The Beavis Audio Research Group just added a great overview for building CMOS Oscillators and LoFi Synths. Starting with your basic oscillating Hex Schmitt Trigger and finishing with a Heterodyne Peyote Space Explorer. The tutorial has clear examples with valuable pieces of information for adding LFOs, Rate LED Indicators, Line Level and Volume control as well as component values for good frequency ranges. Excellent work!

 [youtube]http://www.youtube.com/watch?v=1E2KZCWZsHM[/youtube]

In the previous post we established what a Dub Siren was. Now it is time to make one and start the journey to become a Legendary Dub All-Star.

First you need the parts:

8 Sound Keychain – available here in packages of 12 for $9.50 plus shipping! There is plenty of other sound generating circuits out there which are cheap and in the next few days I will do a keychain roundup. There is also a COB (Chip-on-board ) version of this sound generator without the button traces that would be a lot easier to work with. This particular keychain just sounds so good, its got your sirens and machineguns, which everyone instantly recognizes as being Atari like or 8-bit, plus it loops!

Rotary Switch –  this will be the sound selector. 8 position is ideal, however 12 position ones seem to be most common and are the cheapest from Jameco. This is what all the sound buttons will be wired to and the center pin will be going to a push button that will trigger the sound by completing the circuit with the Ground.

1 Mega Ohm Linear Potentiometer – to control the pitch of the sound being played back. Obviously the sound circuit has to have a pitch resistor, most cheap keychains do. Lets try to stick with just Jameco for the source.

2 Momentary Push Button Switches – One button will connect the ground to the rotary switch and the other one will be used as a Mute by sending positive output from the instrument jack to the ground. Jameco once again.

AA or AAA Battery holder – Mine was from the SAW III sampler toy, you can improvise as long as you don’t go above 3 Volts. Jameco has a good deal on these.

Plastic Enclosure – Plenty to choose from, but I would recommend reusing something and helping to keep the trash out of the landfill. A Cigar box would do nicely.

Output Jack – 1/4 inch seems to be the industry standard.

Rasta Graphics for the front panel – free on the Internet.

2 Knobs – lots to choose from and scavenge if you can, otherwise get some chicken heads from Jameco.

Rubber feet – so the Rasta Box doesn’t slide off that table when the Bass is bumping.

Solid Core wire – stranded core will be a pain to work with, it splinters too easy.

The total for parts should be under $20 and possibly even cheaper if you are resourceful and can strip components from discarded electronics, etc.

The construction process:
Get everything together in one place on a work surface in a well lit and properly ventilated environment.

1. Connect the battery compartment to the 8 sound keychain board. ( Probably best to keep the batteries out for now, so there are no shorts ) No need for power switch because this particular keychain powers down automatically and instantly comes ON when the any button is pressed.

2. Solder the output jack to where the speaker was connected on the sound board. One wire to the Transistor Buffer and the other to the positive terminal? I dunno seems like that’s how they had it. Normally you would connect the Ground to the outside ring of the instrument jack and the Positive output to the tip.

3. Remove the pitch resistor and replace it with a 1M potentiometer. Center and one of the outer taps should go to where the resistor legs used to be. Very important, connect the remaining outside tap to Ground, this will give you the low pitch drop!

4. This step is perhaps the most tricky and easy to mess up, solder 8 wires to the button connection traces. These are very thin traces that interlock and if you build a solder bridge the sound will be stuck in the ON position. It will be hard and the only thing I can recommend is that you lightly sand or scrape the green protective layer and possibly cut the traces that go to Ground from those buttons. Solder with caution and be careful not to pull hard on the wire because the entire trace may come off. At this point you can probably plug in the batteries and test the soldering job making sure that if you connect the Ground to any of the button wires the sound gets produced. If the soldering checks out, drench that thing in hot glue so nothing moves or falls off!

5. Take a break then drill some holes for the controls in your plastic box. Make sure the knobs are spaced such that there plenty of room for them to rotate. After that you should overlay the graphic and then cut the holes on paper for the controls to slide through. To hold the paper in place I just use packing tape, but there are other plastic sheets and laminate options if you want to get fancy with it.

6.  Mount the push buttons, the potentiometer, and the rotary switch in the case if it is big enough to work inside. Otherwise solder wires to the components first and then mount them. Please note that there will be more solder lugs on the rotary than button contacts. I typically just use every other one the first round. The trigger button needs to have a wire going from the center tap of the rotary switch to one of its legs. The other leg needs to be connected to Ground. The Mute button will need to be connected to the tip of the output jack with the second leg going to Ground as well. The potentiometer should just have to be wired to the ground. If you are clever you can make all the Ground connections meet at the same place saving on wire. Mount the output jack and test everything. Makes sound? No…try setting the potentiometer to center and maybe resetting the batteries. Still no? Go back and recheck your connections.

7.  If everything works, use copious amounts of hot glue to hold stuff in place. Close the enclosure and start jamming out. The gate switch should chop up the sound when pressed, doesn’t get much simpler than that.

Next plug this into a delay pedal, turn up some Dub Reggae on YouTube, and start Toasting! The only thing to watch out for is  overusing these sounds because they eventually lose the magic. However the first 30 minutes will be a pure blast! Enjoy.

P.S. Here is the rough schematic of the circuit with the keychain board in the center, note the red Xs, those are to indicate that if you are soldering to the pads make sure they are not connected to any other pads via Ground. I can post a picture of exactly what I’m talking about soon.

* Updated *

I will be making another schematic and a new version of the siren using the Chip-on-board sound circuit. Also if these keychains are not available in your area, for whatever reason, feel free to order them from GetLoFi here.