Fun with Resistive Position Sensors

As part of my continuing adventures in drumming symmetry, I have been working on a dual electronic hi-hat pedal. The idea is to have a single hi-hat pad respond equally well to either a left or a right foot pedal. It is similar to having both left and right kick drum pedals. For the hi-hat, this effect is sometimes realized through the use of a switch, but that requires some extra motion and it's not possible to use both pedals at the same time. These pedals (both an FD-7 and an FD-8) are used with a Roland TD20 drum controller. The hi-hat pedal uses a resistive position sensor to indicate the location of the hi-hat pedal, be it fully up, fully down, or somewhere in between, to the TD20.

So, how do these sensors work? Well, let's go back to some basics. The electrical resistance of a material depends on the inherent characteristics of that material (its resistivity) and its physical layout:

resistance = resistivity * length / cross-sectional area

In other words, if you take a certain amount of stuff and make it very long and skinny in shape, it will have a much higher resistance than if you shape it short and stout. Below is a photo of the sensor used in the Roland FD-7 (the FD-8 sensor is very similar).

At the left end is the flat flex terminus (the end that goes into the FFC jack) and to the right is the body of the unit. Note that large rectangular area. Basically, this sensor has a specific resistance of about 20,000 ohms when untouched. If you press on the sensor, it makes contact with the underside and this shorts out a portion of the sensor (it literally makes the sensor shorter from an electrical perspective) and reduces the resistance. If you press halfway from the end, you get 10,000 ohms. If you press one quarter of the way in, you get 5,000 ohms and so on. Note that there are no moving parts associated with this sensor so it has a fairly high expected life cycle.

The pedals manipulate this sensor through the use of a rubber "foot" that rolls across the sensor as the pedal is depressed. With the pedal fully up, the sensor reads 20,000 ohms. When the pedal is fully depressed, the sensor reads just a few ohms. The drum controller uses the resistive value to trigger an appropriate hi-hat sound when the associated drum pad is struck (the controller is also smart enough to look at the rate of resistance change and trigger an appropriate step sound). The photo below shows the guts of the FD-7 pedal.

So here's the problem: If you simply use a Y cord to connect two pedals together (as you might with kick drum pedals), you'll never get a fully open hi-hat sound. The two pedals together produce a parallel equivalent resistance of only 10,000 ohms which corresponds to the half open sound. That's no fun. Further, you cannot simply add an extra 10,000 ohms in series with the pedals. Although that will achieve a fully open sound, you'll never get less than 10,000 ohms total, so you'll never get the closed sound. What you really need to do is recalibrate the controller for a new resistance range but that's a little unrealistic. Alternately, you can figure out a way to double the resistance of the pedals to 40,000 ohms each, thus yielding the desired 20,000 ohm open resistance for the parallel pair. This can be achieved by doubling-up the sensors, one on top of the other, and combining them in series. The trick here is that you can't just cut and solder the flat flex film as it will melt. Instead, you have to hack away at the original jack board and tie-in a new flat flex connector. In order to keep this as flexible as possible, I decided to add a toggle switch so that the pedals could be used in either normal (single) or double resistance (dual) modes. Extra hi-hat film sensors can be purchased directly from Roland.

Here is a shot of the jack board. Note that the copper trace connecting the flat flex connector (right side) and the 1/4 inch jack (bottom center) has been severed. A wire has been attached to the now-isolated flat flex connector pin. The other end will be soldered to one of the outside pins of an SPDT toggle switch (I actually used DPDT switches because that's what I had hanging around in my basement shop). In a similar manner, the now-isolated 1/4 inch pin will have a wire soldered to it and connected to the center pin of the toggle switch. By the way, I used 24 gauge solid wire for these connections. (Not the greatest soldering job either as the iron's tip was a bit too large for this job.)

The final wiring step is to get an eight pin flat flex connector with 1.25 mm spacing (a four pin connector is used on the FD-8). Roland actually uses two connector pins for each of the sensor input traces (the sensor traces are much wider than the contacts and two contacts touch each trace). I assume they did this for reasons of long term reliability. I will warn you that the flat flex connectors are not the easiest things to find. I used some made by Molex. You should pair-up the connector pins used for each trace and solder a wire to each of the two pairs. One wire will go to the same end pin on the toggle switch that was in step one (red wires in the diagram below). The other wire (white) goes to the remaining unused end pin on the toggle switch.

Now drill a hole a couple inches away from the jack's hole on the pedal for the toggle switch. Remove the backing from the new sensor and position it directly over the existing sensor on the pedal's bottom plate. The sensors are slid into their respective flat flex connectors (grey in the photo above) and the unit is reassembled, taking care to keep the wires away from the rubber foot.

One FD-7 with a brand new switch. The process for the FD-8 is very similar. They seem to work fairly well together and I can now switch back and forth from right-footed to left-footed drumming with ease.