Cheap protection...

[Bernd Felsche]

Mike writes:

>At 06:25 PM 26/3/2000 +0800, Bernd Felsche <bernie at perth.dialix.com.au> wrote:

>>>Well, another option is to use a protected MAX232 RS-232 input/output
>>>chip, this already has lots of ESD protection and overvoltage too,
>>
>>That's novel. A bit pricey though. Works out about twice the cost
>>per input as opto.

>Well that depends - are you doing a one-off or designing a commercial
>product. In production MAX232's can be got for about A$1.50 depending
>on volume ?  Its relative to the cost of EFI controllers...

I wasn't _planning_ on making a hundred thousand, but if you send me
your purchase order, I'll get the wheels turning. :-)

Seriously though; in the spirit of sharing knowledge and encouraging
education; I'd be happy to offer the proven design to somebody who's
be willing to "productionise" a kit. Hands-on experience is a
valuable asset, but a student may well have difficulty coming up with
more than about $20 for a "gadget" that does little more than help
to propel them into geekdom.

Seeing that MAX232's retail for about $8 locally and opto-couplers
at $1.50, it doesn't make sense to use them for something so trivial.

>>I was aiming at $20 in parts for the lot (well, about $25 including
>>the case). The uC is about $6, though I might have to spring for $10
>>on the prototype as the "best type" is not yet available...

>Sorry, I think I missed an earlier email on this, I take it this is not
>for an EFI project, what are you building - is this the WI project  ?

Nothing that complicated - not brave enough to do something
important as my first, blank-slate uC project. It's just a smart
courtesy light control with a bag of other functions thrown in to
make it complex enough to justify using a uC. :-)

The next most ambitious project (if I don't flounder hopelessly on
making a light come on) is to "re-do" the cooling fan and
aircon-clutch control in my car. 

It's an absolute rat's nest of heavy wiring back and forth along the
front of the engine compartment.

That'll look at the analogue voltage from the NTC coolant
temperature gauge as well as the stock switches, allowing me to
control the fan to operate at *my* setpoints at the speed I choose.
(PWM to be added later.)

As you can probably see, I like to build my confidence before diving
deeply into implementing the EFI - though I will obviously go back
and revise the EFI as I learn more about the practicalities of
applying uC technology to the real world.

I've already figured out that I could have an aircon-clutch output
to maximise available engine power while accelerating.

>>>depending on the source impedance you'd probably also want a series
>>>resistor to the MAX232 input.
>>
>>What's the source impedance of a car battery? :-)

>mmmm ! If you are measuring battery voltage then a MAX232 will not work
>as its a discrete signal conditioner not for measurement of an analog
>voltage. Since you want to measure battery volts, then a simple
>divider and large caps would be sufficient...

Well, I'm not measuring analogue voltages. But I will see a total of
about 60A fused potential from the battery if I'm not careful.
Clipping the input voltage is the trick I've been looking for. A
5-cent signal diode is an attractive option.

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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