I am building a weather station with an anemometer. I have an OPB917BOCZ photoswitch on-hand that I would like to use as the tachometer if possible. However, I can't seem to have any luck with it.

The device has five leads: anode, cathode, GND, Vcc, and output. From the diagram it looks like the anode and Vcc can be hooked together, same with GND and cathode. I tried the reverse as well (cathode and Vcc; anode and GND). I tried measuring the output with a multimeter, but I cannot see anything. My father suggested that it generates pulses and doesn't simply open a gate, so I tried measuring those pulses with a DSO but I don't see any pulses either.

He has used one of these devices successfully for a similar purpose, a tach for a spindle on a CNC lathe. He has it wired the same way I tried. Why can't I measure any output? I am getting pretty frustrated. Any suggestions?


  • \$\begingroup\$ how are you measuring the output? \$\endgroup\$ – jsotola Dec 29 '18 at 23:27

It's OK to connect Anode to V+ OR Cathode to 0V but if both are connected, it will not limit IR Diode current and burn out inside. ( Test with DMM in diode test mode for 1V or so with + on Anode)

The proper way to test is to use a series R in either Anode or cathode connection to V+ and 0V (respectively) and operate at 10mA and no more than 20mA using Ohm's Law such as (5V-Vf=3.8V) so 3.8V/380 ohm = 10mA. The LED has a wide (+40%) voltage tolerance of 1.3V typ to 1.8V max @ 20mA and reduces with current. So using 10mA target current is optimal target.

Also it may get wounded or damaged if reverse polarity >2V is applied.

Visible LEDs are similar except have a forward voltage drop that is greater 2V typ for R or Y and 3V typ for B or W.

http://www.mouser.com/ds/2/414/OPB917BZ-962256.pdf datasheet

  • \$\begingroup\$ I already had the datasheet and did not find it very helpful. I also don't understand what you mean by tolerance of 1.3-1.8 V. Does that mean I cannot use 5V between cathode and anode? Did I burn it up? \$\endgroup\$ – Riley Scott Jacob Dec 29 '18 at 22:50
  • \$\begingroup\$ I just don't understand why I am having such a hard time with this. I should put 5 V between Vcc and GND but only ~1.5 V between anode and cathode? I can do that, if that's what I should be doing. \$\endgroup\$ – Riley Scott Jacob Dec 29 '18 at 22:51
  • \$\begingroup\$ How can I specifically use the DMM to test if the IR LED is okay? \$\endgroup\$ – Riley Scott Jacob Dec 29 '18 at 22:52
  • \$\begingroup\$ correct. yup....Test with DMM in diode test mode for 1V or so with + on Anode and Overflow in reverse The Diode test reads voltage in a low current like 0.5mA \$\endgroup\$ – Sunnyskyguy EE75 Dec 29 '18 at 22:52
  • \$\begingroup\$ I described how you measure current using a 380 Ohm resistor with 5V to LED to R to 0V If you have no 380 nOhm but have 1k then using 2 in parallel for 500 Ohms in series and measure the voltage drop. Ohm's law says the current is V/R. But the LED has an internal resistance < 10 Ohms so the current will exceed 20mA when connected direct. then your anemometer slotted disk can interrupt the light to cause Vout got to V+ and pass the light and go to < 0.5V on output. \$\endgroup\$ – Sunnyskyguy EE75 Dec 29 '18 at 23:09

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