I recently found an interesting circuit, by Michael S. McNatt in an old issue of ELECTRONIC DESIGN Magazine, that I would like to share with other PAIA equipment users. It is a variable speed and variable range pseudorandom tone sequencer. As seen in the schematic shown in figure (a), exclusive OR gates G1, G2 and G3 are configured as an oscillator (adjustable through about 2-20 Hz.) which clocks the 4015 shift register IC. The shift register and exclusive OR gate G4 generate a pseudorandom sequence of 127 seven bit binary numbers which are decoded into voltage levels by resistors R1 - R7 and the op-amp (a simple D to A converter.) These voltage levels control the output frequency of an astable connected 555 timer IC. The range pot, which controls the gain of the op amp, adjusts the spread between the minimum and maximum frequency desired. A power-up circuit formed with R2 and C1 introduces ONEs into the shift register during the first few clock pulses. The circuit arrangement avoids a possible all-Zero lock-up state. Diode D4 discharges C1 at turnoff, so that the power-up circuit is immediately available for re-use. The momentary sequence reset switch is used to introduce an all-ONEs state into the shift register as a starting point reference for the sequence. When the Mode Switch is in the Glide position, the 555 moves smoothly from note to note; when the switch is in the Blip position, the 555 is reset on alternate half cycles of the clock causing a staccato sequence of notes. (If desired, this switch can be left out of the design, but be sure to short pin 4 to pin 8 on the 555.)

The output (pin 3 of the 555) can be used directly to drive a small speaker, or can be used as an input to the PAIA synthesizer line. You can pick up the required 9 volts from the synthesizer's 9 volt bus rod (likewise ground, of course.) I found that a small capacitor in series with the output helps to eliminate clicks, especially when in the Blip mode... experiment to find the best value for your set-up. I made an etched P.C. board for this project along with a front panel so that I could mount the unit in PAIA's road case. Of course, I see no reason why other construction techniques wouldn't work as well, perf-board, etc. There are no tricky considerations to watch out for. All parts are standard (cheap) and can be found at almost any surplus dealer or electronics store.

When constructed, experiment with the control settings. Many different effects can be had. Try a narrow tone spread with fast clocking... a jungle of birds, maybe... who knows!

Ed. note: Ken said that he will sell copies of his original PC layout to anyone who is interested in making this circuit into a module as he did. Contact him at the address listed in the Local Happenings section of this issue.

Additional:
Another item I should have mentioned in the article (but neglected to) is the following: the auxiliary output (labelled 'to synthesizer') is a 9 volt square wave which is a bit too high for input to PAIA modules. Though it will still work, I'd recommend lowering the amplitude with a resistive divider something like this: Otherwise, no doubt, the module would probably "clip it"... it will function, but distortion would no doubt result.