#62 SIMPLE DIY ATTINY85 ALARM
Upon experiencing a few break-ins on the farm I decided to look for a simple alarm system to monitor certain door. For such a simple project a full on alarm commercial alarm system would be overkill.. So I endeavored on the short and fruitful DIY journey.
My system is based on the simple circuit from Great Scott on YouTube. The brains of the circuit is the Arduino programmable Attiny85 MCU. A reset push button, notification LED, arm/disarm toggle switch, notification buzzer and magnetic read switch are the IN/OUT components used.
The project runs on 12 volts (12 volts for the siren) which is filtered down to 5 volts for the Attiny85. My plan is to use a 12 volts 7 ampere lead acid battery combined with a smart charge board to power the project effectively.
After soldering the components onto the board everything worked fine. However the siren was very soft and the 2N2222A transistor was getting extremely hot. This is because the transistor has to provide a ton of current to the siren in order to get it working at 100% capacity. A quick fix is to remove the 1k resistor between Attiny85 pin 1 and the transistors base.(replace it with normal wire) This allows the siren to be louder but the transistor is still hot. A possible permanent solution will be to just use a 5v relay or look for a transistor with a higher current tolerance.
In the end I decided to go with a 5v relay module since it’s a quick reliable solution but is more expensive than a transistor. In a future upgrade I will most likely use a BC517 darlington as referenced by this article. It will be cheaper than a relay and provides more than enough current for a 15W 1 tone siren (380ma).