A few summers ago I thought it would be interesting to be able to easily switch around the three pickups on my Stratocaster into whatever configuration I wanted. I figured I could get all kinds of different sounds that are not possible with the standard configuration, although I really had no idea what I would expect them to sound like (and I still don't). I finished designing it, bought all the parts, but never constructed it. It became apparent when beginning to design something that would allow me to do this would not be as simple as it first seemed. But eventually I did come up with something that would allow me to pull off ANY combination. With the standard Stratocaster setup with a 5-way switch, five combinations are possible (obviously). Bridge, bridge in parallel with middle, middle, middle in parallel with neck, and neck. With my setup, I would have the ability to do any bizarre combination I want (bridge and neck connected in parallel while in series with the middle 180 degrees out of phase, neck and middle in series, neck connected in series to 180 degree shifted bridge, etc...). This switcher is not restricted to use in Stratocasters, it will work in any guitar with three pickups. If you have any interest in constructing it, let me know! I'm interested to see how it turns out, and would also be willing to assist in its construction.
For the less technical reader:
A bit about what I mean by series, parallel, 180 degrees out of phase, polarity: There are two ways you can connect two pickups together; series and parallel. When in series, the two coils are in line with one another. One end of the first coil connects to ground, and the other connects to the second coil, and the remaining end of the second coil goes to the amplifier. When in parallel, both coils are connected together so that one end of each coil goes to ground, and the other end of each coil goes to the amplifier. Phase has to do with which end of a coil is connected to ground and which end goes to the amp with respect to the other coils. In the standard Strat setup, when the neck and middle or middle and bridge are connected, they are connected in parallel with the output of each coil in phase with the others. That is, if you were to look at the waveforms from the coils, the peaks and troughs from one coil would match up directly with the peaks and troughs of the others. Swapping the ends of a coil around shifts the phase by 180 degrees (or inverts the signal), so that now the peaks and troughs of one coil match up with the troughs and peaks respectively of another, which has a canceling effect when the signals are added together. This is how humbucking works. Without going into detail, hum induced in one coil is 180 degrees out of phase with the hum in the second, while the string vibration signals are in phase. When the signals from the two coils are combined the hum is significantly reduced (or bucked), while the string vibration signals add up. I may also refer to reversing the connection of the coil as reversing it's polarity. 180 degree phase shift, polarity reversal, signal inversion, all referring to the same thing in this document... I'm sorry.
How it works:
The switcher consists of two buses for connecting the pickups to; a serial bus and a parallel bus. Through a network of relays, the pickups can be inserted into either bus, in phase or inverted, or left out of the circuit. The serial bus and the parallel bus are then connected to one another in series or parallel (phase flipping of a bus can be done by flipping the individual phases of the pickups on the bus). I just called the serial/parallel connection of the two buses the "final output" for lack of any better term. With the ability to connect each pickup to either bus in either phase, and control how the two busses are connected to each other, the switcher is capable of connecting the three pickups together in every possible combinations. The configuration is controlled by 4 switches and displayed by 7 bi-color LED's, and a PIC microcontroller monitors the buttons and controls the relays.
Although the switcher could operate on batteries, I don't expect they would last very long. I planned on using a stereo phono connector in the guitar, with the added ring connector used to supply power. I was going to make a stereo cable for the guitar which would split at the end with a mono phono connector going to the amp, and another connector going to the power supply. This way power could be supplied externally, while still having only one cord go to the guitar.
The switcher has four buttons in addition to the original switch (although I was going to use momentary toggle switches instead of buttons). One is assigned to each pickup, and the last toggles the connection of the two buses between serial and parallel. In addition there are 7 bi-color LED's, indicating the current configuration. Each pickup has two bi-color LED's, one indicating which bus the pickup is on (green for parallel, red for serial), and the other for phase (green for 0 degrees, red for 180 degrees). If the pickup is not connected, both LED's will turn off. The seventh LED indicates how the two buses are connected (green for parallel, red for serial). Switching one of the pickup buttons cycles through parallel bus 0 degrees, parallel bus 180 degrees, serial bus 0 degrees, serial bus 180 degrees, and disconnected. When all pickups are disconnected, the device shuts off, and the standard Strat setup is used with the original 5-way switch (the 5-way does nothing when the switcher device is turned on). The device is switched back on by pressing any one of the pickup buttons.
There are certain configurations which require the final output connection of the two buses to be in either serial or parallel. Such situations are where all currently connected pickups are on only one bus. If all pickups are on the parallel bus, the parallel and serial buses must be in series mode, otherwise the serial bus would short out the parallel bus. And if all pickups are on the serial bus, the parallel and serial buses must be in parallel mode, otherwise if they were in series the parallel bus would act as an open circuit. The microcontroller will automatically switch the final output to the appropriate connection mode when such situations occur.
Should the power ever momentarily fail, or be purposely turned off for an extended period of time, upon power up the device will restore to the last state it was in (or remain turned off if it was already turned off before the power was disconnected). Every time the configuration is changed, the configuration is stored in non-volatile memory, and automatically restored upon power up. This will prevent the device from resetting itself if the cord is unintentionally disconnected, and save time by not having to reconfigure everything in such an accident.
There are certain configurations which will add a good bit of humboosting (all three in series with middle polarity reversed, or all three in parallel with middle polarity reversed are good examples). For such configurations, depending on the surroundings and noise in the atmosphere, a decent amount of interference could result. Noiseless pickups would be a good idea to reduce unwanted hum and buzz in such situations. I have noiseless pickups, and I must say they work surprisingly good. Also, there are some configurations which are functionally identical (in fact, every configuration has at least one additional configuration that is functionally identical). For example, when only one pickup is selected, it does not matter what bus it is on, or if it is in forward or reverse polarity. Or, if two pickups are in series and both are reverse polarity, it is the same as if they were both in forward polarity. Take any configuration, and compare it to an identical configuration with the exception that all connected pickups have the opposite polarity from the original configuration; the two configurations are identical. There are a total of 53 functionally different combinations; 3 using only one pickup, 18 using two, and 32 using all pickups.
A recommendation: I have looked at as many relays as I could find, looking for the smallest high quality relay with good contact conductance and vibration/shock resistance, and have found the Nais AGN2004H to be the best suitable relay (http://panasonic-denko.co.jp/ac/e_download/control/relay/signal/catalog/mech_eng_gn.pdf). For low quantities, the relay costs about $3.50 (about $50 for the 14 relays needed).
Here is the schematic of the device, as well as the code for the PIC16F627 microcontroller used to do all the dirty work. Because I have never built the device, I can't guarantee anything works. Although I am very confident the schematic is fine, as it is a very rare instance for my circuit designs to have flaws, I can't say the same for my programming. Although it is a simple program, I would not be surprised if there are some small errors, as I rarely get my programming correct the first time around. If you decide to build the device and find trouble in it's operation, let me know, I would be more than happy to correct any errors.