What can an RPi-Arduino combo do that an RPi cannot by itself?

Question

A couple of projects utilise the Arduino along with the RPi to do some applications (example). Can such projects be done with an RPi without the Arduino?

I have an RPi-3-B (with Raspbian installed) and wondering whether or not to get an Arduino too. I haven't worked with an Arduino before and so wondering if it would be worth the effort.

What are the reasons would one prefer interfacing their RPi with an Arduino?

• joan: You really need to ask on an item by item basis. There is an infinity of applications and each application needs to be looked at individually. My opinion is a very small percentage of applications would need an Arduino as well as a Pi.
• Milliways: This question has an almost infinite number of answers. You need to approach it from the other end, work out what you want to do then decide what you need to achieve the result.
• Ébe Isaac: @joan I just wanted to look at it from a technical aspect. What makes the Arduino so indispensable (if so) while the RPi provides much more.
• joan: In combination with the Pi an Arduino offers two extras. 1) analogue inputs. To get analogue inputs on the Pi you would need to buy an ADC. 2) more deterministic timing of an input event triggering an output. I.e. if an input changes state you can guarantee to set an output within a few microseconds. Such deterministic timing is very rarely needed by applications.
• Ébe Isaac: Thanks, @joan. It's similar to what Steve posted. So as long as analog signal and hard real time is not a concern, one can be fine with just the RPi, right?

Answer 1

The Arduino has a couple of advantages over the Pi:

1. analog input and output pins. The pi does not have any analog pins, so things like driving a servo or reading an analog sensor may require extra hardware (the Pi does have some PWM capabilities).
2. better realtime performance.
3. most Arduinos are 5 Volt tolerant, while the Pi is not (it uses 3.3V). This means connecting the Arduino to 5 Volt sensors etc. is easier.

Most of these limitations can be overcome with the proper choice of hardware (ADC's, DAC's, level shifters etc.).

You can read more about the differences and tradeoffs here, also this post has an interesting test to help you decide between them. This question and its comments are helpful as well.

• Ébe Isaac: So technically, you mean to say it is feasible to do RPi-Arduino projects without an Arduino, right?
• Steve Robillard: @ÉbeIsaac Not exactly it is possible to do most projects without an Arduino, but it may require more hardware (things like ADC, DAC, level shfters etc.). Projects that need real time support will be very difficult to do using the Pi alone (because of how the Linux scheduler works).