long as he rips it up before he creates the ground plane, anyway. (I don't think vias automagically merge into plane pours. Might be wrong; haven't made anything with a via in many months.)

 

I'd still move the power connector and capacitors around, to shrink the total board area a fair bit.

 

Why rotate the trimpots? Or do you mean rotate them around the relay?

post #7 of 14
Quote:
Originally Posted by Nemo de Monet View Post


Why rotate the trimpots? Or do you mean rotate them around the relay?



Because Vdd goes to the top and gnd to the bottom, creating a clear path for the wiper to the relay contacts (with a ground plane).

post #8 of 14

Al Gore would not be happy with you,  basically what you are doing is not legal.   Your punishment is to provide free eagle cad work for all head-fiers

 

But off course we know this is for off-road use only,  may want to put that in the first post.

post #9 of 14

Any reason why you can't use one pot instead of two?

post #10 of 14

On further review, why not just use fixed resistors as dividers?  Very easy then to make the board single sided.

post #11 of 14

It should be rather trivial to get it one sided even with both pots (and without a ground plane). For example, rotate the right pot 180 degrees. I'd still do the ground plane on the bottom (single-sided).

post #12 of 14
Thread Starter 

 

Thanks for the feedback everyone! 


 

Quote:
Originally Posted by Pars View Post

Do you want this to be a single-sided PCB? Regardless, I would use a ground plane. To do a ground plane, draw a polygon on the board side that you want it on, then name it the same as the GND signal name. You can increase or decrease the amount of spacing of the ground plane around pads and traces using the ISOLATE command. I assume that you intend for the diode across the relay coil to be mounted on the bottom? Also, I would beef up the Vdd trace.

 

My goal is to keep the board single sided and the V.2 the top trace was going to be a jumper. A ground plane sounds like a great idea.


 

Quote:
Originally Posted by Nemo de Monet View Post

On V2, move C2, and you can re-draw the traces such that everything is single-sided, or you can have all the signal traces on one side and a ground plane on the other. Move C1 and the power connector too, and you can shave 10-20% off the size of the board. If not, at least get rid of the vias, which are very unnecessary. I might also rotate things around so that the trimpots are on the end of the relay, minimizing trace lengths and generally making everything neater.

 

As an added exercise in efficient PCB design, you can also probably lay everything out such that the TO-220 voltage regulator fits on the reverse of the PCB, in the gap between the pins of the DIP relay, and save even more PCB space. And once you do *that*, if you're doing a double-sided board, you might as well switch to the cheaper SMD package, and use a power plane as a heatsink.

 

I really can't visualize how moving C2 would allow for all the traces to be on 1 side.  I'll experiment with the component locations some more.
 

 
Quote:
Originally Posted by regal View Post

Al Gore would not be happy with you,  basically what you are doing is not legal.   Your punishment is to provide free eagle cad work for all head-fiers

 

But off course we know this is for off-road use only,  may want to put that in the first post.


Yeah, off-road use only... which is pretty true since I mostly track the car anyways.

 

Quote:
Originally Posted by cobaltmute View Post

Any reason why you can't use one pot instead of two?


Other than having two "separate" loads which would be the O2 inputs at the DME.  I don't know what the current demand is at the DME, can't be much since a O2 sensor is not a current source, but a voltage source. 

 

Quote:
Originally Posted by cobaltmute View Post

On further review, why not just use fixed resistors as dividers?  Very easy then to make the board single sided.


It gives me some adjustability for factors not accounted for.
 

post #13 of 14
Thread Starter 

V.3

BMW E36 SAP SIM Board_v3.png

post #14 of 14

An improvement, though I can't imagine C1 can pass DRC like that. You could probably move it and the power terminal to the lower part of the board, and save half a square inch per copy, or thereabouts.

 

If you post the .sch file, people can probably take a crack at creating a more optimized layout...

 

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