I suppose that before long I'll be sitting in front of a typewriter like this not pushing down keys, but touching pads that represent the keys and watching the print go down on paper. Not a particularly exciting thought to me as I have yet to master this ancient electro-mechanical wonder I'm working with. But the idea of touch pads instead of mechanical switches is a downright nice thought when considering the many applications to which one might put these "Magic Buttons". For instance, suppose you've got your guitar plugged into a box full of modules for some "special effects", and in order to get a certain effect you have to trigger one of the modules. Well, you could step on a mechanical (yuck) foot-switch on the floor, or you could set up an envelope follower that has a variable threshold trigger output and try to control the triggering with the pulse output of the envelope follower. This is not too bad under the right conditions, but you could put a "Magic Button" on your guitar (or any other electrical instrument you may be using), and accomplish the effect much more easily.

A single capacitive type touch switch can be constructed around one IC chip, (like a 4001 MOS quad NOR gate). It can be made to take up very little room, and be placed strategically on your ax so as to be convenient for your picking hand pinky to reach down and stroke whenever it gets the urge to send a trigger pulse to that box of modules, or whatever you want to trigger, (lights maybe?).

The particular touch switch circuit I have in mind is like that found in figure 1. We may need to add a simple differentiating circuit that will provide us with a pulse output. Let's take a look at these circuits, shall we? Using the values shown here the clock should run at a frequency of about 45k to 50kHz. If we pump this clock signal into the remaining circuit as shown in fig. 1, here is what will happen. On the positive or high half cycle of the clock, diode D1 will be forward biased and a positive voltage will appear at the junction of D1's cathode, R2, input pin 6 of gate "C", and the touch pad. The second input to gate "C" is connected directly to the clock output. This being a two input NOR gate, both inputs must be at ground before the output can switch to a high state. If the inputs are in any other condition the output will be low. The touch pad represents a very small capacitance that has to be charged before the voltage at the input (Pin 6), of gate C can rise to a value that will cause this gate to switch. With the very low "on resistance" of D1 during the positive half cycle of the clock as a current path to the touch pad, the time required to charge this "capacitor" is very short. So short, in fact, that the voltage at the input of the gate (Pin 6) can easily reach a value sufficient to switch the gate within one clock cycle. When the clock is in the low half cycle, the only discharge path available to the pad is R2. D1 is now reverse biased and the gate input represents a very high impedance. If the pad can discharge completely on low half cycles of the clock it is of no consequence at this point because all we need is the inverted replica of the clock signal at the output of gate "C". Actually, the arbitrary capacitance of the touch pad assumed in this writing is exaggerated to aid in circuit analysis and, in reality, is significantly small in relation to the other values in the circuit. The high portion of this 50kHz. square wave forward biases D2 and causes a voltage drop across R3 charging C2 to a value near supply. When the output of gate "C" goes low, D2 is reverse biased and the only discharge path available to C2 is R3. The time required to discharge C2 is longer than the duration of the low half cycle, so both inputs of gate "D" (pins 1 & 2), are held high, and it's output is low. This is the off condition of the switch. Now, if a finger is placed on the touch pad, the capacitance offered by the body belonging to that finger is added to that of the pad. The total (several hundred pico-farad) is quickly (within a few clock cycles) charged to a value near supply via D1 during positive clock half cycles, but is more than can be fully discharged through R2 in the 20 microseconds or so allotted during the negative half cycle of the clock. This phenomenon is somewhat dependent on the assumption that some part of the body is at ground potential. So, with a finger on the pad the voltage at the first input (Pin 6) of gate "C" never gets to a low value and the gates output cannot switch in accordance with the signal from the clock, but instead remains low. This leaves us with no means of charging capacitor C2. So it discharges through R3 allowing the inputs of gate "D" to be pulled down to ground, which in turn causes the output to switch to a high level. This is the "on" state of the switch.

In case your clock must drive many touch switch circuits, there should be a buffer between the clock and the inputs to the switch circuits. This will reduce any interaction caused by the increased touch switch load on the clock circuit.

Something else worth mentioning is that changes in clock frequency will cause corresponding changes in the sensitivity of the touch switch. That is, the higher the clock frequency the more sensitive the switch becomes, and vice versa.

Fig. 2 shows a circuit which can be added to produce a differentiating circuit that will provide pulse outputs. This is the circuit that would serve best as a "Magic Button" on a guitar, except that perhaps the RC pulse output section should be placed in whatever cabinetry houses the unit to be triggered, or somehow built into the end of the cord that will carry the trigger voltage from the output of the touch switch to the receiving unit. If you're thinking about incorporating this line into the audio patch cord from your guitar, don't. The resulting snap, crackle, pop would be more than you would be able to handle at any time of day! It is too bad there has to be an extra cord running from your ax, which will no doubt hold some folks in favor of the good old foot-switch but I feel it's worth having an extra cord to tend in exchange for the extra control afforded by the touch switch.

