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Build a POSC Synthesizer 

Make] Projects 

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Build a POSC Synthesizer 

Written By: Sean Michael Ragan 



Diagonal pliers (1) 

• POSC kit M) 

• Hobby knife (1) 

• Battery (1) 

Needlenose pliers (1) 

• Switch enclosure (1) 

Ruler (1) 

• Acrylic (1) 

Soldering iron (1) 

O-rings (4) 

• Third- hand tool (1) 

• Masking tape (1) 

aka helpina hand 

• Epoxy (1) 


I'm a dabbler, really, both at music and electronics. My fellow MAKE staffers put me to 
shame when it comes to circuit-savvy; when I started this kit I can comfortably say it had 
been at least four years since I'd soldered anything. But the POSC went together without a 

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Build a POSC Synthesizer 

Step 1 — Make front panel 

• It was the enclosure that I really geeked out on, ordering a custom-cut and -etched cover 
in orange-coated 2-ply acrylic sign plastic from my pal Angus Hines . Angus is both the 
most competent and the least expensive CNC contractor I've ever had the pleasure to 
work with. 

• The plastic I used cost about $20, including shipping to Angus from the distributor, but the 
actual cutting and etching only cost $8 altogether. Angus still has a bunch of the material 
left, and anybody who wants to have a cover cut from it is welcome to do so. 

• The panel has four openings to install three components: 1 hole each for the pair of zinc- 
plated finger contacts, and 2 holes for the leads of the photoresistor. 

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Build a POSC Synthesizer 

Step 2 — Install panel-mount components 

• Seat all the components firmly against the front of the panel and hold them in place with 
blue painter's tape. (NOTE: Stronger tapes may damage the finish on the plastic.) 

• Turn the panel face down and carefully solder pre-tinned lead wires to each contact and to 
each leg of the photoresistor. Be careful not to overheat the contacts, as conducted heat 
from your soldering iron may damage the plastic cover. Use a heat sink if you're nervous 
about this. 

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Build a POSC Synthesizer 

Step 3 — Secure panel-mount components 

• Slip a piece of the included heat-shrink tubing over each lead wire and cinch them up tight 
against the rear face of the panel. Apply heat to shrink the tubing, but don't overdo it. I 
used my soldering iron for this. If I were going to do it again I might try a cigarette lighter 
or candle flame. 

• I had hoped that the heat-shrink tubing itself would be strong enough to secure the 
components against the cover, but alas, they were still wobbly when I got it in place. So I 
beefed it up with a couple of blobs of 2-part epoxy putty, which works great. Sugru might 
be a better way to go. I gently clamped the panel face-down against my bench when I 
installed the putty, and as it dried, to make sure the components remained firmly seated as 
the adhesive set up. 

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Build a POSC Synthesizer 

Step 4 — Prepare enclosure 

• The enclosure itself is a blue PVC switch box that cost $0.80 at my local ACE Hardware 
outlet. The cover is held on with four black O-rings that slip around the tabs. 

• The switch box actually has two knock-out tabs that are about the right size for the power 
switch and the 1/4" jack the POSC requires. I used a flat-head screwdriver to bend them 
over initially, and then just wiggled them back and forth to fatigue the plastic until they fell 
off. I cleaned up the bit of leftover flash around the edge with a hobby knife. 

• Neither knock-out opening was quite big enough, so I hogged each of them out a bit with a 
round file. This was not an exacting process: The plastic cuts quickly under the file, and I 
just kinda ground on it, poked, looked, and ground some more. Until it fit. 

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Build a POSC Synthesizer 

Step 5 — Assemble PCB 

• Sonodrome's assembly instructions are here . 

• As pretty as Kat and Jim's PCB is, it was too big to fit inside my switchbox with the 9-volt 
battery, switch, and jack in place. So I cut it down to size by scoring it with a hobby knife 
against a steel ruler, then breaking the board across the edge of the table. I was able to do 
this with all the components installed without breaking anything, but obviously it'd be better 
to do it before you attach anything to the PCB. 

• Bend the component legs using needle-nose pliers as needed, insert the legs through the 
right holes in the PCB (make sure you get the LED right way around, and the integrated 
circuits oriented correctly), then turn the PCB over and solder the legs in to the pads. Clip 
off the excess wire using nippers. Solder everything in place on the PCB side, then be 
sure you've got your switch, jack, and panel-mount components in place before soldering 
their leads on. 

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Build a POSC Synthesizer 

Step 6 — Put it all together 

• With the PCB trimmed down, everything just fits into the switch box as shown. I left about 
4" of slack on the panel-mount components, and this spools nicely into the compartment 
and pads the battery. When the cover is strapped on, nothing rattles at all. 

When you first turn the POSC on, the LED will light for a fraction of a second. If you've got it 
hooked up to an amplifier, it'll also squawk for a bit. Playing it is as easy as licking your finger 
and tapping, rubbing, pressing, or smearing it across the two contacts. The amount of incident 
light on the photoresistor controls the frequency, so experiment with playing it under different 
lighting conditions. Jim and Kat have produced a bunch of software for digital post-processing of 
the POSC signal. For more info, see the Sonodrome website . 

This document was last generated on 201 2-1 1 -03 03:32:08 AM. 

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