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I'm trying to make a sensor to detect that a person is within 50cm to trigger a function. But when I use the HR-SR04 sensor, it works accurately on flat surfaces (e.g. walls, books) with a margin of error around 5cm, but not on uneven surfaces (e.g. humans) where the value randomly jumps between 0cm to 1000cm.
I was thinking whether using an IR distance sensor would be better.
That depends on your requirements. An IR distance sensor will work better with different materials, since IR light is mostly reflected in very broad angles (expect for very reflective materials, like mirrors), so that uneven surfaces are also detected. But you will get a significantly worse error margin in your 50cm range. Normal IR distance sensors are just an LED and an IR photo transistor, sometime with some extra components. So the LED is lighting up and the photo transistor is measuring, how much of the IR light is coming back. The IR sensor depends also on the reflectiveness of the materials and the level of surounding IR light.
But as you don't really need to measure the distance, but just want to know, if there is an object in the range, an IR sensor might be enough. You would need to change the threshold of your IR sensor according to your needs (by experimenting with it).
All in all I think, yes, an IR sensor can give you better results for your application, though not for really measuring the distance, but only for the presence in a range with rather big error margin.
I have found IR accuracy to be poor resolution (+/- a few centimeters), as it's highly dependent on the refectivity of the surface. It also frequently has very poor range (a few cm to tens of cm), depending on the emitting power of the illumination source.
I have found ultrasonics, however, even the cheap $0.99 ones you mention, to have much better resolution (+/- mm), and much better range (about 2m), depending on the code driving them. I have yet to see good non-blocking code in any library for one of these, but when I write my own non-blocking code with interrupts and basic digital signal processing, such as a median filter, I get excellent results.
So, I recommend you use ultrasonics, but it requires advanced knowledge to do it well and do it right, such as using interrupts and taking the median of every 3 to 5 samples to remove spurious results.
On very flat surfaces, such as walls, at angles 45 deg or larger, ultrasonics also do poorly, since they end up measuring the distance including the extra reflections off the walls, but contrary to what you experienced, on odd shaped or softer objects, such as humans, I find them to do much better. I have used ultrasonics as the means of detecting humans several times before with good results. The real secret/challenge is your code. And again, I have yet to find a library worth using, and good code on these cheap things makes a HUGE difference in results and reliability and accuracy!
