In virtually every area of electronics there are compatibility problems when using a set-up that consists of items of equipment from a variety of manufacturers. There may even be problems of incompatibility between items of equipment from the same manufacturer if some of the gear is of more recent origin than the rest, and electromusic is not an abstainee from problems of this nature.

The E&MM Universal Trigger Interface is a simple, but very useful, gadget that helps to overcome a host of incompatibility problems. The unit comprises four sections: two monostable multivibrators and two stages which invert, buffer and provide interfacing for processors. Figure 1 is a simplified block diagram of the unit and this shows just one of the inverter/buffer/micro-interface stages, and just one of the monostables.

System description

The inverter is used when an input signal which has a leading edge going from 0V to some positive level must be converted to a signal having a leading edge that switches from a positive voltage down to 0V. Alternatively, it can convert a leading edge that switches from a positive level down to 0V into one that makes the opposite transition. What this stage cannot do is process input signals that are negative going with respect to the 0V rail, but it's unlikely that a trigger signal of this type will be encountered in practice.

The buffer stage enables a sensible output current to be obtained when the source signal is at a fairly high impedance. The voltage remains unaltered apart from a small DC offset appearing at the output of this stage.

In the digital interface mode the signal fed to the input is simply clipped to a nominal level of 4.7 volts; this enables micros (and other logic systems, both TTL and CMOS) to be safely controlled by high voltage input pulses, e.g.: 9 to 15 volt levels emanating from synths.

A switch is used to connect the input and output sockets to the appropriate circuit, and a LED indicator is connected at the output. This is pulsed on when the output is in the high state.

The monostables are negative-edge triggered types, although they may sometimes trigger reliably when fed with brief positive input pulses. The output is normally low and a positive output pulse of adjustable duration (from about 10ms to 2.5 seconds) is produced when the circuit is triggered. Of course, by using an inverter at the input either monostable can be made to trigger on the positive going edge of the input signal, and using an inverter at the output enables negative output pulses to be obtained.

These facilities can be useful where it is necessary to trigger a piece of equipment from a pulse that would otherwise be too brief to give reliable triggering: the monostables can be triggered by a pulse of less than a microsecond in duration. Monostables can also be used as a simple form of frequency divider; with an input frequency of (say) four pulses per second and the pulse duration set at just under one second, the monostable will have an output frequency of just one pulse per second. After the monostable is triggered, the next three pulses have no effect, as the output pulse hasn't ended, and the circuit will not be triggered again until the fifth pulse, this appearing just after the first output pulse ends. An important point to bear in mind when contemplating the use of this type of frequency division is that the output signal is not a brief positive pulse; it obviously has a long duration, and this might preclude its use in some instances. Each monostable has a LED at the output which lights up to indicate the output pulses.

The Circuit

Figure 2 shows the circuit diagram of the two inverter/buffer/micro-interface circuits and Figure 3 shows the circuit diagram of the monostable multivibrators.

If we first consider one of the inverter stages, this uses TR2 plus its collector load resistor R3 as a simple common emitter inverter. Base resistor R2 is included to effectively make TR2 voltage rather than current operated, and to protect TR2 against excessive base current. The buffer stage employs TR1 as a straightforward emitter follower stage, having R1 as its emitter load resistor. D1 is used to clip the input signal at approximately 4.7 volts when the unit is switched to the micro-interface mode.

S1a couples the input sockets to the appropriate stage, and S1b provides the equivalent function at the output of the unit. A pair of input sockets are fitted to the unit so that an unprocessed input signal can be easily taken from the extra input socket if desired. Similarly, a pair of output sockets are fitted so that the unit can readily drive more than one item of equipment.

The second inverter/buffer/micro-interface stage is essentially the same as the first, the only difference being that it has an additional pair of output sockets. LED indicators and series current limiting resistors are wired in parallel with the output of both stages. If the indicators aren't bright enough, don't be tempted to reduce R7/8 below 1k, as the additional current drain will be detrimental to the fidelity of the output pulses. Instead, look for a more efficient LED (red ones are available with high light output) or use a green one (the eye's sensitivity is greatest here). Alternatively, a 'bezel' LED has a fair degree of immunity from high level ambient light, and a wide viewing angle.

