Hi Russell,

Maybe you can help me understand the phenomenon. I did post a message to the
list on what I've found so far, but I'll fill in some answers for your
questions....

> The opto should be more than fast enough. I suspect that you are using it
with a large load resistor and that
> capacitive effects are predominating.

Actually, one of my first mistakes as no load resistor because it drove Vo
to a constant high (9v). Now, this was also when I had the output GND
accidentally floating. Once I *properly* grounded, I needed about 4.7k to
get the wave to go from 0-9v. Using the 1.9k recommended gave me an offset
of 5 volts - so the wave was 5-9v. When I scoped the hall sensor, there was
a clear 0-9v swing (I wanted to mimic the output as best as I can).

> If you use a resistor R from Vcc to opto collector and place opto
> emitter to ground then it will turn on rapidly. When you turn off the
resistor R must
> pull the opto output high by itself. If you have a capacitive load here
the time taken will be several
> times the RC product.

I'm going to have to research this. Unfortunately, I've had a hard time
xrefing the parts on the tach to any known part (it's a '94 vintage unit).
Not knowing what the tach is doing with the signal, I'm extremely lucky that
I got it to work. I'm a little unclear if the tach is a capacitive load - I
do know that the tach input signal to ground is about 2.4Meg ohms.

I do know that if I scope the hall sensor without it connected to the tach,
I get an incredibly dirty signal (of course, no load). Once I connect it to
the tach and then scope the input, I get a nice wave. Without ripping apart
the tach I'm not sure what they've done for signal conditioning.

> How big is your load resistor?

Nothing now. If I add one, I get a flat line.

>You don't say how fast your tach needs to run - lets say it is 6000 rpm =
> 100 Hz. With a 1 megohm resistor you shouldn't be driving more than about
0.002
> microfarad capacitive load. For starters try a much smaller load
resistor - say about 10k. This will
> draw under 1 mA max at 9 volts.

470Hz = 2000
1.4 Hz = 6000

I'm horrible as ASCII, here's a text example

PIC18F84:
RB1 - to 250ohm resistor to Opto anode

Opto:
Anode - from PIC
Cathode - to ground
.1uF cap from anode to ground

9v in to Vcc (transistor base)
Gnd to GND (transistor emitter)
.1uF cap from Vcc to GND
Vo out to tach

Tach load is about 2.4Meg ohms.

That's it. It works. I'm assuming the tach load replaces the load resistor
???

-DO