
This is the full setup. I just use the USB power from an old phone USB charger to power the Arduino setup and the strobe light has its own power cord that needs to be plugged in as well since I only use the Arduino to trigger the strobe instead of powering it. The output of the MP3 player goes to a small guitar amplifier and its stereo headphone jack is converted into a mono phono jack for the amplifier input. The amplifier needs to be plugged into the wall too.

In order to use the strobe light for its normal purpose I chose to install a switch to toggle between modes. In one position the strobe works "normally", being controlled by the knob for the interval. The other position allows our Arduino's trigger circuit to close the relay when we want the light on.

This is the inside of the strobe light that shows the DPDT switch I used to switch the transistor in and out (and the relay contacts out and in, respectively). Note the original strobe light transistor mounted on the switch terminals. This was removed from the circuit board and replaced with wires. It also shows the small 3 volt RW-SH-103D relay I used that is driven by the Arduino via the Darlington transistor on the "trigger shield".
PERFORMING MODIFICATIONS LIKE THIS IS DANGEROUS. DO NOT ATTEMPT.
This is the shield I used but wish I used the one from Adafruit instead. I chose this one because of the price but it turned out the driver software didn't work. At all. All of the demos I found online did not work and the driver software was all several years old. I wound up using the Adafruit drivers for the Adafruit board and then modifying it slightly because the pins were different. I got it to work but wasted a lot of time in doing so.
You also need to buy header pins (and solder them in) so it can plug into the Arduino.

This is the prototyping shield I used because it was readily available (from frys.com) and was still under $10. Unlike the Adafruit shield this one comes preassembled so there is no soldering involved, not even for the header pins.

This is the original strobe light I purchased in anticipation of just using it because it had sound too. What a joke! This is a real turd. When I received it and tried it out I wanted my money back and tried to get it but the supplier REFUSED! So I figured I had nothing to lose by taking it apart and modifying it.

All of the MP3 files I used are "open source", no purchases were necessary and no royalties need to be paid for usage. The project cycles through these three sound files.

All of the MP3 files I used are "open source", no purchases were necessary and no royalties need to be paid for usage. The project cycles through these three sound files.
I wound up using the software drivers from the Adafruit MP3 board instead of the SparkFun one because I could not get the SparkFun drivers to work. Here is the version of the AdaFruit software I used.