LM3914

[BIGED2022 at aol.com]

I made an O2 sensor meter with LED display driver LM3914 for my truck and it's
really neat.  I got the info from you guys, thanks.  I added a transistor to
the LED brightness control section and connected it so the LEDs dim when I
turn on my headlights.  This headlight dimming feature throws the 1volt
calibration off about .1 volt when I turn on the headlights.  How come?  I
have a 2.2k resistor and a 1k pot between pins 6 and 7 that I adjust to
calibrate full scale to 1 volt. Pin 7 goes to ground thru a PNP transistor and
a 680 ohm resistor on the emitter. The base of this transistor is connected to
a wire that has 12v when headlights are on. 
One question on the O2 sensor.  How fast should the voltage swing rich/lean
overall?   Mine swings rich to lean fairly steadily about every .5 to 1
second,  seems slower than what I expected.
Thanks
Eddie Lord