1. The ability to beep at random intervals
2. Minimal parts needed (simplicity/elegance of design)
3. I wanted to use a microcontroller (because I recently started the journey of learning about microcontrollers)
This project was inspired by an article in MAKE magazine about making a similar device with a 555 timer chip.
After making my prototype, I searched instructables.com and found the Raven, which is a similar beeping thing using a microcontroller. I decided to add my instructable because it uses less parts and has random intervals.
Step 1: Parts Required
- ATtiny13 microcontroller (Sparkfun.com)
- AA batteries (3)
- Battery Holder with switch (Digikey part # SBH-331AS-ND)
- 8-pin socket for microcontroller
- Speaker (small 8 Ohm or piezoelectric buzzer)
- Silicon adhesive (RTV)
I made this entire project from parts I had in the house. You can use almost any battery(s) that has a voltage within the microcontroller's range (1.8-5.5 Volts for the ATtiny13). The higher the voltage, the louder the beep.
Almost any small speaker will work. You may want to experiment with different small speakers. I chose the speaker with the plastic membrane because it didn't make a click noise at the end of the beep like some other small speakers.
You can adapt the source code for other microcontrollers. It would potentially only require a change to the timer register settings.
Step 2: Prototyping
Connect the speaker to the ATtiny13: Pins 4 & 5
Connect the battery to ATtiny13: Pin 8 (+) & Pin 4 (-)
So Pin 4 has the negative battery terminal and one speaker wire (doesn't matter which one). Pin 5 connects to the other speaker wire, and pin 8 connects to the positive side of the battery.
I realize that the reset pin (pin1) should be pulled high, but it works without doing that, and this project makes no attempt to be formally correct.
Note that if you wanted better quality sound, you could put a resistor-capacitor low-pass filter on the output pin inline with the speaker. But for this project, we want an annoying sound anyway.
The prototype made me realize that the sound was not very loud. It still might work well for you in a quiet environment (office?). To increase the volume, I changed the battery from 3V (CR2032) to 4.5 volts (3 AA).
Step 3: Firmware
2. Open a command window in that directory.
3. "make program-beep" to program the ATtiny13
You can change the minimum/maximum time between beeps, the beep frequency, and duration by changing the parameters near the top of the source file beep.c. The files in the zip file have the following parameter values:
secMin = 180; // Minimum number of seconds until beep
secMax = 600; // Maximum number of seconds until beep
freq = 6000; // Frequency of beep in Hz
msDuration = 1000; // Duration of beep in milliseconds (1000 = 1 sec)
So it beeps at 6kHz for 1 second every 3 to 10 minutes.
Feel free to experiment with different values. However, extreme values could cause unexpected results. Let me know what values work well for you.
Note that since Pin 4 is used for the programmer and for one of the speaker wires, you must disconnect the speaker from pin 4 when programming.
If you need more information on this step, see this tutorial.
Step 4: Final Assembly
1. Solder parts together
2. Use Silicon adhesive to hold it together and provide strain relief for the connections/wires.
I used a chip socket so I could remove the chip and reprogram the beep parameters (interval, frequency, and duration). So that the socket sits flat on the battery case, I bent the used pins (4, 5, and 8) horizontal, and cut off the other socket pins.
Your choice of battery and speaker might depend on how you want to use it. I originally wanted a very small package so I could hide "anywhere". I had considered using 1.5 button cell batteries (3), but I couldn't think of an easy way to make a battery holder. I ended up liking the 3 AA solution. The case is the right size to mount the microcontroller and the speaker.
It also works well to attach Velcro. I found during testing that hiding it on the underside of a table or desk was convenient. The 3 AA case allows me to connect both sides of the Velcro, remove the tape covering the sticky side, and slap it under a table. Then when I want to retrieve it, I can simply reach under the table and rip it away (leaving the 'hook' side of the Velcro still under the table).
Have fun, show us a picture of your beeper, and tell us the story of your victim.