What value of resistor should I use with this reflective optical sensor

[gilperon]

Hi, the reflective optical sensor I am using is this:

https://www.vishay.com/docs/83760/tcrt5000.pdf

It is an infrared emitter and a sensor that collects the infrared. My question is this: I am a total beginner and I want to connect this sensor to my 5v supply. How do I know the resistor that I have to use to the infrared emitter and to the receiver? I know it's like a led, so I should take the voltage forward of the led and the current. But I dont know where to get this information in that link cause it has many numbers like forward current but also forward surge current... What value of resistor I should use in the emitter and what value in the receiver?

 

[barry]

Basically, you want the resistor large enough so that you don't draw enormous current when the transistor is on, and small enough so that the "dark"(off) current won't cause too much drop across the resistor. The data sheet says the sensor has a nominal output current of 1mA and a dark current of . Thus, if you were to use a 10K resistor, the dark current voltage drop would be insignificant (2mV) and the on current would be 0.5mA with a 5 volt supply. Obviously, different supply voltages might necessitate a different resistor. Also, look at figure 7; that shows you what the saturation voltage vs collector current (and emitter current) is

The resistor value might also affect the speed of the device, but the data sheet doesn't even talk about speed, so my guess is it's a fairly slow device.

 

[esp1]

hi
The datasheet states Emitter forward voltage drop max = 1.5V and Imax of 100mA.
The basic characteristic table says 60mA.

So for a 5V supply I would use Rser= [5v-1.5v]/0.05 = 70R, Use a preferred value of 82R.

Assume for the detector a collector current of 1mA.

E

 

[GrandAlf]

Presume you mean the series resistor for the led. I think 60mA DC is probably around the max. If you want to play around I suggest around 220R is a safe starting point no lower than 56R if you need more output. Transistor Emitter to ground, Resistor from collector to +5v 10K is a good starting point but not critical. Output to logic/amp is from collector/resistor junction (active low). Range probably a couple of mm or so.

 

[D.A.(Tony)Stewart]

It depends what you wish to detect and how you are going to sense the voltage. A pulse current gives you more sensitivity with higher current at low duty cycle, then you need a sample and hold or a peak detector to look if there is any current in the collector load resistor.

Using the specs... SENSOR Collector current VCE = 5 V, IF = 10 mA, D = 12 mm Ic = 1 mA
Vcc-Vf= 5-1V= 4V thus R=4V/10mA ~ 390 Ohms as closest value.
For Vce to drop below 0.7 , Vc=4.3/1mA = 4.3k or bump up to 4.7 or 10k.

You can also increase Rc to any value you want to extend the range or detect darker reflectors. IR contrast might be less than optical contrast. so target such as carbon or laser printed paper works well as well as Sharpie black stripe on a metallic rotating disk for detecting rotation.

YOu can increase drive current up to 100mA if you use pulse mode but try to keep average current = 10mA and rep rate >> 1kHz.




Low drive input and expecting low voltage out on collector means it is detecting a reflection with collector current. ( Low side logic tends to drive more current in most logic families but not all e.g. ALVC2 logic in ARM chips. )


You want a high signal to noise ratio or high reflectance light object to background IR light levels. But light level drops off with inverse squared path loss for both emission and diffused reflection, so path loss is 4th order with distance. Increase the collector R makes it more sensitive to low light and noise. Choosing the right value of the each resistor determines the equivalent threshold for object sensing and lower levels are more prone to noise, stray reflections etc.

First define what it is you want to detect. How big a gap and how reflective ( shiny to IR or white... or dull and dark )
Then when it is moving and how fast. The RC load time constant affects the response time.


• Operating range within > 20 % relative collector current: 0.2 mm to 15 mm
• Typical output current under test: IC = 1 mA

Say you are using 5V logic to drive the Cathode low with Anode to +5. The LED drops 1V so for

 

[gilperon]

Thank you so much! Your 3 explanation were very different in terms of results... I got more confused but I agree more with the sunnyskyguy cause his values are in the datasheet. Is there some reason for your explanation being so different?

 

[D.A.(Tony)Stewart]

I have designed and produced many opto products and extensively tested the reliability. e.g. reflective for Automated Meter Reading (AMR) of elect. meter disk rotation and Interruption type for automated bottom of the basket (BOB) shopping cart scanners. My design approach is from basic specs and then explore the bleeding edge.

My favourite was using IRDA transceivers in a 1 meter path with 5mm deep slot apertures that could detect something as small as a resistor wire crossing the light beam path, not using lasers.


Each person has a different set of experiences and Optos have wide variation in performance.

Consider it like a simple transistor with low beta ( less than 1)