There are really two versions of this circuit:
1. If you want to plug two mono pads into a piezo/switch input then you would use this diagram. This diagram can also be used in the same way #2 can be used.
2. and if you want to create a dual piezo pad (such as a snare or dual zone cymbal) that you can plug into a piezo/switch input on your module you would use this diagram. This diagram cannot be used to connect two mono pads to a piezo/switch input. R1 should be replaced with a 100k trimpot (a trimpot is a small adjustable resistor. You can easily adjust it to get any resistance between 0 ohms and 100k ohms)
A little background on this circuit and how it works (bear in mind that I know almost nothing about electronics or electricity...). Essentially what this circuit does, is it takes the voltage from the switch piezo (i.e. edge piezo) and used that voltage to "flip" the transistor from open to closed. This will momentarily close the circuit which tells the module that you have hit the edge, bell, etc. The two circuits shown above differ just a little. The first one also uses the edge piezo to tell the module how hard you hit the pad (that is why the edge piezo is also connected to the tip and ground of the jack). The second diagram gets its velocity information (i.e how hard you hit the pad) from the head piezo (as you can see the switch piezo is not connected to the tip of the jack).
Since the circuit can be used for many things (edge or cymbal, bell or cymbal, rim shot on tom, rim shot on snare) I am going to refer to this piezo simply as the switch piezo - because it is the one that closes and opens the transistor switch.
Now, for those of you that know nothing about electronics (and I put myself in that category!) the first attached picture shows circuit #2 with a visual representation of each component on it. As you can see you don't need much to make the circuit. If you are getting ripped off you still won't be paying more than $5 for these components
So what exactly do all thos letters and numbers on the diagram mean?
R1: Resistor 1 - 100k ohm resistor (we will be using a 100k trimpot though)
Q1: Transistor BC337 - in north america it's known by a different model number but if you ask for the BC337 or equivalent the electronics shop will give you the right one. The Radio Shack number is: 276-2058 (thanks RoadKing)
R2: Resistor 2 - 1M (1 meg) resistor. I believe this is used to provide a voltage divider, so for this reason you can use a larger resistor if you wish (2 meg, etc)
D3: 1N4148 Diode - a diode only allows electricity to flow in the direction of the arrow.
Putting it all together:
Really there is no trick to putting the circuit together. You have to know which way to hook up your diode, which post on the transistor is which and that you only use two poles on the trimpot. That's about it. It's really easy to assembly the circuit by putting it on a project board (make sure you get one that doesn't have traces - unless you know what they are and how they work). Put all the components on the board roughly how they are laid out in the circuit. Don't worry about bending the wires on R2 or D3. You have to be a bit more careful with Q1 but you can bend the posts quite a bit before they break off. Once you have all the components layed out on the board get some thin single strand wire that is sheathed in plastic. Cut pieces of the wire to length in order to make all the connections for the circuit. Trim off the edges of the plastic sheathing so you have something to solder to. Now, run the wire on the same side of the board as your components are on and push the wire though the board as close to the component that you are wiring up (you may even be able to push the wire though the same hole the component is going through). Solder up all the components and the wires and your circuit is done! Time for some testing.
Tape both piezos onto a single piece of cardboard, this will allow the vibrations on the cardboard to move both piezos. Hook up the circuit to the module. Set the module trigger type to something that is a piezo/switch - personally I use a pd type pad (pd-7/8/9, etc). Hit the head piezo. You should get a response. Now hit the switch piezo. Did the rim light on the module light up? If it did, then you are good to go! If it didn't then adjust the trimpot all the way to one side and try again, then move the trimpot all the way to the otherside and try again. Still no rim light lighting up? Hit the switch piezo very hard (), but without damaging it - we have to make sure that the vibrations are closing the switch and reaching the head piezo. If it still doesn't work then you have to make sure that there is nothing in the circuit that is shorting it out, like some solder that is accidentally creating a loop somewhere it shouldn't. Then make sure that you wired all the components up right. If it still don't work, then cry, watch some tv and come back to it later. If it still doesn't work then post here and we'll try to help you out.
Hope that helps some of you out there. Sorry there are no pictures, I didn't have any transistors left over so I couldn't build the circuit to show you guys how to do it step by step.