06-21-10
Open Source Hardware Ctd.
Well, I got the PIC16F88 connected to my Voltage Sensing circuit, and it works great. I am running the ‘F88 on it’s internal (8MHz) oscillator, and have disabled the Brown Out Reset (BOR). It operates between 2.0V and 5.5V quite well. I am still using the DS1803 100K digi pot. Attaching fixed resistors to the High and Low end of the potentiometer increases the resolution. I am using the following:
+V – 100K – 220K – H0pin
L0pin – 33K – GND
Those value gave me the best range between trigger points of the pile of resistors that I have on hand. I am up in the upstate with limited resources 
Here’s a graph of the datapoints. It’s pretty darn straight!
I created the graph here.
I will be able to get much more range when I use a lower value digi pot. Also worth noting: the voltage divider on the TL7700 sense pin (which the digi pot is a major part of) sets an internal hysteresis based in part on the value of resistance on the high side of the divider. Higher the resistance, larger the hysteresis (simply put, RTDS). I tried to use a much larger value resistor connected to the H0pin and was unable to get a trigger in the lower voltage range. The effect of the internal hysteresis appears! One more reason to go with a lower value digital potentiometer.
Next up: Solar Recharging Circuitry! Stay tuned…
