I recently started to work on encoder sensors and circuits.

I found one optical encoder sensor from vishay electronics that suits my requirement,

I found a circuit diagram online and it works like a charm ! but i have following questions about its working:

1 : Why do we need MOSFETS on the output terminals of sensor ? shouldn't we directlyget quadrature output without MOSFETS at points OUT1 and OUT2?

2 : Can we use BJT like BC547 instead of CJ2102 ? Wil it work ?

Attached : schematic enter image description here Thank you

  • \$\begingroup\$ @DKNguyen So are those MOSFETs Optional ? \$\endgroup\$
    – Mr.Sky
    Commented Nov 25, 2019 at 16:08
  • \$\begingroup\$ Yes................ \$\endgroup\$
    – DKNguyen
    Commented Nov 25, 2019 at 16:08
  • \$\begingroup\$ @DKNguyen I tried without the MOSFETS but circuit didn't work. i tied outputs to arduino and arduino didnt detect the quad output. \$\endgroup\$
    – Mr.Sky
    Commented Nov 25, 2019 at 16:10
  • \$\begingroup\$ You probably have some other problem them. You only need the stuff onthe left side, nothing on the right side. Unless you Arduino can't accept 5V (or whatever you are using to power the output). I would check alignment if this is a setup with a separate encoder disc...those can be really finicky. Also, decoupling caps. \$\endgroup\$
    – DKNguyen
    Commented Nov 25, 2019 at 16:14
  • \$\begingroup\$ Will do that ! thankyou @DKNguyen \$\endgroup\$
    – Mr.Sky
    Commented Nov 25, 2019 at 16:15

1 Answer 1


Q1 and Q2 MOSFETs are being used as a level translator. The way that type of level translator works is a little unusual compared to how MOSFETs are typically used but it works. If you don't need to do that then you don't need them (or even if you do, you can use other methods).

  • 1
    \$\begingroup\$ I should also add that this kind of level shifter is bidirectional. Not a good fit for this specific application. \$\endgroup\$
    – Maple
    Commented Nov 25, 2019 at 20:20

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