View source for Roland FD-8 Issues: Hall Sensor Modification

This page describes one of two solutions to common issues with the Roland FD-8 hi-hat pedal. Both solutions replace the film resistor with a Hall effect sensor. This solution uses discrete components and you may find it easier to construct. A low-cost microcontroller-based version is described [[Digital FD-8 pedal|on this page]].

Both editors of the Justintime pages have a Roland TD-3 electronic drum kit. This kit plays quite nicely, but after some years the responsiveness of the Hi-hat pedal seems to worsen. We are not the only ones who have noticed this; google for 'roland fd-8 issues' to find widespread complaints and hacks/solutions people have come up with. Most solutions have to do with adjusting the mechanical pressure on the variable film resistor in the pedal. 

However, I wanted to completely get rid of the film resistor and replace it with an electronic alternative that has no contact issues AND had the possibility to adjust the half-open range to personal taste. The obvious choice for the detection was a (linear) Hall Sensor, but this would have to be translated to a variable resistor for the TD-3. The first design thoughts were MCU based with an ADC and programmable variable resistor, but after some more (clever) thinking it became clear that a circuit with two comparators and two analog switches would work just as well and might be a lot simpler. Another important consideration is that it might be easier for the average FD-8 owner (other than the two of us) to recreate.

Although the film resistor is very variable, the TD-3 hi-hat recognizes three positions from the resistor: 
* closed (short, 0R to 5K)
* half (5K to 15K)
* closed (15K and up)

==Electronics==
Finding a linear Hall Sensor can be hard. Pay attention: most Hall Sensors are on/off type and have hysteresis circuit inside. These can not be used, because we have to detect at least two switching points. I used a Honeywell SS490, but I suspect any Linear (Ratiometric) Hall Sensor should do. Also be aware that the sensor is polarity-sensitive; the circuit is designed for voltage increase when the magnet draws near (4.5 - 8.5V), but if you flip the magnet the voltage will decrease!!

For powering the circuit we branch off the TD-3 power supply. Be careful: the Roland power supply states 9V output, but it outputs about 11.5V. Since my Hall Sensor max was 10.5V, a regulator was necessary. 9V seemed a good pick. Using a small Neodymium magnet the sensor had a working range of 4.5V to 8.5V. I used a plastic Lego brick because it was available and about the right size, and most of all because it was sturdy plastic. After starting of with a metal construction, this was abandoned because I was worried the metal would interfere with the magnetic field and influence the sensor readings, so plastic seemed a better pick.

By making the comparator inputs adjustable with potmeters, the sensitivity range of the FD-8 half and closed settings can be made very flexible. The circuit is not very complex, so it could very well be made on veroboard or stripboard. I have made a SMT PCB myself, because I prefer working in SMT. Check out the [[Hall-High-Hat-Hack-Schematic|schematic]], [[PCB Layout]] and [[Media:HallHighHat Eagle Files.7z |Eagle Files]]. 

The prototype circuit works very well and makes the Hi-Hat a lot easier to play e.g. provides a much more reliable closing action. The only 'downside' of the circuit is that the foot-sound is always on full volume. I suspect that this can be adjusted by increasing the 0R value, but this has yet to be tested.

==Construction==
Below are some pics. These pictures were made before the  DC connectors had arrived. These 2.1mm DC Power plugs are intended to make use of the original TD3 power supply; the pedal will feature 2 parallel plugs, one for power supply input and the TD-3 module can be connected to the other power plug via a cable.

Below are some pics of the not-yet-finished build. Mechanically, this is exactly the same as in the [[Digital FD-8 pedal|microcontroller-based concept]].

[[File:Hall Sensor and Strip.JPG]]

The Magnet sticks to the metal by magnetism, but is also glued with superglue to avoid shift. The Hall Sensor and Lego brick are set using doublesided tape and [[secured by a tie-wrap]]. Also visible is the original variable film-resistor. This is left intact, so it is always possible to return to using the original mechanical film-resistor mechanism. The output of the hall-sensor-resistor-circuit is routed to a new jack output on the other side of the pedal, as can be seen on this picture, since the original output is on the left side.

[[File:External Pots.JPG]]

The external pots are for adjusting the sensitivity of the closed and half-open state. If you do not intend to tweak trimpots would do fine too.

[[File:Internal Wiring.JPG]]

The Hall Sensor is connected by a pin header connector, so it is easier to take the pedal apart. Fortunately there is enough spare room in the pedal for the circuitry.

[[File:FD8 with Knobs.JPG]]

Disclaimer: Use this info at your own risk. We take no responsibility for any damaged equipment!

==Comments? Questions?==
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