Midi lights for EWS

Putting MIDI lights on your EWS front panel

You had to ask

For all you folks on the EWS mailing list, here is how to fix up midi lights to your EWS64 XL. Please don't try it unless you have reasonable experience in electronics - don't make this your first ever project!

Shopping list

One small piece of stripboard
One 14-pin IC socket
One 74HC04 hex inverter chip
One 0.1uF ceramic disc capacitor
Four 680-ohm resistors
Four high-brightness LEDs
Double-sided sticky foam pads
Approx. 1 metre of thin plastic-insulated wire

Tools

Small soldering iron with fine tip, and solder
Drill
Centre punch
Drill bit to make holes for your LEDs (3.5 or 5.5mm)

Test equipment

5V power supply (3 x 1.5V batteries will do)
Multimeter with current measuring facility
4 x 4.7K resistors

Explanation

Adding MIDI lights is quite simple. The MIDI signals are already present on the front panel, and there is a 5-volt power supply too. However, the signals can't supply enough current to light an LED. I've just used a common logic chip, the 74HC04, to boost them.

The MIDI lines all pulse 'low' when they are active. The 74HC04 is an inverter, so its outputs will go 'high'. Therefore, you connect the LEDs between the outputs and ground. Since the 04's outputs are only rated at a few mA, you need to use high-efficiency (aka high-brightness) LEDs. Resistors are used to restrict the current to 5mA.

Making the circuit

Put the bits together into this simple circuit. Remember to make a row of holes in the stripboard under the IC. Note that the unused gates in the package have inputs grounded. It's a hassle but if you don't do this, the chip can be damaged. The shorter leg on the LEDs connects to 0V.

The arrows in the circuit indicate connections to the EWS front panel PCB. Connect wires to your stripboard at these points. Use thin wires, because you will be soldering them to tiny surface-mount components. Also, you will probably have to put the LEDs on flying leads, for instance if you want one above each MIDI socket.

P.S. The IC is static sensitive.

Warning! Mistake in diagram

Pin 12 (3rd from top, right-hand side) of the IC is shown connected to 0V. This is wrong. Pin 12 should be left open, and pin 13 (2nd from top) should be connected to 0V.

I didn't want to just change the picture, in case it caused confusion. Sorry.

Testing

Mistakes in the circuit might toast your front panel or even your EWS itself. So, it's a wise idea to test the circuit thoroughly. If it passes this test, it should be fine (but see Disclaimer on top page)

Have a tea break. Then double-check your work against the circuit diagram. Look for solder bridges, chips in upside-down, etc.

Connect wires A,B,C, and D to the +5V wire, via one 4.7K resistor each. Then connect the 5V and 0V wires to a power supply. Measure the current draw with the multimeter. No LEDs should light, and the current draw should be only a few hundred microamps. If it is more than this, switch off quickly and check your work.

Now test the LEDs one at a time. Remove one of wires A,B,C or D (why not try A :) ) from the 5V supply, along with its resistor. Connect it to 0V, again via the resistor. Check that the corresponding LED lights up. Reconnect the wire and resistor to 5V, and try another one.

Installation

Needless to say, this will invalidate your warranty. So be careful not to break the front panel ;) I'm assuming you want to put the lights on the front panel itself, but you can put them anywhere on your machine - as long as you keep the wires from the front panel to the stripboard short. (use long wires from stripboard to LEDs)

Unplug the front panel, take it out of the computer, undo the screw in the middle of the front panel PCB, and lift it out of the silver chassis. Careful, the circuit board is static sensitive.

Solder the wires from your stripboard to these points on the front panel PCB. Use a fine soldering iron, and be careful with B and D; too much heat can make the SMT components fall off the board.

For goodness' sake don't get them mixed up! You could toast your EWS (see Disclaimer)

Now use a centre punch to mark where you want each LED. This stops the drill bit skidding. Clamp the chassis securely and drill the holes.

Secure the stripboard to the front panel PCB, using double-sided sticky pads. If you have a daughterboard fitted, it might be a bit of a squeeze. Replace the PCB in the front panel chassis, and stick the LEDS through their holes. Fix them in with epoxy glue, or use special panel mounting kits (if you're posh)

It's ready to go

Hook the front panel back up and give it a test. If you get any trouble, switch off immediately. Y'all have a blast!

Coming soon (subject to demand)

Accessing MIDI-2 on the EWS64 L

Modifying the EWS for balanced ins/outs and reduced noise (hardcore hackers only)

e-mail me: "steve at scopeboy dot com"

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