So apart from a 100-in-1 electronics kit I had when I was little, I've done nothing with electronics, and after running across Dave's blog I got the itch. I pulled some parts from said kit and played with them and made LEDs flash and all that, but I want to make something useful, if not cost effective. So I'm building a USB temperature sensor.
Here's what I've got so far:
http://partiallystapled.com/~gxti/circuits/2010/08/12-project.pngOn the left is a LM35 temperature sensor, in the middle is a MCP3301 ADC with SPI output, above it is the reference voltage, and to the right is the FT232RL USB interface which I am going to use to bit-bang SPI instead of dragging in a microcontroller. Vdd should be 5 volts considering it will probably (eventually?) leech off USB.
If anyone could point out what I've done horribly wrong, or even what might improve precision, I'd be very grateful. I'm not worried about everything to the right of the FT232RL as I'm actually getting a Sparkfun breakout board that has the USB port on-board -- no sense trying to solder SMT parts on my first real project. However, I am worried about the reference voltage (U1), and whether that together with D1 will allow me to accurately measure somewhere in the ballpark of -0.3 to 0.8 volts (corresponding to -30 to +80 degrees Celsius). I don't know much about ADCs but I'm assuming that Vref corresponds to the maximum positive voltage measurable, and I'm not sure whether what I've designed can measure (small) negative voltages.
Thanks for any tips or corrections!