Expression Pedal Meter

The EPM (Expression Pedal Meter) indicates the position of an expression pedal using a bar graph of 10 LEDs. It connects inline between an expression pedal and a keyboard's expression pedal input. It has a switch to change polarity (Yamaha or Roland wiring) and a recessed potentiometer to calibrate the meter.

Table of contents

Introduction

Sometimes it can be important for a keyboard player to know the exact position of an expression pedal - adjusting the pedal to a known value before playing a note, for example. It is surprisingly difficult to guess the exact position. Most keyboards will not display the MIDI value that the pedal generates.

There is therefore a need for a hardware device that indicates the position of the pedal to an adequate accuracy. There are three approaches:

  1. Extract the position from the MIDI message that the keyboard generates and somehow display that in hardware (like the Kenton LD2PRO);
  2. Visualise the MIDI controller message in software;
  3. Measure the voltage that the expression pedal produces in hardware and display it on a meter of some kind.

The first option has a couple of significant drawbacks. MIDI monitors like the LD2PRO area quite expensive and require a spare 5-pin DIN MIDI output on the keyboard. If you don't have that spare MIDI output, then you will need a MIDI thru box. It gets complicated (and expensive) quickly, but it could work well if your keyboard rig is already complex and a couple more devices (and power supplies) won't make it much more so. But if you have two or more expression pedals, then it becomes quite cumbersome.

If your keyboard eventually plugs into a computer via MIDI, then the second option is a possibility. It would be straightforward to display the value of a MIDI controller such as an expression pedal by mapping it onto an unused fader, or maybe even building a simple Reaktor instrument that can be inserted as a plug-in.

If you're not using a computer, or do not want to use it to display the position of the expression pedal, another approach would be option 3. To my knowledge, there is no commercially available device that does this, even though it is a simple electronic solution that should be much cheaper than the LD2PRO. It wouldn't need a spare MIDI output, nor a computer screen. The expression pedal meter described below uses this approach.

For the sake of brevity, I will refer to "expression pedal" as EP, and the "expression pedal meter" as EPM.

How does an expression pedal work?

Expression pedal and keyboard simplified schematic

Expression pedal and keyboard simplified schematic

First let's describe how an EP input jack on a keyboard works. It is a 1/4" stereo jack socket. It has three terminals: ground, reference voltage and voltage input. Usually the reference voltage is somewhere in the region of 3V to 5V - it doesn't really matter what it is, although it is important to note that it is not a low impedence output (signified by R2 in the schematic above, more on this below). The voltage input determins the MIDI value that is generated, and is measured by the keyboard in relation to the reference voltage. It is the ratio of voltage input in comparison to the reference voltage that is significant, not any voltage in isolation.

An expression pedal, therefore, needs to output a voltage that corresponds to the expression pedal position, in relation to the reference voltage. All expression pedals use a potentiometer to achieve this (RV1 in the schematic above). A potentiomer is really just a rotary variable resistor (like a volume knob). It takes ground and a reference voltage as an input and outputs a voltage relative to the position of a wiper as the knob is turned.

Sometimes additional resistors are present. R1 protects the keyboard against in case it is expecting a different wiring. RV2 sets a lower limit to the voltage the wiper outputs.

The ground and reference voltage terminals of the EP should connect to the terminals of the same name in the keyboard's EP input. The EP wiper should connect to the keyboard voltage input.

Expression pedal wiring

Expression pedal wiring

The ground is always connected to the sleeve of the EP jack plug and the keyboard's EP input jack socket. The reference voltage and wiper/voltage input can connect to either the tip or the sleeve (see diagram above for the "Yamaha" and "Roland" wiring standards).

Design considerations

The EPM must allow for both the Yamaha and the Roland wiring standards.

The EPM must not affect the MIDI value generated by the keyboard by drawing any significant current from the voltage reference or voltage input terminals of the keyboard. It should therefore have a high input impedence.

The EPM should be a small as possible, whilst providing a clear indication of expression pedal position from a wide viewing angle and in a range of lighting conditions.

After some experimentation, it was found that some keyboards reach their maximum MIDI value before the pedal has reached its maximum position. The EPM must have a method of calibrating this so that the EPM display corresponds to the MIDI value, not the expression pedal position. It is what the keyboard sees that is important, not the absolute position of the pedal.

Schematic

Expression pedal meter schematic

The EPM uses the LM3914 bar graph display driver IC. Page 8 of the datasheet shows a simplified block diagram of the IC. It is basically a resistive ladder with one input of a comparitor connected to each stage. The output of each comparitor can turn on an LED. The top end of the ladder is effectively connected to the reference voltage of the keyboard's expression pedal input, the low end is connected to the keyboard ground, and the other input of each comparitor taps off the voltage at the keyboard voltage input. When the input voltage rises above each stage of the ladder, an LED is lit. This creates the bar graph effect. The LM3914 is somewhat more sophisticated than the above description - each LED output is a constant current that is set by a single resistor, R4 in the schematic.

