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I was playing around with an obstacle avoiding sensor and want to share my experiences with you.
What is it.
An obstacle avoidance sensor is mostly used in robotics.
The sensor has an infrared led. This led sends out lightin the infrared spectrum which we can not see. the light normally shines streight on.
Next to the infrared led there is a photodiode. That photodiode detects infrared licht.
When the sensor approaches an obstacle the infrared light is reflected back and detected by the photodiode.
Like said, this is mostly used in robotics. The sensor is mounted on a robot and when the robot comes close to a wall or any other obstacle the photodiode sends a signal to the robots controller to alter direction.
The sensor
This is how the sensor looks in real life. Let's have a look at the different parts of it.
On the left you can see the photodiode at the top and the IR led beneath it. They are next to eachother so the there is a maximum chance of detecting any reflection.
In the middle there is a screw with witch you can adjust sensitivity. That means by turning the screw you can adjust the maximum distance at what the reflection is detected.
Next to the header pins there are two leds. The top led is the power led and that is always on whenever the sensor has power. The led at the bottom is off and goes on when an obstacle is detected.
A test setup
To test the sensor I build a simple setup on a breadboard. I connected the sensor to a Raspberry Pi Pico. Here is what the breadboard looks.
The led (I used a blue one) is connected with a current limiting resistor to GND and to GPIO15.
Pico's GND (pin38) is connected to the sensor's GND. and Pico's 5V (pin 40) is connected to VCC. The sensors output (OUT) is connected to GPIO16.
Test program in MicroPython
To test the sensor I wrote a simple program in MicroPython.
import time
from machine import Pin
obstac = Pin(16, Pin.IN)
led = Pin(15, Pin.OUT)
while True:
print(obstac.value())
led.value(not obstac.value())
#led.value(0)
time.sleep(.2)
In the loop the program constant tests the out pin of the sensor. When that pin's output changes from 1 to 0 (obstacle detected) the led on GPIO15 goes on.
Copy this program, paste it in Thonny and save it as main.py on your Pico. It will work on the Pico, Pico W and Pico2.
Saving it as main.py makes the program run immediately when the Pico is powered up so you can do some tests with a powerbank, phone charger or batteries.
If you want to learn about MicroPython on the Raspberry Pi Pico or Pico W please consider buying one of my books that are listed at the bottom of this page.
First test.
This should immediately work. Just hold your hand above the sensor and move it towards the sensor. At a certain moment the led will go on both on the sensor as well as the blue led.
Now you can adjust the screw in such a way that the leds will go on at your desired distance.
Distance
According to some specifications I found you can adjust the screw so the distance at which the led will go on can be set from 2 to 20 cm.
Just be aware that there are environmental issues that might influence the detection distance. Here are some things you might like to take into consideration.
- Temperature may affect the distance
- TL light might affect the distance
- Sun light might affect the distance
- Direct light shining on the photodiode might affect the distance
The only thing I want to say is that you should adjust the sensivity screw in your real-life setup.
Some tests
Here are the results of some tests I performed.
All leds are OFF because there is no obstacle in front of the sensor.
A shining transparant box was put in front of the sensor and at a distance of 8 cm the obstacle was detected.
A black piece of paper was put in front of the sensor and at 6cm it was still not detected.
The black paper was detected at 2.5 cm.
This means that the black paper absorbed (did not reflect) a lot of infrared light so it was not detected until it was at a short distance.
A white piece of printer paper was detected at 8 cm distance. The white color reflects the infrared light more so the paper was earlier detected.
A black shining object was detected at 4cm. This means that the black color absorbed some infrared light but the shing surface reflected also some infrared light.
A book (yes my book about the Pico) with a shining green cover reflected the infrared light at a larger distance so it was already detected at about 8.5cm
Water ???
Solid obstacles: OK
But how about fluids.
If you want to reproduce this test yourself just make sure the electronics don't get wet. Water and electronics don't mix and you might damage your sensor or pico beyond repair.
I filled a white bowl with a small layer of water and help the breadboard above it.
The led stayed off: no obstacle detected.
I used a bottle of water to gradually fill the bowl and yes !!!
At a certain moment the led went on. So water reflects the infrared light.
Concluding
First let me state that obstacles are detected very well.
In the demo program you can lower the delay and that will not affect the working of the sensor. So in real life you should use an interrupt for testing for an obstacle which is the fasted method.
The obstacle avoidance sensor detects obstacles but does not give any indication at what distance the obstacle is 'seen'. The difficulty lies in different materials reflecting the infrared light more or less.
If you need to measure distance then use an HC-SR04 ultrasonic sensor.
This means that you should adjsut the sensivity screw so that the obstacle is detected at a larger distance when you are working with a fast moving object like a remote controlled car. That is because you will need time to shut the motor down. But you also need to take the obstacles material in count as black obstacles are seen later.
I was specially impressed when I realised that the sensor also detected water as an obstacle.
I did not try but wonder if a black bowl would make a difference.
And remeber: you can always use this sensor inverted. Meaning that an alarm is given when an obstacle is removed like the lid of a box is taken off.
Till next time
Have fun
Luc Volders
