Update resistor values to match what works. Add LED.

Testing revealed that the phototransistor appears to need quite a bit
of current to get the rise time within reasonable limits. Change the
resistor feeding it to 680 ohms (from 3.3k). This also required
changing the op-amp bias voltage from 1/2VCC to 90% VCC. Swapping in a
1k resistor for the 10k there fixed that nicely.

Signed-off-by: Keith Packard <keithp@keithp.com>