If we now consider one of the monostable stages, this is based on the familiar 555 timer IC used in the standard monostable configuration. Taking the monostable based on IC1, the output pulse length is determined by RV1, R11, and C4. The pulse length can be adjusted over the approximate range stated earlier by means of RV1, with minimum resistance through RV1 corresponding to minimum pulse length. Pin 2 is the trigger input of IC1 and this is biased to about half the supply voltage by R9 and R12. In order to trigger IC1 this input must be taken below one third of the supply voltage, and it must be taken only momentarily below this threshold level, otherwise the trigger signal will significantly affect the length of the output pulse. The input signal is coupled to the trigger input by C3 and R10. The low value of C3 when compared with the input impedance into which it feeds ensures that only a brief pulse will be supplied to the trigger input of IC1 however long the input pulse may be.

LED indicator D5 and its current limiting resistor R13 are connected at the output of IC1, as is preset potentiometer RV2 which enables the output signal to be attenuated somewhat if desired. As the 555 can source ample current, reducing R13/19 slightly to make the LEDs flash more brightly should present no problems. Again, pairs of input and output sockets are fitted to the unit. The second monostable circuit is identical to the first one. S3 is the on/off switch and power for the unit is provided by an external (readymade) mains power supply unit. The current consumption of the circuit varies somewhat according to the states of the invert and buffer stages, but is in the region of 20 to 30mA.

Construction

The unit is housed in a diecast aluminium box having external dimensions of 190x110x60mm, and the sockets are fitted in two rows along the lower part of the front panel. There is not a great deal of space for the sockets, and it is necessary to fold their tags over slightly in order to fit them into the available space. Fit the sockets one pair at a time, wiring them in parallel before fitting the next pair. The five controls are mounted in a row along the centre of the front panel, and the five LED indicators are fitted in a row along the top part of the panel. On the prototype the LEDs were mounted in fresnel lenses, but ordinary panel holders can be used if preferred, although this will give the unit a less neat appearance. Alternatively, a recessed 'bezel' LED may be preferred for its ruggedness.

The two inverter/buffer/micro-interface circuits are accommodated on a 0.1in. matrix stripboard having 43 holes by 9 copper strips; details of this board are provided in Figure 4. The two monostable circuits are fitted on to a 0.1in. matrix stripboard of the same size, and this board is detailed in Figure 5. Construction of both boards is quite straightforward and there should be no difficulty in producing either of them. There are numerous breaks in the copper strips of both boards; note that the main row is placed obliquely to avoid weakening the board excessively. Care should be taken to ensure that none of the breaks are omitted or incorrectly positioned.

The completed boards slot into vertical guide rails in the case, and it might be necessary to file down the ends of the boards slightly before they can be fitted in place. They must be fitted into guide rails lying to one side of the switches, so that there's no danger of the components or copper tracks coming into contact with the tags of switches. An entrance hole for the power cable must be drilled in one side of the case, and this hole should be fitted with a small grommet. Alternatively, for stage use, a nylon cable gland or latching power connector may be preferred.

Note that R7, R8 and R20 are not mounted on the boards, but are hung between the appropriate LEDs and sockets (or LED and switch in the case of R20). Wiring details of these three components, together with details of all the other point-to-point style wiring of the unit are given in Figure 6 and the accompanying wiring chart.

The unit can be powered from any supply that gives a suitable voltage, is capable of providing the necessary current, and has a reasonably well smoothed output. The prototype utilises a four voltage (6, 7.5, 9 and 12V) supply, which enables the unit to be used in virtually any set-up. In this circumstance, use the minimum voltage that provides reliable triggering. A supply voltage of not less than 5 volts is needed, and the absolute maximum permissible supply voltage is 15 volts, which is the most the 555 ICs can withstand.