Probably wouldn't really be considered old-school soft circuits but probably more of a hybrid, electronics in a softshell case. I still have a bobbin of conductive thread I need to do something with. Note that this was a first attempt in trying to do a mod with stuff I had on hand but I will offer suggestions on what you should do to get a more satisfactory result.
Ruby's partner in crime Mho the Resistor plushie has been modded as an analog resistor value decoder.
Step 1: Stocking Stuffers...
To mod your plushie, you need:
A sound reactive LED circuit. I do not know how to put one together from scratch, well I could try but I'm lazy. I am also cheap. Sourcing parts can also take a lot of time. I was in Radio Snacks one day and saw these Velleman Sound to Lights Electronics Kits MK103RS on clearance. I don't think they stock this one anymore. I got two of them because you know you always have to have backup in case you mess up one. Look up "color organ" for these types of circuits.
These beginner kits are great to see if you have the skills to solder and to see if you can follow simple directions.
I am going to mod those kits a little bit to make it work for my application. For ease of placement in the plushie, I wanted to extend the leads of the battery pack and microphone. I also wanted to swap out all the red LEDs that came with the kit and replace them with brighter white LEDs.
Soldering is fun. Until you burn yourself. Learn how to solder safely.
You also need a bit of sewing skillz. Yup, you need to do a little bit of plushie surgery to implant the bionic devices.
If you are a sewist (seems people really didn't like being called sewers) you should have a seam ripping tool, needle and thread.
An x-acto knife or utility knife would suffice too.
Pointy things are pointy and sharp things are sharp. Pointy sharp things are extra pointy and sharp. Be careful.
Step 2: Red Light, Green Light, 1, 2, 3...
Assemble the kit according to the Fn instructions.
It took about an hour to sort out all the parts, pick and place each component and solder.
No matter how careful you are, no matter how low you set the temperature of your soldering iron, melting off a trace is inevitable. Note that the circuit was simple enough for me to follow and affix a jumper in place when I could not bridge the gap with solder. I may have toasted that pad too hot along with the trace that lifted off. A superfine tip may not have been well suited for the larger through hole connections. This was the same problem I encountered with some tiny Arduino breakout boards I used before. Oh wells.
The changes I made were to have "sockets" for the microphone and the LEDs. I clipped/sawed/sliced and broke off 2-pin segments of female header strip and soldered those in the positions for the microphone and 4 LEDs. This would allow me the flexibility to place the LEDs and microphone anywhere I needed if I soldered extension wires to male headers that plug back into the circuit.
The battery pack originally is designed to screw on to the circuit board forming a compact unit. I extended the power leads and used some heat shrink tubing to seal the exposed wire splice connection. This would give me flexibility in placing the battery holder if the unit was too big to fit in one position.
After the kit was assembled, I tested it out with the original 4 red LEDs. The LEDs will light up with the sensitivity potentiometer adjusted. It gets brighter as the sound input is louder.
I then wanted to start swapping out the red LEDs with brighter white LEDs. Note that I sanded my white clear LEDs to get a better diffused light. I swapped each LED one by one. I found that the power requirement of different LEDs only allowed me to swap out 3 of the red LEDs for white LEDs. I guess one could recalculate the load and adjust the value of the resistor to accomodate a set of 4 white LEDs. The LEDs are hooked up in series so it worked sufficiently with 3 white LEDs and 1 red LED.
I used some ribbon cable to construct the harness that goes to the LEDs. Use electrical tape to insulate and cover all the exposed leads so that they don't short out when you stuff it into the plushie.
Step 3: Sew Easy...
On the bottom of the plushie, find where it was sewn up by hand after it was filled with stuffing.
You need to cut the threads that hold the seam together to open a hole big enough for your electronics to fit through.
A seam ripping tool is well suited for the job as it has a pointy probe tip to get into the crevice and can cut the threads in its path. Dang, sonic screwdriver is in the shop again, along with the Jaguar.
We will sew it back later or maybe even add some velcro or snaps to be able to fasten the opening so that we can get in to change the batteries or adjust the electronics when needed. In the meantime, you can suture it up with a safety pin or staples.
Note that there are armature wires for the "legs". Just bend them out of the way as you push the fiberfill stuffing aside.
Step 4: Silicon Implants...
You can bend the soldered leads of the LEDs at a right angle so that they are facing out when stuffed inside the plushie. Just create a spacer/mounting plate out of more electrical tape. It holds the positions of the LEDs so you can just slide the assembly into the plushie.
Fit the electronics in the plushie. You can thread the microphone through one of the "leads" or just wrap it around outside. You can also place the microphone stuffed inside the plushie.
No other way to put it. This is now an LED enhanced plushie.
Stuff the rest of the electronics in the plushie.
Crank up the volume of the sound source and adjust the sensitivity of the sound to light device.
Watch the blinking lights.
WHAT TO DO BETTER:
I was trying to go for the effect of a light up mouth/teeth.
The light output is not really bright enough to illuminate the plushie. The thick plush fabric material does not transmit light well.
Placing the LEDs deeper to use the fiberfill as a diffuser did not work out as it diminished the light output. The LEDs are placed right up to the fabric causing the hotspots and puckering. More like a light up acne effect.
To really mod the plushie would be to cut out the mouth portion and replace it with a more translucent material. That might change the look too much but then again, make it a Domo mouth.
Maybe use EL panel cutout of the mouth to illuminate it but then you need to fit in the EL panel power inverter and the sound driver circuits.
You would need a better circuit that can drive more LEDs to get the light output needed for a good glowing effect. Or get higher output LEDs that might work.
Think about adding voice chips or other electronics like sensors. Make Ruby into a proximity alarm or a temperature probe. Ruby blushes when it gets hot flashes.
This would be great to add the heartbeat sensor where you grab the two legs/leads to complete the sensor circuit. Mod the ends with conductive fabric or conductive thread pads. Body fat reader or lie detector?
So there you have it., hack a plushie. It always leads to something new.