I own a used Clavia ddrum4 SE drum module. It employs a fairly advanced technique to detect the position of the hi-hat; on the stand is mounted a magnet, and in the hi-hat pad is a Hall effect sensor circuit.

It’s exceedingly difficult to find working hardware for the module, so a number of years ago someone reverse engineered the circuit and posted it online. I have tried to replicate the circuit on a breadboard, with one exception: I couldn’t find a retailer selling the original sensor model, so I tried my best to find a replacement unit with characteristics that are compatible with the original. I fear I have failed, because the test circuit isn’t working. I am not an electrical engineer; I only know enough to be dangerous and I’m in over my head. Let me walk through the details:

  1. here is the circuit that was posted: Hall effect sensor circuit

  2. the “508” is a Micronas HAL 508

  3. I substituted a TT electronics Optek OH180U
  4. the ddrum4 SE unit provides +4.75V on the ring of the connection jack, ground is sleeve (tip is for the piezo trigger)
  5. I have confirmed that simply placing a 10K pot across +4.75 and ground (ring and sleeve) lets me control the hi-hat position. It’s extremely touchy with a narrow range; if I bridge a 10K fixed in parallel and recalibrate the unit (a built-in function to set the “closed” point) it makes it slightly more responsive…anyways, the point is I have confirmed the head unit works.
  6. in the circuit, ddrum must have fine-tuned the 10K trim pot at the factory and then sealed it, because users’ pictures show it covered with glue. I’ve tried adjusting it all over the place and recalibrating often with no success.
  7. I've double-checked pinouts for everything, especially diode polarity, op-amp pins and Hall effect sensor pins too.

So my questions:

  1. can a real electrical engineer look at the HAL 508 and the Optek OH180U and see if they’re in the ballpark? Perhaps my choice of replacement Hall sensor is just completely wrong. The 508 is a unipolar switching sensor, on typically at 18 mT, off at 16, hysteresis 2. The OH180U is also unipolar switching, on at 18mT, off at 14, hysteresis 4. I've used some fairly strong magnets...not rare-earth, but some stacked together. It should be enough to get something out of it. I tried both N and S by the way. =) Perhaps I should be making a test circuit to see if the magnets are triggering the sensor? (Links appreciated!)

  2. I’m really confused about the circuit itself; in that there are only two leads (+4.75V and ground) provided even for the op-amp supply. I’m assuming that it’s forming a parasitic load on the voltage based upon the magnet distance—which the unit measures—but that’s its supply too. Can someone explain what this should do? I’ve tried to match the design to a number of basic op-amp circuit designs on the web, but almost all of them have a separate output, so I don’t grok this pattern.

  3. It’s also conceivable that the circuit was reverse engineered improperly…I’ve looked at the pictures that were taken, and tried hard to trace everything to confirm correctness. The problem is, the guy didn’t take apart or damage his unit (obviously! it still worked) so it’s possible we missed a connection. Does this circuit look “really close” to something familiar, but is messed up a little bit?

  4. I traced the original op-amp’s unit 2 circuit, and it looks like they didn’t terminate it properly—the output is wide open and in+/in- are tied to ground. I terminated my unit 2 circuit properly with 10K resistors from in+ to supply and ground, output to in- like mentioned here. Is it possible that me changing this design would influence the first circuit? I don’t think so, but I’m grasping at straws without a formal education in electronics so…

Thanks so much in advance for any help.



I forgot the link to the op-amp #2 termination I used instead of their "dangling" approach: here it is. I used version (a) because I don't have a negative supply.

responding to Scott: I was very confused about the choice of Hall effect sensor as well. The sensor data sheets clearly state that they are switched sensors, i.e. on/off. However, the Clavia documentation (and several other sources) state that there are at least 8 levels of magnet position sensor detection. It's actually also implied that they interpolate between these, most likely in software. But 8 levels is quite a bit more than 2! =)

At the bottom, I am attaching some relevant pictures of the working device someone disassembled, in case that gives anyone a Eureka! moment.

I assumed the zener diode is a pop protection for when the 1/4" stereo cable is inserted into the jack; the contacts kind of jump until the cable is all the way in.

I also assumed the capacitor between signal and ground acted like a low-pass filter, but I don't really get how it plays with the op-amp. I kind of thought you'd want a high-pass filter to smooth a noisy sensor; having a magnet move close to this circuit by means of me pushing my foot on a pedal is pretty much the definition of low frequency so why would we filter that?

responding to Russell: great questions...let me try to elucidate some things (but claim ignorance on many others!)...

The ddrum4 SE brain is powered from 120VAC; literally a two-prong US polarity plug. Inside it must have a regulated power supply of some sort...I'm reluctant to open the unit because if I break something I'm pretty much toast on fixing it. (I did open it; see below).

