How to use a reflective optical sensor with Raspberry Pi.

A reflective optical sensor is a sensor that irradiates infrared light onto an object and detects the light reflected from it.

This article shows how to use a reflective optical sensor with Raspberry Pi.

Parts required to use a reflective optical sensor

In the beginning, I show parts required to use a reflective optical sensor.

reflective optical sensor

Breadboard

This breadboard is a best seller on Amazon.

Jumper wire

This product is a set of male-male, male-female, and female-female.

Resistor

This product is amazon's choice.

sponsored link

Calculation of resistance value

I show how to calculate the resistance value to be connected to a reflective optical sensor.

Datasheet of a reflective optical sensor (RPR-220)

A datasheet of a reflective optical sensor is required to calculate the resistance value.

The data sheet can be downloaded here.

Circuit diagram of a reflective optical sensor

This is a schematic of the reflective optical sensor, the power supply, and the resistor.

From this schematic and the current constraints on the GPIO pins of the Raspberry Pi, the value of the resistor is calculated.

the current constraints on the GPIO pins of the Raspberry Pi.

Calculation of resistance value A

From the data sheet, the VF of the LED is 1.34V.

So, 1.96V ( = 3.3V - 1.34V ) is applied to resistance A.

Maximum current per GPIO 1 pin is 16mA.

It is only a maximum value, so it should be 10 mA or less with a margin of error.

From Ohm's Law

R = V / I = 1.96[V] / 0.01[A] = 196[Ω]

When the resistance is 196 Ω or higher, the current is 10 mA or less.

I used a 330 ohm resistor that I had.

With a 330Ω resistor, the current value is 6mA.

Calculation of resistance value B

To calculate the value of resistor B, the Low / High threshold voltage of the GPIO pin of the Raspberry Pi must be considered.

From the Low / High threshold voltages of the GPIO pins, two equations can be established.

1.3[V] < Ic min × R ・・・Equation 1

0.8[V] > Iceo max × R ・・・Equation 2

Ic : current of collector, Iceo : dark current, R : resistor value

Equation 1 : This condition is recognized as High on Raspberry Pi even when the current of collector "Ic" is minimum.

Equation 2 : This condition is recognized as Low on Raspberry Pi even when the dark current "Iceo" is maximum.

From these two equations, the resistance R is calculated.

According to the data sheet, Ic min is 0.08mA.

However, since the value of "Ic min" are based on the IF=10mA condition, it is necessary to recalculate "Ic min" to match the current conditions.

Under the present condition, IF is 6mA.

Ic min = 0.08[mA] × (6[mA] / 10[mA]) = 0.048mA

According to the data sheet, Iceo max is 0.5uA.

Based on the above conditions, R is as follows.

27[kΩ] < R < 1.6 [MΩ]

I used a 47kΩ resistor that I had.

sponsored link

Connection of each part

I show how to connect each part.

GPIO and the 40-pin Header on Raspberry Pi 4

Here is the pinout of the Raspberry Pi.

quotation : Raspberry Pi Documentation

Connection of each part

This is a schematic of the reflective optical sensor and Raspberry Pi.

A reflective optical sensor on the schematic is replaced by two LEDs.
( There is no part of reflective optical sensor on "Fritzing". )

On this schematic, white LED is "LED" and gray LED is "phototransistor".

Phototransistor is input to GPIO 17 (pin 11).

Picture of breadboard after wiring

sponsored link

Python sample code

Every second, the output of reflective optical sensor is read at the GPIO pin.

Press "ctrl" + "C" simultaneously to stop.

### sample code

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM) #Specified by GPIO number
#GPIO.setmode(GPIO.BOARD) #Specified by Pin number

gpio_sensor = 17 #Specified by GPIO number
GPIO.setup(gpio_sensor,GPIO.IN) #Specified by "in mode"

time_start = time.time()

try :
    while True :

        ### ------------------- get_time ----------------- ###
        time_end = time.time()
        time_elapsed = time_end - time_start

        ### ------------------- GPIO input ----------------- ###
        input_sensor = GPIO.input(gpio_sensor)

        print('Time : {:.1f}s, GPIO_input : {}'.format(time_elapsed, input_sensor))
        print("")

        time.sleep(1)
except KeyboardInterrupt:
    print("\nCtrl+C")
    GPIO.cleanup(gpio_sensor)

This video shows the output of reflective optical sensor is acquired by Raspberry Pi.

sponsored link