Automotive Supply to 5V + 3.3V+Analog Protection

I was hoping someone out there might be able to take a look over this schematic I've made. Its for a power-input board and 3.3V supply for my project. This module is designed to be the top most shield of an arduino-shieldA-shieldB1,B2,B3 type sandwhich.

Its my very first schematic from scratch, first use of Eagle and first attempt at a PCB layout. It is based on a similar schematic from designer2k2, but I had to change the reg and add another reg for 3.3 etc. I also added some analog voltage dividers along with some protection diodes. I have tried to follow all the 'good practice' PCB rules and circuit rules etc.. but no doubt i've got lots screwed up or missing :)

I've made it on protoboard, and it works as I expect it too.. but I'd love some feedback.

Heres the schematic:

and heres the PCB layout (less important, but I'd like feedback anyway)

A more involved circuit explanation is at my blog on the projects development, http://slida.canberrariders.org.au/2009/08/10/first-schematicboard-slida-ext-io-ext-pwr-board/

Big thanks to designer2k2 for his original circuit!

All sorts of feedback is graciously welcomed :)

I think you need to rethink the analog protection circuit. Those Zenders will affect the readings within a volt or so of the rated cutoff and the resistors present a much higher impedence then that recommended in the ATmega datasheet ( I think 20k or less suggested for maximum performance)

What are you expecting to connect to the analog inputs and what is the highest voltage you need to measure accurately?

A Throttle Position Sensor which is basically a variable resistor and the input for reading RPM.

I'm probably replacing the RPM circuit with a LM2907 circuit... not sure as my ignition personally is TDI.. but most other bikes are CDI and want this to be as generic as possible... was thinking even an inductive coupler (capacitive involves ugly things with TDI/CDI) to a transistor feeding an 555IC .... it doesn't help that my bike has a micro-processor controlled dash unit that takes the tacho data via some sort of proprietary Yamaha bus from the ECU... (for the love of a tacho input wire!!! :) )

but the TPS.. not sure what to do.. its always just a variable resistor.. I want to read the sensor, leave it also connected to the ECU on the bike as well (as its a fly-by-wire throttle it needs the circuit as unaffected as possible). Basically looks like its a potentiometer of another means - just 3 legs, 12V rail, GND and Signal.. For the TPS, I was thinking maybe an optocoupler?? I believe basically I need to have a silly high resistance to prevent any effect on the existing sensor readings right? just like a multimeter ? (i.e, in goldilocks conditions I'd have infinite resistance ??)

There is so many options.. I really dont know where to start with that. More importantly - don't fully understand the options ;D

I just took the voltage divider values from another persons project - that part hasn't been used yet... so any suggestions would be welcome.

The ratio of your resistors is good but the values are too high. I suggest using 10k instead of the 100k and 4.7 k instead of 47k.

This will give you just under 4 volts on the Analog pin when there is 12 volts on input to the divider

I'd put the choke and the Zener "downstream" of the 1N4004: if there's a negative voltage spike in the electrical system, the Zener will wind up being forward-biased, and creating a path to dissipate it. It's probably better to just block it from your circuit.

Ran

Thanks Ran.. I've made those mods. Appreciate your feedback!

Now I think I need to re-examine what I had planned for the RPM/TPS reading....