When it came time to design this project, I had two choices:

I could either create something useful and practical or...I could create something that would no doubt drive others out of their minds and provide plenty of entertainment in the process. The choice was not hard.

A little background:

We have all experienced it. For some reason, around 2am, the battery in your smoke detector ends its happy (and previously quiet) life and lets you know by that horrific death rattle we shall call a "chirp." It doesn't stop. Despite you wandering around in your pajamas with your head cocked to one side muttering to yourself looking not unlike a zombie, stopping and listening below each of the smoke detectors, and praying harder than you have ever prayed that you either have a spare battery or that the battery in your TV remote has enough juice to last through the night, you still end up listening to the same chirp over, and over, and over.

Imagine if you had the power to get other people to do that. Just think of the fun you could have.

Before we delve into this glorious little device, a warning:

Smoke detectors save lives. It is generally a bad idea to do anything that would encourage someone to take a smoke detector out of service. When your little joke is over PLEASE make sure that your smoke detectors are not disassembled, or not in their proper place, or smashed into pieces, or otherwise out of service. While you are at it, change the batteries.

Now, on with the show

Step 1: Gather Your Parts

In its most basic form, this is little more than a simple timer that sends out a chirp every 90 seconds. What makes it special is the motion detector that prevents beeps when someone is near so that it is even harder to find. When the motion sensor detects movement, the clock resets to about 1.5 times the time of a normal chirp. This delay lets the device remain hidden and not chirp when someone is close by.

First we will gather our parts. We will program the micro-controller and assemble the parts. Last, we will sit back and enjoy the chaos that ensues.

Let's get started.

Parts

Parts wise, you only need a few bits and pieces. Below I have listed the parts that I used as well as a few comments.

Micro-controller
- For this project I used a cheap Arduino Nano knockoff. Normally, I prefer the real Arduinos because I like the company philosophy and I feel that I should support them when I can. Because this is very likely to end up on the wrong end of someone's anger, I'm using the cheaper part.
Breadboard
- Either a breadboard or a PCB. I started with a breadboard because that is what I had on hand. To fit the project into my box and just make it a little neater, I transferred the whole thing over to a stripboard. You can stop at the breadboard if you want
Jumper wires
PIR Motion Sensor
- These can be found all over the internet and are not expensive. Adafruit has a great tutorial on how these work. Click here to check it out.
Passive Piezoelectric buzzer
Project box
- If you decide to put this in a box and wrap the thing up like a present, you will need a box of some sort. I used a good 'ole Altoids tin.
Battery
- You need a power supply of some sort. I used a cheap rechargeable power bank.

That's it for parts. Let's setup the micro-controller.

Step 2: Program the Microcontroller

The sketch I provided isn't anything special. It is pretty well documented so any changes you need to make should be fairly easy to implement. Make any changes you need.

Following the instructions for your specific device, upload the sketch to your micro-controller.

Chirping_Box.ino
Download

Step 3: Get It Together

There isn't much to assembling the actual device. The PIR sensor needs to be connected to a 5v power supply and it needs to share a ground with the micro-controller. It also needs a line in on the micro-controller so that the micro-controller will know when the motion detector senses movement. If you use the sketch that I have provided in the next step, you will want to connect the output pin of the PIR sensor to pin number 4 on the micro-controller.

Depending on the PIR sensor that you are using, you may need to adjust the settings for the sensitivity, the pulse length, and the timeout length. Adafruit has an excellent tutorial on these nifty little devices if you need more information.

Connect the piezoelectric buzzer in a similar fashion. The positive goes to a 5v supply and negative connects to the shared ground. The communication line for the buzzer needs a PWM input so connect it to any PWM pin on your micro-controller. For the sketch that I am supplying, I used pin number 8.

I ended up using fairly long jumper wires. The reason for this is that I wanted the buzzer and the PIR sensor on the lid of my project box. The sensor and buzzer needed to be able to move without coming disconnected if I ever wanted to open the lid

If for whatever reason you cant use these pins, the sketch is well documented so it is easy to change the settings that don't work for you.

Once you are all wired, double check your connections and let's try it out.

Step 4: Go the Extra Mile

Now, stopping at the previous step is perfectly acceptable. If your beeper is going to get tossed, crushed, or otherwise destroyed, it would make sense to not spend a lot of time putting it in a box. Skip to the last step and get ready to not be someone's best friend.

If you want to spend just a little more time on this project, you can make it prettier, neater, and all round more devious.

The only extra parts that you need are a prototype board, some tape, a couple of drops of hot glue, and some kind of container.

Step 5: Setup Your Board

Get out your soldering iron and let it warm up. Decide where you want your micro-controller and if your board is a strip board (all the holes running across the board are connected, you will need to break the connection between the two sides of your micro-controller. A sharp knife or a razor will do the trick.

Make sure that the strips are no longer connected all the way across by checking for continuity. If you get any resistance at all, your strips are still connected and need to be re-cut

Check the location of your micro-controller again and solder away.

Once your micro-controller is in place, solder the leads from your PIR sensor and buzzer to the appropriate places.

Step 6: Build Your Box

I wasn't going to put this in a project box because I wanted to be able to move the PIR sensor so that it was effective but still able to be hidden. I happened to come across an empty Altoids tin and my plan changed.

The tin needed a little work to make the whole thing fit.

First, cut two holes in the lid of the tin. One hole will be for the speaker and the other will be for the PIR sensor. Both holes need to be fairly tight.

Fit your components into the holes and drop a bit of hot glue on the sides of the components to keep them in place.

Second, cut a hole for your power cable if you don't have a power supply that will fit into your box.

Since using a metal box for an electronics project is just not that great of an idea, I needed to add a strip of non-conductive material to the base of the box.

Step 7: Stuff Everything in the Box and Seal It Up

Once your components are in place, drop a bit of hot glue on the corners of your parts to keep them in place. In the next step you will be sealing this thing up so you don't want anything coming undone or shifting out of line.

Step 8: Set Your Trap and Enjoy the Show

Seal your box with whatever you have on hand. I covered mine with black duct tape. I poked a hole in the tape where the buzzer was so that the sound was not muffled.

You are now ready to cause a little chaos. Plug in your power supply and get ready to enjoy the show. You have several seconds to place your device before it will first go off. As long as your are moving, the timer will reset every time that the PIR sensor detects movement. This should give you plenty of time to find the right spot and get your sensor set up in the right direction.

For placement, one of the most frustrating places to put this little guy is in a HVAC vent. Not only is it nearly impossible to find, the darn thing sounds like it could be coming from any number of rooms. It's a little more work to get it in there but definitely worth it.

Thanks for taking the time to check out this Instructable. If you make this thing, I would appreciate a comment letting me know how it went. Have a great day.