How do you use accelerometer measure tilt angle?

Answer 1

First, you have to make sure you're measuring the right thing. Tilt isn't specific enough. You'll want to measure pitch and / or roll.

Make sure your accelerometer is capable of measuring tilt. Read the datasheets carefully. For example, the ADXL322 is a 2-axis accelerometer which is capable of measuring tilt. The ADXL78 does not repeat NOT have this in the datasheet. You could try to use the 78, but it might not work at all.

You must mount the accelerometer so that the sensitive edges are parallel to the ground. In other words, it has to be lying flat on its back. ("How flat?" It must be as flat as you want your accuracy to be. ) In such a configuration, the chip listed above will give pitch as pitch = arcsin( Ax / 1g) and roll as roll = arcsin( Ay / 1g ). Ax and Ay are the two outputs. Read the datasheet. (I can't stress this enough.)

While you would usually put the - and + outputs into an instrumentation amplifier, the 322 chip listed above does not have such a thing. You would have to run it through some kind of gain circuit, and probably one with an adjustable gain and offset.