Mystery Encoder

I think this is some sort of rotary encoder, but it doesn't seem to act like one (no quadrature output from the pins).

Tests I've run: 5V input in each pin, monitoring the other three on an O-scope. I rotated through until I ran out of pins to put the power into...

Images of the whole assembly are below.

Purpose of This Question

Thank you to user28689 for pointing out that I had not stated the purpose of my question. Here it is:

I am seeking to drive the motor assembly with a micro controller (Arduino to start with). At this point I am seeking to use only the necessary parts from the assembly, in this case I think the sensor and the outrunner motor are the only things that need to be kept which is why I am trying to find out what kind of sensor it is and how to read the data from it. I have four of these motor assemblies that are relatively unmolested and being able to use them in projects would be useful.

Assembly Pictures

Here's a picture of the part in assembly. R19F 2E in assembly of RH7-1495.

Here it is by itself. R19F 2E removed from assembly. R19F 2E removed from assembly.

A closeup of the sensor 'head' for added clarity. Sensor head 1. Sensore head 2.

Assembly Details: As per request: the assembly is identified by PN: RH7-1495. It was pulled from an HP Color Laserjet 4650. The board logic runs on 5VDC while the outrunner DC motor runs on 24VDC. There are 7 pin outs on the end of the board.

This afternoon I probed the whole assembly and tentatively identified all 7 pins. I say tentatively because I haven't found anyone else who is looking for information on the assembly (RH7-1495) or the specific part. I can post images of each test from my multimeter if there is interest. RH7-1495 printer assembly. RH7-1495 assembly pinouts. RH7-1495 bottom aspect.

As an added bonus I dug out the position sensors that go with these motor assemblies. The top one has its cover, the bottom one I stripped to see what was inside. RH7-1495 position sensor.

Printer Details So that no one has to go haring around the internet for more information on this printer I've included as much as I have here.

This link says that it is part of the fuser drive assembly (LaserJets have a heater that fuses the ink to the paper, click for more on LaserJet operations theory).

The service manual, on page 219, calls these assemblies (RH7-1495) 'drum drives'. The printer contains five of them. One I've torn down to get a good look at components. The others I've left intact for testing and (hopefully) later use.

Service Manual: drum drive assembly.

The service manual also mentions that the homing sensors are not part of the overall assembly (page 222).

Below is the DC control board (I think) for all of the drum drives (RH7-1495 assembly). DC control board. On the rightmost side of the image is the power header, a 5 prong receptacle that is labeled for 5V, 3.3V, and 24V.

[edit] Here is the inside of the motor housing. The purple ring is non-magnetic, the metal housing is magnetic and the inner ring is the motor's magnet. enter image description here [/edit]

  • 1
    \$\begingroup\$ Is the thick brown band magnetized? \$\endgroup\$ Sep 8, 2013 at 1:02
  • \$\begingroup\$ I just checked it with a magnet. The metal under it is more magnetic than it is. I'm uploading a picture of the inside of the motor so that you can see it. \$\endgroup\$
    – Albert
    Sep 8, 2013 at 1:08
  • \$\begingroup\$ Most likely a hall effect sensor, it seems to me. Did you probe it in circuit? Is there more than one of these? What is this in? \$\endgroup\$ Sep 8, 2013 at 2:00
  • \$\begingroup\$ Erik, I didn't have an opportunity to probe this while the printer it was in was in operation. I'm adding the specifications of the printer and as much as I know about the assembly it resides in. \$\endgroup\$
    – Albert
    Sep 8, 2013 at 2:57
  • \$\begingroup\$ I notice three of the chips on the board are also marked with part numbers of the form RH<n>-<nnnn>. I suspect these are custom parts (or off-the-shelf parts built to a custom specification) made specifically for HP and you won't find them in the manufacturer's catalog under these part numbers. \$\endgroup\$
    – The Photon
    Nov 8, 2013 at 0:14

1 Answer 1


Its a differential Hall sensor that picks up each rotor pole as it moves and is used to measure rotation like a tachometer. Most Tachs use either a coil or Hall sensor.

  • \$\begingroup\$ So, a hall effect sensor it is. Here's what I did wrong: I was testing the sensor without powering up the motor, this left me with 0 magnetic field to trip said sensor. My apologies for being quick to jump to a conclusion. \$\endgroup\$
    – Albert
    Sep 8, 2013 at 21:03

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