using an analong meter for diy-wb

[Phil Hunter]

Enzo wrote:
> what do you guys think of using an analog meter for 
> the diy-wb?  I can get an 0-3Vdc analog meter and 
> relabel the voltage to the actual a/f ratio and it could 
> be easier for tuning. Just wanted to know if any of guys 
> have done this before and if it works ok..

Moving needle meters are deflected by magnetism, or more
precisely electro-magnetism created by current, not voltage.
Assuming you are talking about a plain meter and not something
w/ electronics between the input and the meter such as a
Volt/Ohm Meter, consider this:

Cut the trace between U4 pin 6 & pin 7, jumper pin 6 to J8,
and substitute a 10 turn potentiometer for R15. This will
make U4B into a voltage to current convertor. The resistance
of the pot depends on the sensitivity of the meter, but 5-10K
s/b in the ballpark for a 1mA meter, 50-100K for 100uA.
Positive meter lead goes to J8, other to J12 (ground).

Since there are some spare IC locations on the PCB, you may
want to wire up another op-amp instead, leaving U4B & J8 for
their original purpose. To keep things simple to explain, get
another IC just like U4, let's call it U6. Jumper U4 pins
4 & 11 (power) and 5 (signal) straight across to U6 pins 4,
11 & 5 respectively. U6-7 goes to one end of the potentiometer.
The center (wiper) lead of the pot goes to U6-6 and to the
positive (+) lead of the meter, the other meter lead goes
to J12 (ground). 

To cal the meter ( O2 sensor disconnected ), using the table
of voltage to A/F ratio from the docs, put the "leanest"
voltage on pin 5 of U4 and adjust the pot for full scale
deflection of the meter. Since the meter will read backwards
from intuition, consider turning it upside down.

Disclaimer: no, I haven't tried this...yet.

rgds,
philh
(digest)

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