Calibrating the EPM to keyboards that generate a maximum MIDI value before the pedal has reached its maximum position is achieved by connecting a variable resistor in series with the LM3914 resistor ladder (R2 in the schematic). This allows the EPM to reach its maximum (all LEDs lit) before the pedal reaches its maximum position, just like some keyboards.

The power input section is merely a reverse voltage protection diode as the LM3914 has an absolute maximum supply voltage of 25V, although it can only dissipage around 1.3 Watts. This means that the EPM can be powered directly from a 9V power supply. 9V is a good choice because it is most likely higher than a keyboard reference voltage, and it is low enough to keep the power dissipation to minimum.

The LM3914 is fed via op-amps to keep the input impedence high to ensure it has minimal loading on the expression pedal. The op-amps are in turn fed via a switch to invert the polarity to suit both the Yamaha and Roland wiring standards. Setting this switch incorrectly will cause the EPM to malfunction, but it will not be damaged by it.

Hardware design

The main goal of the EPM is user convenience - it should work with nearly any expression pedal, no patch cables required, easy calibration - and reliability. A flying lead removes the need for any further cables. The diecast aluminium enclosure makes it very robust.

Several good-quality brand-name LEDs were tested for even brightness and diffusion. Of particular concern was matching the brightness of the red LEDs to the green LEDs.

Through-hole components (most are only available as through-hole, anyway) for ease of user servicing and modification (e.g. adjusting the brightness of the LEDs by replacing a resistor).

Compatibility

The EPM works with the Yamaha FC7 and Roland EV-5, and any other EP with similar wiring to these. It should also work with the Boss FV-500 if it is also connected to a keyboard or guitar processor (and not just used as a volume pedal - see below).

The EPM is not compatible with the Line 6 Ex-1 Pedal because it is wired differently.

I have received a few emails from people asking if the EPM can be used with a Boss FV-500 pedal, either version. The FV-500 is a volume pedal with an additional "expression" output, equivalent to the Roland EV-5. If you use the pedal plugged into a keyboard or a processor EP input, then yes, it should work. But I recently came across a case where the FV-500 was being used as a guitar volume pedal (nothing currently plugged into the expression output) and the user wanted to use the EPM plugged only into the pedal's expression output to see an indication of the pedal position. This will not work as the EPM needs to be connected to both an EP and an EP input on a keyboard or guitar processor. Read on for a possible solution for this.

Improvements

A fix to enable the use of the meter when it is only connected to an EP and not a keyboard would be to use a high value resistor divider inside the EPM to provide a voltage reference via the normalising switches on the jack sockets. This way, when no keyboard is present, the meter provides its own voltage reference. And when a keyboard is connected, the resistor divider is disconnected by the jack socket's normalising switches. I have not tested this yet, but if it works, it will be a feature of a new version.

Can the cost be lowered further? The two main costs are the diecast aluminium enclosure and the time to assemble it. Other than the cost, the enclosure is ideal - it is as small as it can be whilst providing a clear indication of pedal position, and it is very, very rugged. It should last a long time of rough use. (And when the cable fails, it can easily be replaced - the area of the cable where the cable gland clamps onto it will probably be the first thing to fail.) But it is very fiddly and time consuming to assemble. Whilst the circuit board itself is quick and easy to assemble, preparing the enclosure isn't. The circuit board is fixed to the underside of the top panel so the holes on the side for the jack socket, power socket, polarity switch and calibration potentiometer must be drilled very accurately. The flying jack lead needs to be stripped, tinned, heat-shrunk and soldered to the board. An improvement might be to discard the flying jack lead and instead have two jack sockets instead. Mounting the board in a transparent enclosure would do away with having drilled holes on two sides of the enclosure that are so difficult to drill accurately. But it may look less professional than the current design.

Whilst every keyboard I have tested has the 'ground' of the EP input jack at the same potential as the keyboard's power supply ground, it would not be impossible (but very odd) for this connection not to be at ground at the keyboard. With the current EPM design, this connection is tied to ground at the meter end. So it could be possible that a condition exists where the 'grounds' differ. Normally this would not be a problem because most 12V plug-in power supplies have a floating ground - they are not tied to mains earth. But if they weren't floating, then the meter might not behave as expected. A solution to this unusual scenario would be to either power the meter circuit from an isolated DC-DC converter (adding considerably to the expense of the device) or decouple the pedal 'ground' from the meter ground. It is not clear from the LM3914 data sheet if the low end of the divider network can be fully disconnected from the pedal ground because presumably the grounds have to be common for an accurate measurement. This needs further research and testing. But it is not a priority since the situation just described would be highly unusual.

Purchasing

I usually have a few ready-made EPMs available for sale. The price is £54 plus shipping. I accept Paypal payments. If you are interested in purchasing an EPM, please send me an email with the shipping address so I can provide a quote for shipping.