New 2009 - Cheap motor with Rom-Enco magnetic encoder

Optical encoders

If a DC motor must spin at a constant speed, one way is to measure its speed and ajust the power on the motor accordingly. The quadrature encoder gives the direction and allows for precise positionning. DIDEL provides the low power optical sensor and slotted gears required for miniature applications. See our price-list,

Quadrature encoder principle

Microcontroler interface

It is easy with any processor to sample and decode the quadrature signals and update an up/down counter. The routine needs only 10-20 instructions, but must be executed frequently enough not to miss a transition. You can find more explanations on page 15-16 of our PIC programming document.

H7001 encoders

The H7001 is encapsulated in a light emitting diode package, but the chip is on the flat side, and not on the lens side.
There are four photodiodes inside which are connected to differential amplifiers and a latch. The diodes distance is 0.15 mm, which match slots with 0.623spacing (400 slots per inch). The circuit will work with any slot size larger than 0.xx mm, but the spacing of transitions will not be even. With correct slots, the interface will give a 4 time resolution.

The H7001 is packaged in two different ways, depending on if the photodiode line matches vertical slots, as in the next figure, or horizontal slots. There are color marks, but the best way is to look at the orientation of the tags. Pay attention, the electrical pinout is different for the H7001H (see below).

Electrical specifications
Voltage 1.8 to 5.5
Supply current 0.05 microAmps
Transfer time 5 to 15 microseconds
Output current high -200 microamps
Output current low 2 mA


The H7001 must be as close as possible from the slotted disk. The photodiode is preferably an IR led with a small angle, at a distance and position that will illuminate the two slots in front of the H7001. We developed specially the G3360 and G348 slotted gears to make microrobot applications easy. Having the encoder directly on the motor axis is possible, but one should check the maximum frequency of signals, according to the microcontroller program, since small motors spin very fast.


The H7001 has 2 power lines (3 to 5V) and 2 signal out lines, TTL and CMOS compatible. The light emitting LED needs an important current (20 to 100 mA), but this current can be switched on by the microcontroller 10 microseconds before reading the H7001 outputs, and switched off immediately after. The average power consumption is hence very low.


Examples of design

Other encoders

Several years ago, the SFH910 was sold with a 30mm 96 slots coding wheel. Several dual channels optical encoders exist (Hewlett Packard ). Hall encoders are available from Allegro, but it is not easy to get the magnets that will give precise square waves, if they are required.
Mechanical encoders are easy to get from many sources, but they have an important friction and bounces. Let us mention also the many industrial, fully packaged optical encoders, high resolution and a high price.

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Last update: 08.05.02/jc 26.01.03/us