The connection between the hi-hat pad/controller and the brain is a tip-ring-sleeve 1/4" stereo cable. That's it. Only three wires. One is definitely common signal ground, and another is also well-known...

The way that pad trigger interfaces almost always work, is that you have a piezo device attached to a metal plate covered with rubber. The physical hit of a stick causes a voltage spike across the common ground (in my case, the sleeve) and the trigger signal wire (in my case, the tip). This signal goes through an amplifier gain stage which is fed into an ADC. The ADC is scanned at a very high rate, so the unit can take slope readings of the change in signal over time in order to produce an accurate "note on" MIDI message—which includes "velocity" or signal strength—before the piezo trigger spike actually peaks. The ADC is most likely 10-bit, since their product literature claims "more than 1000 levels of velocity" (even though MIDI is only 7-bit, internally they apparently use 10).

Additionally, ddrum specifically designed the interface such that a standard 2-wire (mono) 1/4" cable can be used on this input, in case it's just a normal pad trigger. That means that the device works fine if the ring (middle wire of 1/4" stereo) is connected to sleeve (common ground)! When I measure potential across ring to sleeve, it's +4.75 V.

But what's most puzzling is this: the Hall effect sensor circuit—with the LM358 amplifier—is both powered by and communicates its output with the same two wires...sleeve (ground) and ring (+5 V).

I'm guessing some things here—again, I'm not an electrical engineer—I'm assuming that they're probably using another ADC to measure voltage across ring and sleeve. They probably have a +5 reference voltage in a comparator circuit (or some other scaling device) to detect how far this circuit above drops the +5 it's being given. Maybe that's the trick...the circuit is supposed to drop the voltage, but only by a small amount so as not to make the op-amp fail below nominal supply voltage?

Or perhaps...forgive me if I'm talking utter nonsense here...don't resistors drop both voltage and current? So maybe the circuit acts like a power sink, consuming a variable amount of current based on the distance of the magnet? If that's the case, then maybe they're somehow using an ADC to measure the current draw of the circuit?

Without opening her up, that's pretty much all I can surmise. I know for a fact that it senses the position of the magnet with two wires, and those same two wires act like ground and supply for the op-amp that's part of the circuit. I know if you bridge that supply to ground, nothing bad happens and if it's in hi-hat mode it acts as if the magnet is infinitely far away (this could be a software mode though). I know I can get a trim pot across the supply and ground to make varying levels, although the reaction band is very narrow. I could try to estimate that range if that would be useful to someone.

It's starting to sound like I should open up the main module and try to trace the circuitry and get more information.

...and I did...it's a fair amount of surface mount stuff, traces on both sides, routing all around and then over a cable to the main board. I took pictures, but I don't think I can figure out what's happening. The hi-hat jack is J10.

The interface/trigger input panel: hi-hat interface board

The wires from the original controller: wires from original controller

The original circuit board: original circuit board

Identifying the Hall effect sensor: Hall sensor type

  • 2
    \$\begingroup\$ Welcome to EE:SE! +1 from me for a well-asked question. \$\endgroup\$
    – Greg d'Eon
    Commented Apr 28, 2015 at 0:22

2 Answers 2


The HAL508 and Optek OH180U are both Hall Effect SWITCHES with hysteresis. The HAL has an open drain output ant the Optek has an open collector output, both on pin 3. When you look at the magnetic specs in the data sheets, after converting Gauss to milliTeslas, you'll see that the two sensors are very compatible.

Since switches are on/off, I assume that the function is to say whether the Hi Hat is open or closed, not to read out the linear position of the hi hat.

The circuit is rather confusing. Why you would tie an open drain high using the output of an op amp is simply beyond me. It's hard to believe that anyone would want to handle a switch in this fashion, even if you need to do inversion and/or level shifting. I suppose its possible that this funky circuit forms some sort of oscillator that when low pass filtered gives you a distance measurement, perhaps explaining the capacitor, but we'd really need to know what is being used as the output of this circuit to know.

Is it possible that the ORIGINAL original circuit used some sort of ratiometric Hall sensor, and not a switch?

