I am looking to program and install a feature in my car where when i turn the ignition on, an audio message plays "Engine On". I don't want to integrate this into my cars stereo system. A small speaker hooked up to a micro controller would be fair. I am guessing i will need a trigger wire from the 12v ignition switch to the micro controller so activate the sound.
It has been a few years since the last time i messed around with Arduino. I need a refreshment on how this will be programmed and wired.
The Arduino doesn't have a DAC so there's no true analog output. And there's not much memory for audio and it can't directly drive a speaker.
So look for an [u]audio board[/u] or "audio shield". These have a memory slot, a DAC, an audio clock, and enough processing power to read & play an audio file. Some of them have built-in MP3 decoders and some have amplifiers to directly drive a speaker.
The Arduino only acts as a "controller" and in your application you shouldn't even need an Arduino because it's only playing one sound file.
I suggest buying from a reputable supplier (such as Adafruit or SparkFun) where you can get complete specs & application notes. If you buy the cheapest thing you can find on eBay, " you're on your own..."
There is nothing really "Arduino"about this... Unless you really want to go to the effort of decoding the audio via a microcontroller it's probably easier to buy one of those cheap sound modules and a small all-in-one class D amps. Now you only need to trigger it when you're ignition is turned on. Extra note, technically that does not need to mean the engine is on...
You can put a sensor to an ADC pin from the ignition relay. You would need to limit voltage and current with resistor divider. Try 100k and 50k to ground. the arduino ADC in goes in between.
If voltage is sensed you would have it trigger a DF player or some such to play a sound... Hopefully this project does more than this.
If this is all it does, you can get away with using a ATTINY