This project demonstrates how to create a simple DIY Car Light Turn Off Reminder using an Arduino Pro Mini, a DC-DC Converter and scrap parts. The acoustic alarm helps prevent leaving the car lights on and draining the battery. The device can be installed behind the dashboard and is meant to be used in older/classic cars, which do not have a builtin alarm.

• Arduino Pro Mini (1-2€)
• Mini-360 DC-DC Adjustable Buck Converter (1-2€)
• Small speaker (can be salvaged)
• A few resistors (only a few cents)

Total cost: 3-5€

• Buzzes based on the state of two logical 12V inputs
• Device is meant to be supplied with power when car lights are on (e.g. power input is connected to the light switch).
• Alarm sounds when ignition is off (IGNITION_PIN LOW) and door is open (DOOR_PIN LOW).
• The device is designed for door switches which tie the signal to GND when the door is open.
• DOOR_PIN has a pullup resistor which ties the GPIO high when the door is closed, preventing the input from floating.
• Makes a somewhat pleasant bell sound by using the Mozzi library.

• Voltage dividers are chosen for 3.3V logic. High/Low GPIO states might not be recoginized correctly when using an Arduino with 5V.
• Current draw is about 25mA@12V (0.3W). Choosing higher resistor values might reduce this.
• No protection against voltage Spikes from alternator, ignition coil, etc. -> Expected lifetime/robustness in automotive environment unknown.
• The device does not use a dedicated amplifier. However, when choosing a suitable loudspeaker and enclosure the output power of the Arduino should be sufficient to reach a decent volume level. You can experiment with different enclosure sizes to get an efficient sound transmission.

The following pictures show a cheap quick and dirty build using a scrap "Ricotta" cup as a case. A speaker salvaged from an old PC is used. The circuit was built on a stripboard.

The video below shows the function of the device installed in a 1973 VW T2 van (unmute video). Door switches where not used in this case, as they need fixing. The alarms sounds when the light is on and ignition is off. In the background, unrelated to the alarm, you can hear birds singing, if your're wondering :)

Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, as used by Mozzi library