Can I offer a word of caution on adding modern micro electronics
to automotive electrical systems.
There are some excellent application notes written by electronics
companies on the characteristics of automotive electrical systems
which show that voltage spikes of +/- 60V of quite long duration
are common on a modern well designed and maintained system.
In addition the electronics should also be designed to survive a
battery disconnection and a 24v jump start on a 12v system.
To ensure that your electronics will survive this environment
does require a certain amount of care in the power supply environment.
In general modern microcircuits require a 5v or 3v power supply
and this is derived from a voltage regulator.
It is very important that a regulator suitable for automotive use
(Nat Semi LM2940 series or similar) is used rather than some
of the more commonly available regulators.
Ideally there should be additional protection of a clamping device
(GE Transorb or similar) on the input to the regulator and an in
line filter is a nice luxury.
Just scratching the surface of this one.
Contact me off list if you need more info
John Dowson
At 05:41 PM 7/4/1999 -0700, The Falcon Glen, TFGTV wrote:
>
>As I said previously the simple way to do this is to get someone who's
>skilled at electronics construction to build a simple divide by 2 circuit
>on a piece of veroboard or similar. Dividers are standard ICs and the only
>other components would be input and output buffers either in the form of
>single transistor amplifiers or operational amplifiers.
>
>Any first year electronics student should be able to rig up such a circuit
>for you.
>
>Since a tach does not measure a linear quantity like pressure or level, but
>a frequency, there are no 'scaling resistors' or suchlike. Essentially a
>tachometer is measuring the number of pulses in a period of time.
>
>Matt
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