My houses' alarmsystem was broken when I moved in. The siren outside of the house however was still working. It is able to produce some 110db noise and a red flash. My new alarmsystem (which is part of my home automation) is capable of operating a flash and switching 12V on and off in case of an alarm. However, it does not produce any siren wailing which is why I decided to use an arduino for this task.
I found an old question on this forum with sort of the same idea which is called "Siren from an AC Horn Speaker via Arduino".
However, I didn't get it to work in my context, so I did a lot of research and came up with a solution that seems to work pretty well. However, since I am not an expert, I am not sure if I am missing something.
I already asked on StackExchange if this approach makes (electronically) sense. I am crossposting my question and this project here as well as a reference for people looking for a similar solution. I couldn't react to the original post on this forum to append my thoughts on it, because the topic is already closed. Hence this new one.
Forget TIP120, much better modern alternatives are available, such as IRLZ44. This is a MOSFET and will result in a much lower voltage drop and less risk of overheating. TIP120 is an N-P-N Darlington, which is an older type of transistor with several drawbacks.
When using a MOSFET, add a 10K pull-down resistor on the Arduino pin. Keep the series resistor between the Arduino pin and the transistor.
Powering the Uno with 12V is on the limit. If the only thing the Uno is powering is itself, and maybe a few low power components, then fine. But if any significant current is drawn from the 5V pin of the Uno, there is risk of overheating or damage. If you give more detail about other components connected to the Uno, we can advise if the risk is significant.
One final point. If you modify a professionally installed alarm system yourself, you may invalidate your insurance and any claim might be rejected by the insurer.
I noticed that when I put a bigger Resistor in between the Digital Pin of the Arduino and the Base of the Transistor, the siren sound is also quieter. Is a potential overheat the only trouble I could get into when continueing with using the TIP120 or are there any other advantages when using an IRLZ44?
Regarding any other components: There aren't any. It's really just the Horn driven by the Arduino using a Resistor and a Transistor. Nothing else.
As you noticed, there is a voltage drop with TIP120 which reduces the siren loudness. If the resistor is too high, the siren is quieter and the transistor could be damaged. If the resistor is too low, maybe the Arduino pin could be damaged.
How does the Arduino know when to switch on the siren?
It doesn't. As mentioned in the post above, the alarmsystem itself switches 12V on when it believes it should trigger an alarm. This +12V is powering on the Arduino which runs some code that only generates a tone on Pin8. The Arduino is really only a siren effect generator.
I was about adding a comment in my recent reply that people might ask why I am using an arduino and don't implement a siren generator directly in a respective circuit instead. That would be cheaper and an arduino might be some kind of pearls before swines. However: This way I can easily control what's happening and even change the sound in a matter of seconds if wanted... also, I had a spare Uno board.
You could use one of PJRC's Teensy boards to sample and create any sound you like not just simple tones or siren like sounds.
You could have some real fun with all kinds of sounds including voice.
Paul has some really cool tools to help create the data.
If the sounds are not too long you won't need an SD card to hold the data. https://www.pjrc.com/teensy/td_libs_Audio.html
I've used several of the Teensy boards on various projects.
They are nice, and are well... really small so they can fit in very confined spaces.
I've worked with Paul on a few projects in past, great guy and he has great support for all his products.
