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I am trying to build an IR sensor module for line following robots, and I used QTRX-MD-06A module as my reference. The principle and PCB photos are attached here.

enter image description here Module PCB

However, the component with a circle mark that is beside 47K resistor on the PCB is something I never saw before. I don't think it is a zero ohm resistor, because if it is, it means the light emitting diode will be directly connected to +5V and GND without any current limiting resistor. It would burn the diode, wouldn't it?

Unknown principle

Would anyone please let me know what it is? How does it relate to the current source symbol in the principle photo? Thanks in advance!

[Answer]

It turns out, there is a dedicated LED driver IC connected to the cathode of the light emitting LED. I guess this IC controls the LED current, and thus there is no need to use a current limiting resistor. So that component is indeed a zero ohm resistor. enter image description here

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    \$\begingroup\$ it is a zero ohm resistor \$\endgroup\$ – jsotola Feb 1 at 5:20
  • \$\begingroup\$ related: electronics.stackexchange.com/questions/417930/… \$\endgroup\$ – Nick Alexeev Feb 1 at 5:44
  • \$\begingroup\$ Why don't you think it's a zero ohm resistor? That's exactly what it looks like. \$\endgroup\$ – rothloup Feb 1 at 6:00
  • \$\begingroup\$ The reason I don't think it is a zero ohm resistor is that if it is a zero ohm, it means the light emitting diode will be directly connected to +5V and GND without any current limiting resistor. Wouldn't it burn the diode? \$\endgroup\$ – roTor-roTor Feb 1 at 6:33
  • \$\begingroup\$ If you are dealing with electronics for robotics, you should have a DMM for sure. What kind if impedance does it show across the "o"-marked component ? \$\endgroup\$ – Ale..chenski Feb 1 at 6:59
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Based on your schematic, and the fact that you can tell the o and the 47k are in series by the zoomed in picture, I'd say it's a 0 ohm resistor.

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\$\begingroup\$

It turns out, there is a dedicated LED driver IC connected to the cathode of the light emitting LED. I guess this IC controls the LED current, and thus there is no need to use a current limiting resistor. So that component is indeed a zero ohm resistor. enter image description here

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