  • \$\begingroup\$ I updated the original with much more detailed information...thanks for the questions. \$\endgroup\$
    – Mike Bell
    Commented Apr 28, 2015 at 21:09
  • \$\begingroup\$ @MikeBell I can't spend time just now - but - you are not conveying the whole picture. Assume (with certainty) that I know less about the use of a hi-=hat controller in this (or any) contaxt than you know about electronics. Some web meandering indicates that the controller uses the position signal to alter or select what sounds or sorts of sounds are output. Why this is so I know not. Maybe hi-hat position influences this usually and the electronics is trying to emulate reality. How would I know? A: Only if somebody told me. Q: Why does this matter. A: How would I know - but it might. .... \$\endgroup\$
    – Russell McMahon
    Commented May 2, 2015 at 2:52
  • \$\begingroup\$ This page seems to suggest an alternative method: jlegler.com/archives/254 - I don;'t know where or if Max for live fits in but if I understaood what he was doing it may suggest other ways of doing it. \$\endgroup\$
    – Russell McMahon
    Commented May 2, 2015 at 2:55
  • \$\begingroup\$ .... | This suggests that a pintech controller may be relevant but I know not (and link is dead) unofficially-ddrum.proboards.com/thread/46/diy-hihat-controller | \$\endgroup\$
    – Russell McMahon
    Commented May 2, 2015 at 2:55
  • \$\begingroup\$ re eg "What the circuit is supposed to accomplish is a magnetic position sensor. It should give a range of values, somehow, to an ADC indicating the pedal position of the hi-hat" -> And? Now you say "pedal position ... ". Pedal posn = hihar posn? Or linearly related or Tangerine or ? | And we seem to have totally lost " ... change in signal over time in order to produce an accurate "note on" MIDI message—which includes "velocity" or signal strength— ..." - is that related to hat position? Does it use the same sensor? How do they relate? Does it taste good? Octarine? -> .... \$\endgroup\$
    – Russell McMahon
    Commented May 2, 2015 at 3:02

Comment re stack exchange EE policy.
At one level this could be seen as being dangerously close to a repair question. I don't see it that way (of course :-) ) as the initial "design" sounds very suspect and it should be possible to meet the need more elegantly and probably with similar complexity.

We will be easily able to answer the question once we know what it is.
You have provided good information BUT it is very hard to determine exactly what it is meant to DO functionally.

Please tell us EXACTLY what the circuit DOES considering it as a black box.
eg the phrase "detect the position of the hi-hat" is low on information - does it output a voltage that changes linearly with hi-hat linear position, or give an on/off output or ... ?

Also you say it only has two leads but as shown these are power supply and ground and the circuit may as well be a 5 volt powered brick - ie 5V supply is "stiff" in the absence of information to the contrary and there is no "possible" signal output. Presumably things are not quite as shown overall. Knowing what it should do functionally in the context that it is used is much more liable to be useful than trying to understand a circuit of suspect provenance.

Where is the output taken from?

What does it DO?
Stand back from the circuit and give us the big picture and a good solution should be available.

This started as a comment but, while relatively contentless answer wise it should set the foundation to build an answer on - very possibly a complete new circuit.

Below is an image of top and bottom sides on your sensorboard with the bottom side flipped and rotated and almost properly aligned. Would it be possible to draw connection information for the hidden tracks and the pot on that image? I'm not 100%confident that the traced out circuit is correct. Would it also be possible to add component names on diagram to match the PCB silkscreen names.

I assume Z3 is the zener?

If the device draws variable current with hi-hat operation then a series resistor in either supply lead would provide a voltage proportional to current.

enter image description here

  • \$\begingroup\$ I updated the original with much more detailed information...thanks for the questions. \$\endgroup\$
    – Mike Bell
    Commented Apr 28, 2015 at 21:10
  • \$\begingroup\$ I think you're right, I believe there are interior traces and the diagram the gentleman created is incorrect. Unfortunately, I don't have the original circuit, so I can't refine/correct it myself. The imagery he has doesn't help elucidate it. I think I'm going to drop it and give up. Thanks everybody for your help! \$\endgroup\$
    – Mike Bell
    Commented Apr 29, 2015 at 23:55
  • \$\begingroup\$ Mike - STOP! - no need to give up. Not yet anyway. You have given us lots of detail. Now a very high level description would be useful. Can you explain in broadest terms what all the functions of the circuit are? It initially sounded like it was a hi-hat position sensor, then a hi-hat signal pickup - and may be it is both with the position / height / ? changing sensitivity or effect in some way. A black box functional overview may well help. Given such a description and the device it plugs into I, and others here could design a cct from scratch. Without the target box its harder but may ... \$\endgroup\$
    – Russell McMahon
    Commented Apr 30, 2015 at 6:51
  • \$\begingroup\$ ... be doable. Worth doing? | Do you know anyone in Auckland, New Zealand who has one by any chance ? :-). Is this functionality similar to what is found on other gear? Is it common? Is ... \$\endgroup\$
    – Russell McMahon
    Commented Apr 30, 2015 at 6:52
  • \$\begingroup\$ OK...I will at least post a link to more media here. These are the original photos taken by the gentleman four years ago: plus.google.com/104759823397760776679/posts/CTdt7K2si6s ...and these are the photos I took inside the brain unit the other day: plus.google.com/104759823397760776679/posts/CTdt7K2si6s I'm adding them here in case the original hi-res versions help anyone. What the circuit is supposed to accomplish is a magnetic position sensor. It should give a range of values, somehow, to an ADC indicating the pedal position of the hi-hat. Thanks again for all help! \$\endgroup\$
    – Mike Bell
    Commented Apr 30, 2015 at 23:34

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