Hi John,

The O2 sensor will have plenty of current to drive the transistor - at low
temperatures it's high impedance, but when it's hot the impedance drops.

I have not tested O2 sensors driving loads for long periods of time(2 or 3
hours is the longest), so it may be that there will be some damage done to
the sensor, since it's not like a thermocouple. It produces voltage as
oxygen "passes through" the sensor from the outside(exposed to air) to the
inside(exposed to exhaust).

They seem like pretty tough little critters, though, unless you crack them.
And the car's computer is not infinite impedance, it's usually 1 to 10
megohms, so you'd think that a small amount of current can't have any
cumulative effect. You could probably use a resistor in series with it and
the base of a transistor and be fine.

You've probably seen my O2 sensor web page, but just in case:
http://www.bobblick.com/bob/auto/o2sensor.html

Cheers,
Bob

At 08:50 PM 10/8/98 -0700, you wrote:
>I have installed an o2 sensor on my car and want to build an LED output for
>it. I was thinking that I could just drive the LED through a unity gain amp,
>maybe a small FET or bipolar? And the output of the O2 sensor would light
>the LED at .67 Volts, showing me a rich condition? Do LEDs light at .67
>volts? Would a small signal bipolar have enough impedance for an O2 sensor?
>Any help on this would be welcome. I want this to be very simple and fast to
>build.
>Thanks
>John
>

http://www.bobblick.com/