Finding your tent in a large festival or campsite can often be pretty tricky! Miles upon miles of similar looking tents, dark paths between them and as often as not (for me at least) a beer inside you to fuddle your tent finding prowess.

Enter the festival tent finder! This super simple circuit, when strapped to a mobile phone, detects the illumination from a mobile phone backlight and turns on a small battery powered relay. This relay can be used to turn on a set of battery powered LED lights fastened to your tent.

All you need do to find your tent, is phone it! When the phone in your tent rings, the lights will turn on. Easy!

This project is a collaboration* between Kiteman and myself.

(*Kiteman adds: I had the basic idea ("Wouldn't it be cool if you could phone your tent to find it?"), but Jayefuu did the actual work designing and building the circuit. I had something much less elegant, and more expensive, in mind.)

Step 1: Circuit Design

Functionality:

The objective is to design a circuit that can be easily triggered from the backlight of a cheap, old mobile phone. As you ring the mobile, the screen backlight will turn on and our simple circuit will trigger a relay. This relay can easily be attached to some battery powered LED lights or a lantern on the outside of your tent, allowing you to easily see your tent.

Components:

1 x breadboard
1 x TEPT4400 phototransistor
1 x 2N2222 NPN transistor
1 x 1N40001 diode
1 x SPDT (single pole, dual throw) 9V relay
1 x 100 Ohm resistor

Step 2: Veroboard Layout

I first prototype the circuit design on a breadboard to check it would work.

Now we know the circuit works as expected it's time to transfer it to something smaller and more permanent. I decided on a quick build using stripboard instead of a PCB since I needed to get it to Kiteman asap.

Unfortunately, in a rush I didn't get any mid-build photos. The second photo shows the direction that the stripboard tracks are running.

Components:

1 x TEPT4400 phototransistor
1 x 2N2222 NPN transistor
1 x 1N40001 diode
1 x 100 Ω resistor
1 x SPDT (single pole, dual throw) 9V relay
1 x SPST switch
1 x screw terminal

Step 3: Testing

To test it, connect a multimeter* in continuity test mode across the common (C) and normally closed (NC) screw terminals which connect directly to the relay. It should beep.

Now connect the mutimeter across the common (C) and normally open terminals. It shouldn't beep.

Finally, place a mobile phone, screen down on top of the relay so that it is above the phototransistor. Ring the phone, the screen will light up to show there is an incoming call and the circuit will activate. Upon ringing the phone the relay should switch, causing the common and normally open terminals to connect and the multimeter to beep.

(*Kiteman adds: if you don't have a multimeter, just make a simple series circuit with a battery and a light, and connect loose ends into the terminals as desired. If the relay works, the light will work.)

Step 4: Enclosure

An enclosure for the electronics is essential to hold the phototransistor in front of the phone screen as well as to exclude light.

I chose to make this one from 1.5 mm white styrene after seeing a Youtube video by Jimmy DiResta. It allows cutting easily and cleanly using just a few scores from a knife before snapping. It glues quickly and cleanly with a dab of DCM (a solvent). The edges can then be scraped and sanded flush before painting to achieve a seamless, clean look for a custom sized box. Alternatively, squeeze it into a match/raisin box or altoids tin.

Step 5: Using

This is where Kiteman took over, having taken possession of an interesting plastic bag on a rainy day in Huddersfield...

Finally, to use the tent finder, cut and strip the live wire on a set of battery powered LED lights. Attach the two ends to the tent finder with the C and NO screw terminals.

If you are working in daylight, it is easy to check things are working - connect everything, switch it all on, and, when you cover the phototransistor, the lights should go off.

Attach the enclosure to your receiving phone with an elastic band (if you can think of a more elegant version, feel free), then test it works by ringing the phone!

Safety: Do not use this circuit for switching mains (120/240 VAC) voltage. The stripboard layout and enclosure aren't suitable for this.

Step 6: In Action

Because a contest deadline loomed before my next camping trip, here's a quick demonstration of the tent-finder being used to find model "tents" in my shed.

Step 7: Other Uses

Because the tent-finder can be used to trigger most battery-powered devices, you could also use it to do more than just flash lights in a tent:

It could wake you gently - if your phone flashes when the alarm goes off, you could set a silent alarm that triggers a gentle breeze to wake you up without annoying flat-mates.
You could build a battery-operated door-lock, so that anybody who knows the phone number can get into a particular locked room.
You could set up a hallowe'en display, where you put the number of the tent-finder on a sign outside your house, and anybody ringing the phone will start off some animatronics or sound-effects.
There are other, more sinister uses as well, but I'll leave those to your imagination...