Two easy-to-build circuits from Paul White kick-off a new series of simple projects this month. The lead checker will doubtless prove invaluable to all those musicians still wondering whether or not faults are attributable to their connecting cables, while the S-trigger converter answers many a synthesist's prayer, since it offers the facility to trigger Korg and SCI synths from Roland drum-machines, and at a ridiculously low cost-price.

S-trigger Converter

It has now become clear to me that the Tower Of Babel was only a minor exercise in inept industrial communication. The outcome of that escapade was only a small tragedy compared to the potential commercial disaster that may yet be caused by the major synth manufacturers' lack of ability to agree on a workable standard (prior to MIDI, of course) for trigger interfacing levels so that any synth or sequencer can just be patched to any other, by anybody, at any time.

The advantages of one uniform standard are obvious. Instead of trying to untangle all the problems he has unwittingly inherited, the musician would be free to concentrate on the business of actually creating music. However, the problem of incompatible triggering standards remains as important and as unfathomable as ever: perhaps we should all be glad that the makers have at least agreed on a fixed number of semi-tones per octave, though there is a rumour of a Belgian synthesiser with only ten. Well, that's decimalisation for you...

Anyway, to the matter in hand. This little gadget was devised when a friend of mine came to me with a problem. Her Korg Polysix arpeggiator would not trigger properly from a Roland TR606 Drumatix. The Korg trigger input has what is termed an 'S-trigger' input which in layman's language means: shorting the two contacts together causes a trigger. Several models of synth, including some Moogs, have these 'S-trigger' inputs although it is not unheard of for the same manufacturer to change his trigger arrangements every year or so, just to keep electronics designers and constructors on their toes.

Roland equipment produces a 14-volt trigger output which goes positive with respect to 0 volts and, in the case of the TR606, lasts some 20ms. Other manufacturers have trigger pulses which go negative with respect to 0 volts, while Sequential Circuits compromise by using a positive trigger pulse of only 5 volts or so.

What had to be produced, then, was a device that would accept any positive - or negative - going pulse of any reasonable amplitude and convert it to a switching action. Several obvious circuits sprang to mind but what I finally arrived at is probably the simplest circuit that will do the job reliably.

The DIL reed relay output device was chosen for two main reasons. First, the switch contacts are completely isolated from the rest of the circuit, thereby avoiding level-shifting problems or ground-loop hum; and second, the resistance of the contacts when closed is as near to zero ohms as makes no difference, so even the most pedantic synth has no excuse not to trigger reliably. If you come across an 'S-trigger' that wants to operate in reverse, ie. it operates by shorting out all the time except during a trigger pulse, then substitute a relay with normally-closed contacts or, better still, change over contacts so that you can have both types available should they ever be required.

Circuit Operation

The incoming pulse is rectified by the bridge formed by four IN9916 diodes, and consequently becomes positive-going. Providing the pulse is high enough to overcome the forward voltage drop of the diodes and the base-emitter voltage of the transistor, the BC107 will turn on and pass current through the relay coil. In practice, any pulse over about 2.5 volts should operate the circuit, and in any event, I don't know of any synth that produces pulses of less than 5 volts, so you should by alright there.

The LED is wired in series with the relay coil and gives a visual indication of correct triggering. The voltage drop across the LED ensures that the maximum safe relay coil voltage is not exceeded, and the relay coil provides the current-limiting resistance required for the LED. The extra diode between the base of the transistor and the positive supply rail is purely a mark of paranoia on my behalf and could probably be omitted in most cases.

Assembly

Construction is best carried out on a small piece of 0.1" veroboard, and the circuit should work first time without any problems, so long as you don't solder the LED in back-to-front. The whole board should fit neatly into a plastic instrument-case and no difficulty should be experienced in obtaining the parts either from usual mail order outlets or from your local component shop.

One last tip. If you want to trigger your Sequential Circuits synth (we used a Pro One) from a Roland trigger-pulse, simply pot down the Roland output using a couple of 1K resistors. It should then work directly into the SCI trigger input.

(Click image for higher resolution version)

[Note: the various circuit images are mixed up between this project and the lead tester. We have corrected the images in the article.]