I have these old motion sensors that i've hooked up to analog pins on a duemilanova. They have three wires, GND, V, and output. The only info I have on the sensors is that they take 10-30V and output max 200mA. I'm giving them a regulated 12V. On the out wire, when I put it on a multimeter and activate the sensor, I get around 14V that's going into the analog pin and it gives me a reading of 1023 in the serial monitor. So, I assume this is way too much and I'd like to lower and regulate the voltage.
I can think of two ways to do this...I could use a 7805 or I could get a couple resistors and make a voltage divider (a little cheaper and easier). I tried making a voltage divider but I was only getting a .5v reading...which is weird. I used a 1kOhm resistor with a 500ohm resistor. I connected the out wire on the sensor to the 1k resistor, another wire going from 1k resistor to arduino and also going to a 500ohm resistor that's going to ground. Is this not the correct way to do this? I was just following what I could find on the internet like the wikipedia page: http://en.wikipedia.org/wiki/Voltage_divider
According to the voltage divider formula I should get getting around 4.6V...so I know I'm doing something wrong. Would a 7805 just be better and more reliable anyway?
Any advice/guidance? Thanks!