# Ri and Ro - What do these acronyms mean?

I am trying to make sense of the below circuit diagram.

What do Ri and Ro mean?

What purpose do the resistors after the Pinouts of Pin 3 and Pin 2 serve?

This is the datasheet for the sensor: http://www.ge-mcs.com/download/turbidity/920-480B-LR.pdf. It says its rated current is max. 30mA. In order to test the current in the circuit I built for this sensor (which you can see at this question: https://electronics.stackexchange.com/questions/207096/help-with-turbidity-sensor-circuit-and-reading-voltage), I would just use a multimeter between my Vcc source and Pin4 to measure the current?

I was also trying to see if I accidentally fried the sensor somehow. I used a digital camera and pointed it at the photo TR, but I could not see anything (this was to see if there was any IR light coming from the photo TR). I tested the camera first with a TV Remote to see if I could see IR light coming from the remote, and I could. I am not 100% sure if it is an IR LED on Pin 3, however. Are there any other ways to see if I fried the sensor?

• Probably "input resistor" and "output resistor". Commented Dec 21, 2015 at 23:19

Ri might mean "Resistor, input". In any case, it is the current limiting resistor for the LED.

Ro is the output load resistor - the current passed by the phototransistor detector will develope the output volotage across this resistor.

• Thanks. But how is Ri the current limiting resistor for the LED? In the circuit Pin 4 is supplying Vcc (5V) and is directly connected to the photo TR. Ri comes after Pin 3. So how could Ri limit the current to the LED if it comes after the LED in the circuit? Commented Dec 21, 2015 at 23:50
• Prof. Kirchoff tells me that the current is the same at all points in a simple series circuit. Ri and the LED are in series, so both must pass the same current. Ri will control that current, whether it is "before" or "after" the LED (regardless of which way you think current flows.) Commented Dec 21, 2015 at 23:55

Your question seem simple, clear and straight foreward. You just want to know what the abbreviated term of RO and RI with respect to resistor mean and nothing more.

RO according to goolge search with respect to resistors ( circuit resistors) refer to "resistor output" of whatever the instrument or device happen to be, that is all. From that I guess that this resistor is placed after the particular instrument as current passed through it and whereever it goes.

And RI is the opposite naturally, in other words "resistor input" of the device. This I assume mean that before current gets to the device, incoming that is, it has to pass through this resistor.

This is my conclusion. I could be totally wrong.

[This remark is not related to the question but I will throw it in: There is a given knowledge within the serious students that there is a voltage drop across a resistor no matter where you find them and that goes with it so to speak. It is just an extra info, aside from other effect which can be stated in other ways.]