DIY-WB LED problem (KC-5300 display)

[Peter Gargano]

Phil Hunter wrote:
> 
> Looking briefly at the schematic, here's my suggestion: remove
> IC1 from it's socket, nothing to the sensor input, on IC2a short
> pins 1 & 2, this will make it a unity gain buffer where pin 1
> follows the voltage on pin 3. 

IC2a functions as a comparator feeding into a simple digital input
at RB0. RB0 cannot be made an A/D input (there isn't one on the 16F84)
and the software does the A/D conversion.

> Vary VR3 from end to end, pin 1 should
> vary from about 1/4 to 1/9 of 2.5V. Remove the short and jumper pin 1
> to the sensor input, this will turn it into a 3X amplifier, now
> varying VR3 should have pin 1 varying from 3/4 to 3/9 of 2.5V. This
> should give you confidence that all the input circuitry (with the
> exception of VR2, the 3.3K & the 560K ) is correct.

The PIC's (IC1) RA3 output produces a PWM signal that appears as a
voltage on pin 3 of IC2a. It's compared with the voltage from the
sensor in a successive approximation fashion to produce a firmware
8 bit A/D converter. Any kind of serious testing probably requires
a 'scope, but I've used one of the sound card input attenuator boxes
and CoolEdit to see that everything is working as it should.

Make sure IC2b (in the same package) is working by adjusting VR1
(the light/dark auto-control preset) to dim the display - if it's
working okay then IC2 is probably working - if not ....

Peter.
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