CRT5000 sensor works with infrared sensors, a biography of a sense. TCRT5000 with an infrared emission control and an infrared receiver tube the receiver tube resistance will change after the launch tube infrared signal receiver tube receives the reflectedthe form of a voltage change in the circuit generally reflected, and after the output results obtained after the treatment after the ADC conversion, or LM324 circuit shaping take on the change in resistance from the received infrared signal strength of the receiver tube, often show the reflecting surfacecolor and the reflective surface of the receiver tube from two second aspect
By the way all I am going to be doing in this is working through this document which the kind folks at Vishay have provided – it just outlines all the uses of the sensor, its actually pretty good and well worth reading.
The first thing you need to know is your reflection factor- this is how well the material you are going to use is going to reflect, for my application I am going to use matt black and matt white so I can select the 2 reflection factors from the table provided (if you aren’t using either of these then they provide a big list of other reflections so check that out).
The table doesn’t mention matt black card but from looking at the other values I am assuming the perfect matt black would have 0% reflection. I know that I am not going to be using perfect materials and so I will assume my reflections to be as follows:
White: 90% – 100%
Black: 0% – 10%
The datasheet says that the optimum current for the LED is going to between 20mA and 40mA however if you are planning to use the sensor long term then 40mA can damage it. However for my application this robot is not going to be running for 1000’s of hours so I am going to go all out of 40mA.
So from looking at the circuit diagram it is obvious that RS is going to be 5/40*10^-3 = 125Ohms.
From the above graph you can see that the actual current measured at a distance of 3mm is going to be nearly perfectly the available current but as you move further away the ratio at 6mm for example goes down to about 0.6 of the maximum transistor current.
For our application then with a 6mm max distance the collector current we can expect will go as low as 0.6 of the amount calculated.
To calculate IC max you have to take into account 2 factors LED current and the coupling factor k.
The coupling factor is the ratio of LED current to phototransistor current and it will be different for all materials. The diagram below shows the maximum k value for a current of 30mA on a 90% reflective material (which is pretty much what we are expecting for our value for white card)
It shows that the k value is 6% for this material.
So our IC max value for the white is going to be 40mA * 6% which comes out at 2.4mA but remember this is in the ideal situation at the ideal distance, we learned that the current will drop to 0.6 of its value at 6mm and so our minimum current will be 1.44mA.
to calculate the black k value we multiply the black reflection by the 6% too which gives us 10% * 6% = 0.6%
and then 40mA * 0.6% = 240uA.
I am not an expert at this but from experimentation these values have worked, I may have made a mistake I am only a student after all so don’t take this as a technical document.
So now we know how much current is going to be going through the transistor we now need to make that work for the Schmitt trigger I am going to interface with the microcontroller.
Schmitt triggers work by going high at a certain voltage and then not going low again until a lower voltage is crossed. For this project I am using the 74HCT series of Schmitt triggers and they go high at about 2.5-3V and don’t go low again until they cross about 0.5-0.8V.
To be safe I am going to set my limits as 0.5V and 3V which should definitely make sure the transistor switches on the trigger.
so on the low end RE needs to be lower than 0.5/black current = 240uA and on the high end it needs to be greater than 3V/white current = 2.4mA
so RE needs to be somewhere between 2.083Kohm and 1250Kohm.
So I am going to select a value of RE to be about 1.8 Kohm.
Remember that these values are for perfect materials, What I suggest you do is put the 125ohm resistor on the LED but then put a 15-20kOhm potentiometer in RE as I have found that once I have calculated everything by the time I get it on to the real track I need to boost RE by as much as 10 times to get the Schmitt triggers to fire. So just having a pot is nice to tweak the values.