The brakedrum is a rather complicated device. It's by far the most complicated thing i have made. Actually its more building than bending since the bending part of this project merely consists of the pitch change mods i made to the 5 toys. But the building part consists of a preamp, a pulse shaper, a mixer, a "multi voltage power supply", the drum pads and the two enclosures.
The above picture of the front panel, shows 5 sections, each corresponding to a bend toy inside the box. The first 4 sections house 4 almost identical steering wheel toys, you can see how it originally looked here. These 4 toys are triggered from the 4 piezo drum pads. The last toy, behind the rightmost section was a small drum toy called "Rap band", it was very much like the infamous "Happy rock" drum toy. This toy is not triggered from the pads, but can play simple drum patterns to play along to.
All 5 toys have pitch control, the former fixed pitch resistors have been substituted with pots. All 5 modules also have individual volume control via a small 5 channel mixer that's built in to the box. The first 4 modules each have a 5 position slide switch that selects which of the toys sounds gets triggered by the coresponding drum pad.
Here is a block diagram of the whole thing, the powersupply have been omitted for clarity :
Here's a picture of the back side of the front panel, welcome to difficult soldering hour.
I made the stupid mistake of making the front panel too small because i forgot that pots and switches take up more space on the back side of the panel than on the front, So i had very little space for all the wires that connect everything, and the soldering was a pain in the neck. I hope i don't ever have to fix anything here.
Here is the the "utility" circuitry, first the Preamp. When i started this project i tried to trigger the toys directly with the piezoes, but that didn't really work. So found a design for a small 8 way preamp on a website with a MIDI drumpad system, the only change i made was to scale it down from 8 to 4 channels. The preamp is very simple and i just build it on a piece of Vero board. Here is a picture of the preamp, and the schematics:
Next thing is the pulse shaper as i have named this circuit. After i finished the preamp the toys triggered ok, but when i made the pitch change mods to the toys, i found out that when they were pitched down, i could no longer trigger them with the piezoes. This problem took me a long time to figure out. I made a lot of trial and error experimentation, and in the end i realised that what really happens when the toy is pitched down is that the internal clock in the black blob is running slower, this also causes the scanning of the toys switches to be slower and the trigger pulses was simply so short that they were missed. After consulting the benders group. Paul Perry suggested that i could use a 555 timer in monostable or "one shot mode". This way you can have the short spiky pulse from the preamp trigger the 555, and have the 555 spit out a nice regular pulse of any length you desire. So i designed and build a 4 way pulse shaper, complete with it's own PCB.
The four resistors "between" the 555 IC's are not in the schematic. They are part of a failed attempt at adding a led to each drum pad that would flash on each hit on the pad. The resistors are not used and not necessary. While working on this thing i found a very cool little piece of software, to help with the design of 555 based circuits. The 555 designer can be downloaded for free here :
this little program is highly recommended to anyone messing with 555 designs, lots of info on the various modes and applications of 555 timers.
If anyone should be interested the PCB layout and the component placement diagram can be downloaded here. Just remember that the LED doesn't really work as i expected.
Here's a picture of the preamp and the pulse shaper mounted underneath the piezo pads. In th upper right corner of the picture you can see the cable that connects the pad box and the control box, this cable takes the trigger pulses from the pulse shapers to the sound select slide switches in the for toys. It also brings power from the control box to the two circuits in the pad box.
Next is the mixer. I found a design, complete witth PCB layout and everything, at the General guitar gadtgets web site. It's a relatively simple design, but it works fine, and is easy to build. I changed the design to allow for up to 8 input channels in stead of the original 4. The General guitar gadtgets web site kindly asks people not to post their designs on other websites, so if you want the schematics or the PCB layout, please follow this link. If you ever need a small easy to build mixer i can recommend this one. I build an 8 channel version as a submixer for all my bend stuff. It works great, and can run from a single 9V battery. I forgot to mount the opamp in the socket before i took the picture. The volume potentiometers for each channel is mounted in the front panel.
The last circuit is the power distribution module. For the BrakeDrum i needed serveral different voltages. The mixer runs on 9V, the preamp and the pulse shapers at 5V. three of toys were originally running at 3V, an two at 4.5V. I made some tests that showed that all the toys could run at 4.5 without becoming hot or burning out. So i build this small circuit out of two voltage regulators, 0ne 5V and one 9V. I could not get a 4.5V regulator so i used some diodes to make a voltagedrop of about 0.6V. So i ended up with 4.4V, 5V and 9V. I forgot to take a photo of the thing before i put it in the box. But You can download a zipped file of the PCB layout an schematics here. it's very simple and could easily be build on a piece of verobord.
The pads are made up as a three layer sandwich of two different kinds of foam material, and a thin (approx. 1.5mm) sheet of metal. With the piezo between the metal sheet and the lower foam layer. Here's a cross section drawing.
The top foam layer is a piece of a mouse pad, the kind that feels almost like the material wet suits are made from. It has a reasonably good bounce. The bottom layer is much thinner, i don't really know what it is, it was part of one of those wrist supports you put in front your computer keyboard. I used the part that would normally go under the keyboard.