Introduction: The Raven, High-pitched Torture

This is a little device that I designed for the simple purpose of being discreetly annoying. It waits for a predetermined amount of time, and then it starts emitting high-pitched beeps. I have programmed mine to take advantage of an interesting property of sound. That is, in general most people above the age of 25-30 can't hear very high-pitched tones (say, 17 KHz for instance). This means if you were to (hypothetically of course) place it in a classroom, it would start bugging the heck out of the students while the teacher/professor will (most likely) be completely unaware of the source of the disturbance.

Step 1: Parts

Not all of the parts I used here are necessarily required. For example, you could use a resonator instead of a crystal, or hypothetically neither. But I won't be going into detail about alterations, if you change something I'll assume you know what your doing. If you don't know what you are doing, you might want to stick to what I've outlined here until you are a little more comfortable with AVR's.

In this project I used an ATtiny25 microcontroller, a handy little chip. I picked this one for this project because it's small and has 2 timers, which will make the programming easy. In order to complete this project you'll need a programmer, there are lots of methods on the internet for programming AVR's (look up "AVR programming" on instructables or google). I personally use the USBtinyISP that Ladyada sells.

Except for the AVR you can proabaly scalvange most of this stuff from old electronics. I needed to make an order to Digi-Key anyway, so I just got this stuff there.

What you need:
*I used a 3.6 V lithium battery, you can use something else, just as long as the voltage is between 3 and 5.5 volts (the weaker the battery, the more quiet the speaker will be).
**You don't need the IC socket, but it will make it very difficult to reprogram the AVR afterwards if you don't use one.

Step 2: Prepare the Circuit

Now that we have all of the parts, we can start putting things together. I recommend you put everything together on a breadboard first, this will make it easier to program the microcontroler and you can get a feel for how the circuit works before soldering anything together.

The parts should be placed according to the schematic I have attached, it's a pretty simple design. The big block in the center is the AVR, and the number beside all of the wires coming out of it corresponds to a pin. To figure out which pin is which, look at the microcontroller. You should notice a little circular indent beside one of the corner pins, this marks pin one. You count the pins going around the chip in a counter-clockwise direction starting here. In the picture the part marked G1 is the battery, C3 is the 0.1 uF capacitor, C1 and C2 are the 18 pF capacitors, Q1 is the crystal, and SP1 is the speaker. All of the GND's connect together (to the negative terminal of the battery).

Step 3: Program

Once you have put the circuit together you need to program the microcontroller with the attached program. Because build environments can vary wildly based on you choice of operating system, programmer, and software, I won't get into setting that up here (For some help, you might want to check out Ladyada's page on this topic). For the purposes of this tutorial I'll assume you are using the same sort of setup in the above link, the USBtinyISP programmer, and a linux (or unix-like) operating system. If you are using a different setup (especially if you are using a different programmer) you will likely have to edit the Makefile (again, too much to get into here, try reading the comments in the file).

Once you have everything set up, and the programmer attached, it's time to program. I have found that the speaker can't be attached while programming, so you should unhook that for this step. It is also very important that the crystal be attached (along with the 2 18 pF capacitors), as it will become necessary after burning the fuses.

DO NOT perform the next step (burn-fuse) if you are not using the crystal, once you do the AVR will require a crystal to operate. If you burn the fuses and you don't have a crystal your out of luck until you can get one.

Now, we need to burn the AVR's fuses (see a tutorial on AVR's for more information, and the above warning). To do so, run this command from within the same directory you have the files: main.c and Makefile.

make burn-fuse

If you are programming this thing some other way, the fuse values are:
lfuse: 0xFD
hfuse: 0xDF

Next run the next two commands to compile the program and program the AVR:
make program

If you want you can run this command to clean up the excesses files that were created by the make command:
make clean

And there you go! Hopefully there weren't any errors and your AVR is programmed and ready to go. By default the AVR will wait for 15 minutes before doing it's thing, then beep for 1 second every 10 minutes for an hour. The beep frequency will be 17 KHz (quite high pitched). Check out the main.c file for more information and to change these values. I made sure to add lots of comments.

Step 4: Finish Up and Enjoy!

Now it is just a matter of fixing everything together a little more permanently. I used a little prototyping PCB I got from Radioshack a while back, you can use anything you want. A word of caution tough, this circuit doesn't have an on/off switch. I originally wasn't sure if I would be able to retrieve this after I... tested... it, so I didn't bother including one. It would be a good idea to add one, because at the moment the only way to turn it off is to remove the IC from the socket (and every time you do that you risk bending/breaking the leads).

When you finish with that, it's time to test out your new toy. Have fun!

Credit goes to Ladyada (Limor), who's site has been extremely helpful getting me started with AVR microcontrollers.

Step 5: Enhancements

This is a very simple device, and is open to many easy modifications and enhancements. One such enhancement would be to add an on/off switch. You might also consider adding some sort of control that will allow you to vary the timing and delays of the beeping. Perhaps a potentiometer, or some jumper pins.

One good modification would be to find a way to increase the speakers volume. You could, for instance, drive the speaker with 2 AVR pins. Alternating which pin is on and off to provide the maximum possible voltage amplitude range to the speaker. You could also have the speaker go off at random intervals to provide maximum annoyance while making it even harder to locate.

I'll leave the details up to you, get creative and have fun!

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