Phone-In-Car Reminder

I hate forgetting my phone in the car. I used to put the phone on my lap, in my pocket or on the passenger seat, but since I’ve installed a dock, I keep forgetting the phone because I’m not used to it being there – There must be a solution.

After some thought I got it – An alert system similar to that of the car’s headlights, which beeps if you forget to turn them off. I wanted the solution to work on any phone and any car so It mustn’t run on the phone (software combined with NFC, for example, needs a lot of maintenance to support all devices).

Generally, there are two conditions that have to be met in order to trigger the alert – The engine must be off and the phone must be docked.
Determining whether or not the phone is docked can be done in many ways, I chose the burglary alarm approach – a ray of light being broken by the phone. This is implemented via an IR LED and an IR photodiode.
Engine-off detection is a little more tricky, I didn’t want to tap into any of the car’s systems because I wanted a simple implementation and model independence. Finally, I noticed that when the engine is off, the 12V power socket is disconnected (and that’s true for most cars). This provided a solution to one problem but created another – I still need a way to power the system when the engine is off.
Rechargeable batteries are one way to go, but they will die very soon because they will be kept at 100% most of the time (not good for Li-ion batteries and I don’t want to get into charging circuits). Supercapacitors were the ideal solution for me, constant charging doesn’t effect lifespan, they can hold enough charge for a few seconds after power is cut and they are inexpensive and small enough.
Eventually, I used four 3.3F capacitors rated 2.5V in sieries, providing 0.825F and 10V rating (it’s recommended that the rating of the capacitor be about double the operating voltage)

Initially, I thought about using an Attiny85 running Arduino, that will be a very simple implementation with very few components and just a couple of lines of code, but since i’m using capacitors as a power source I need to conserve as much power as I can.
A few iterations in, I came up with the circuit in the pictures.

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I used very inexpensive and simple components, no IC’s, no controllers, no code. Minimum power consumption.
The circuit will be connected to a 5V USB car charger with a current limiting resistor.

The LED and photodiode will be positioned on each arm of the dock’s clamp. When the phone is docked, it blocks the light and the photodiode opens making the transistor close (there are two implementations shown in the drawing for the same circuit based on the transistor type). The green LEDs in the prototype indicate that the voltage source is connected and that the light is broken, a kind of hardware debugging.
The beep will come out of a piezo-electric buzzer.

The system hasn’t been installed in the car yet because I’m going to replace my car in a couple of months. rest assured it’ll be installed in the next and a post will follow.

If you want more information regarding the circuit and the job of each part, leave a comment or Email me.


    1. Well, I tried your app and it doesn’t do what I want (or even what it’s supposed to do).
      If you read the article, you must have noticed I want a platform independent solution. What I’m trying to do here is a hardware solution that has nothing to do with the device itself. This means that my solution will work with any operating system out there, the only requirement needed is for the device to block light (and even that can be bypassed by a micro switch when transparent devices will be invented).

      The app you suggested not only requires software to be installed (duh), but it’s not cross platform and it doesn’t confine its activity to the car. Whenever you connect and disconnect a BT device (be it a speaker phone, media player, or any other BT device, even my BT controlled sims costume) the app will sound the alarm. This is not the functionality I’m looking for.
      Moreover, it requires a BT kit that costs much much more than a phone mount and a couple of transistors.


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