Are you thinking radio? Yeah that would be neat, but if we're going to do that let's just put the audio signal on the air too. And maybe a few effects, and... No. Let's just keep our clunky old patch cord so we can talk about touch switches. Besides, not everyone plays guitar! (Or do they? Sometimes I wonder.) In fact one would expect that there are more keyboard freaks reading this than guitar thrashers anyway. It would probably even be fair to say that a good number of those people are Oz owners. So, for those of you who presently fall in this category, as well as those of you who intend to join the club, here's a fun way to exploit the use of "Magic Buttons" on your Oz.

TOUCH SWITCH OCTAVE SELECTION FOR OZ

With three IC chips (two 4001 quad NOR gates and a 4052 dual four channel multiplexer), we can add a couple of "Magic Buttons" to Oz and have touch octave selection. Fig. 3 shows how the 4052 is incorporated into the Oz octave selection circuitry so as to replace the 5 position rotary switch, S2. If you're not familiar with the 4052 check out fig. 4. This chip serving as a multiplexer will pass one of the three output signals from the 4024 clock frequency divider or the clock output from IC6b, to the clock input of the MK5024U top octave generator. The lowest octave can be omitted without too great a loss. Selecting which channel is activated is a function of the two digit binary number applied to the address inputs A0 and A1. This number is a representation of the status of the touch pads since it is in fact composed of the voltages appearing at the outputs of the two touch switch circuits. Thus far we have described the action in 1/2 of the multiplexer. The other half works in the same manner. The two address inputs are common to both four channel multiplexer networks in the chip. That is, if the binary number 00 is presented to the address inputs, channel Y0 in the A section is connected to the Za pin while channel Y0 in the B section is connected to the Zb pin. This B part of the multiplexer we can use to switch in the appropriate LED octave indicator for whatever octave has been selected. So if you arrange the connections as shown in fig. 3, Oz will be in it's center or third octave position when no "Magic Buttons" are being touched. Under these conditions, both touch switches will have low outputs presenting a binary 00 to the two address inputs of the 4052, enabling channel Y0.

There are two gates left over in IC1. Waste is a drag. We ought to give them something to do. We can use them in a third touch switch circuit, but we don't need another address bit for the 4052. Maybe this third switch could serve as a trigger, second to and independent from the existing "keyboard activated "trigger. Might be a handy thing to have if your Oz spends much of it's time as a pitch source for a larger synthesizer system. Fig. 5 shows how the circuitry looks with the third switch utilized as a trigger.

Another little goody that you may find interesting is shown in fig. 6. The binary counter circuit can address the multiplexer so that neighboring octaves can be sequentially selected automatically. This is a neat little cheater gadget because you can just hold a chord and do some fancy arpeggiation by letting the counter run. This also gives us something else to do with the third touch switch. Fig. 6 shows how the 4052 can be addressed either by touch switches or the counter. Diodes are used to AND the outputs of the touch switches and the counter circuit into the address inputs of the multiplexer. The third switch is used to enable the counter. When this switch is activated the output of Touch 3 is switched high. IC3A inverts this high state to a low. This holds the one input of the clock buffer, IC3D, at ground which enables the buffer to switch at the clock frequency. IC3B and IC3C form a low frequency clock which drives the counter circuitry. This, in turn, causes the automatic arpeggiation through 4 octaves of transposition. The output of IC3D is binary bit 0 of the counter circuit and also drives the Cp input of the flip flop. The flip flop divides the clock frequency by two and output Q1 is the second bit (bit 1) of the two bit binary counter circuit. (By the way, there is an unused flip flop of this type on the Oz P. C. board. Half of IC1 is not committed and if you don't mind cutting some foil and doing a little point to point wiring, you can save yourself the cost of a 4013). Notice that the first two touch switches are still functional even when the counter is enabled so that they may be used in conjunction with the counter.

When the third switch is released, it's inverted output at IC3A goes high pulling the input of IC3D high to disable the switching action of this buffer and hold it's output at ground. The counter is stopped. This would be all that needed to be done except that we don't know whether the Q1 output of the 4013 will be high or low when we stop the clock. In order to control address input Al of the 4052 from the second touch pad the Q1 output of the flip flop must be low when the counter is not running. To insure that this is the case the output of IC3A is also connected to the "Cd" (clear direct) pin of the 4013. When this line is high the Q1 output is switched to ground regardless of other input status. With both outputs of the disabled counter at ground, diodes D3 and D4 serve to isolate the counter outputs so that the touch switches 1 and 2 can have complete control of the multiplexer addressing.

Steve Wood is Director of Technical Services at PAIA Electronics, Inc., Oklahoma City.