One of my current projects: Arduino police siren w/simple, custom transistor amplifier/speaker driver circuit

By Gabriel Staples Written: 16 Jan 2016
Last Updated: 2 Oct. 2017
History (newest on top):
2 Oct. 2017 - link to source code added
16 Jan 2016 - first written

A Few Other Articles:

• The Power of Arduino
• 100+V Arduino/AVR Microcontroller Input/Output (I/O) Pin Over-Voltage Protection Using a *Single* Resistor!
• Restoring/Recharging Over-discharged LiPo (Lithium Polymer) Batteries!
• Parallel Charging Your LiPo Batteries
• Beginner RC Airplane Setup
• Thunder AC680/AC6 Charger & Computer Data-Logging Software

So, a few days ago I got a car horn *and* a car audio speaker to play a siren sound!

In these two videos, I do a quick desktop demo of playing a siren sound through first a "fixed frequency" car horn, and then through a standard car audio speaker, rated at 120W peak, 60W RMS, and 4 Ohms. Using an Arduino, a relatively simple, custom transistor-based amplifier circuit, and the Arduino core tone() function in my code, I drive the horn and speaker to play a siren sound by sinusoidally varying the driving frequency.  I also show the signal to the devices on an oscilloscope, and briefly discuss and hook up a Schottky diode in a "snubber diode" configuration to knock down the inductance-induced voltage spikes created each time the square wave has a falling edge.

First, watch the "fixed frequency" type car horn play a varying-frequency siren sound:

Calculations For Additional Capacitors Necessary in a certain 21-motor Airplane Setup

These are my calcs, in regards to Posts 540~543 (here: http://www.rcgroups.com/forums/showthread.php?t=952523&page=36 and here http://www.rcgroups.com/forums/showthread.php?t=952523&page=37).  Basically, I tried to figure out how many additional capacitors are necessary on each of the 21 ESCs in order to offset the voltage ripples caused by increased battery lead length (which can destroy/damage the existing capacitors in your ESC if you don't add capacitors to help absorb the magnetically-induced voltage spikes [refer to post 1 in the above thread for more info about this]).

I am only putting this spreadsheet here because RCGroups won't allow it as an attachment on their website.
To download the file, click here then go to File --> Download:  https://docs.google.com/file/d/0BwHItOaHOOvdeFQ3WTd2T1Q5Y2s/edit.
The above link is to the old spreadsheet.  I have corrected some errors, and the NEW SPREADSHEET LINK IS FARTHER BELOW!

FOR NEW SPREADSHEET, CLICK LINK BELOW
Update: 20 March 2013 - Spreadsheet corrected and now available here (go to File --> Download to save):  https://docs.google.com/file/d/0BwHItOaHOOvdQmp3c3gydzV3SU0/edit

Update notes:  I added the "Total Capacitance Needed" (Row 25 in the new spreadsheet) in the calculations, and I removed the row called "Additional Length, beyond ESC's design, for which you must add Caps (in.)" (Row 22 in the old spreadsheet).  I then added Row 26 as well, and corrected the formulas in Row 27.  This allowed me to account for the additional current in the entire wire station length, not just in the length beyond that of what the ESC was intended to handle (Cell Q14) at its max rated continuous current (Cell Q11).

Here is a table of the differences in end-results between the old and new spreadsheets, for this one particular example:

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For jaccies (referring to Post #579 here)
Download spreadsheet here (go to File --> Download to save)