After digging around on the Internet, I found a schematic detailing a circuit to decode Audiostrobe signals. I won't post it here because I was unable to contact MagicJim, the author, to get permission. I based my circuit upon that one; however, since he simply used the example circuit included in the LM567 datasheet, I'm fairly comfortable that I'm not infringing upon any copyright by posting this.



Audiostrobe works by encoding a high frequency (19200hz, apparently) signal into the audio stream. Most humans can't hear it, but a tone decoder can detect the signal and apply power to the LEDs when it is present. I'm not sure if the commercial decoders are this simple; maybe there's additional data sent with the stream, perhaps like a serial data line, only in audio. I was pleasantly surprised to find out that some software has brightness control built in, and it works with this circuit. After reading the tone decoder's datasheet, I'm clueless as to why. The decoder is a simple switch; if the tone is present, it connects the LEDs to ground, with no current or voltage regulation. The only thing I can think of is that the software uses PWM to drive the LEDs. In any case, this circuit works.



MagicJim commented that you need two circuits, one for each eye. Although dissimilar auditory stimuli work well, as in binaural beats and other techniques dealing with the two hemispheres of the brain, I've seen no studies showing any benefit to separate stimuli for the left and right eye. In any case, I was more interested in having selectable LED banks than in seeing how massive a migraine I could give myself by stimulating each eye separately, so both eyes are synced.



The decoder unit runs off of a 9V battery. Due to the inefficiency of linear voltage regulators, combined with the low capacity of 9V batteries, I don't expect battery life to be phenomenal. I originally planned to use the MintyBoost circuit to power it from two AA batteries. I didn't have the parts on hand, however, and I didn't feel like waiting for parts to arrive. I haven't been using the device long enough to get any real battery life data, and at any rate, I've been using old batteries that I had after replacing the ones in my smoke detectors, so it wouldn't be accurate anyway. If it turns out that battery life sucks, I may redesign the circuit to include the MintyBoost.



I've made a few changes to the original circuit. I added some ripple protection caps to the 7805. They're probably unnecessary since we're using a battery. I had planned on having a DC jack with contacts that broke when a plug was inserted. That way it'd run on batteries until you plugged in a source, at which point it'd automatically switch to the supply line. I scrapped that because there were too many cables going into the unit as it was. If you don't happen to have them on hand, you should be able to leave C6 and C7 out without any issues.



I also added a diode to protect against inserting the battery incorrectly. I placed a SPST switch for power, and a SPDT switch to choose between output lines. I also added a power indicator LED and accompanying resistor. If you don't want the indicator, LED1 and R3 may be safely left out.



The sound and signal input is supplied by a male to male audio cable. This cable can be hooked up to you computer's sound card if you're using entrainment software, your cd player if you're using commercial cds, or the output of a light and sound machine. I've used it with several different entrainment software packages, and it worked with all of them, but I haven't tried it with cds or l&s machines. I'd be willing to bet it works with cds, Hemisync for example, but I can't promise anything. L&S machines, on the other hand, are iffy. If they output a bog standard Audiostrobe signal, it should work fine. But if they utilize any special features, they may not work.



The cable for the glasses is a standard audio cable with a 3.5mm stereo plug. You can either cut one off of a pair of headphones or cut one end off of a male to male patch cable. I'd recommend the patch cable, as they tend to be a bit beefier, and can handle a bit more abuse.



There is a headphone jack included in the unit. Most entrainment software and cds require that you wear headphones, rather than speakers, while using them. I was originally going to just use an external cable splitter, but decided that the likelihood of me losing it and being unable to use the unit was great enough to justify the extra $.50 cost of an additional stereo jack.



Instead of using a 9V battery snap connector, I added some spring contacts. I've always hated snap connectors, and I've broken quite a few over the years. Unfortunately, I couldn't find any sacrificial electronics that were willing to give up their contacts, so I had to improvise. See step 4 for more info.



There are numerous design flaws in my unit. The actual glasses, for example, were built while I had MagicJim's circuit breadboarded, before I started making changes. I placed one set of LEDs on the glasses, wired them and expoxied everything into place, all the while thinking how great it would be if I could stick a second set of LEDs on the glasses. No, I thought, I don't want an extra wire dangling. It wasn't until roughly twelve seconds after the epoxy finished curing that it finally occurred to me that I could've used the second signal wire to power a second set of LEDs and use a SPDT switch to choose which one to use. That change made it into the circuit, but I haven't built a new pair of glasses yet.



There are several similar changes that could be made to make this unit better, but I'm going to detail the construction of the project as I made it. I know the unit I built works, but I can't guarantee any changes I suggest won't break it. In addition, by pointing out problems that occurred for me I might save somebody else some grief. At the end of the instructable, I'll suggest any changes I feel are necessary.



I'm not going to go into excruciating detail in certain areas that have already been covered well by other instructables. I'm not going to talk about etching the PCB, for example, because there are numerous instructables detailing exactly that. Nor am I going to go into extreme detail on things like drilling the holes for your jacks and whatnot. This is basically just the outline of what I did, what went wrong, and what I would change so that somebody else can do a better job their first time than I did.



I should point out that I am not an electrical engineer, nor do I know anything about electronics (a fact that will likely become painfully clear shortly). I don't know anything about audio, either, so I make no claims about the safety of your high-end sound card while using this. There's no amplifier in this circuit, and I've no idea how much of a drain it places on the signal. It's possible it can cause an impedance mismatch, and if so, your equipment might be toast. Additionally, there's no line isolation between the circuit and the signal input. You're placing an electrical circuit into a metal box and plugging it into your computer. If you were sloppy, you're going to have a bad day. You've been warned. If something bad happens, it's not my problem. Do not build this circuit if you can't take responsibility for what might happen. That dire warning aside, I've been using it for several weeks now, with two computers and a cd player, and haven't had any problems.



Oh, and for God's sake, if you've got epilepsy or some similar light-induced disorder, don't build a freaking machine designed to pulse light and then try to sue me, okay?

