You should not think of them as low outputs in the sense of voltage, but of active pull down signals. The undriven state of a pi output is high impedance. If the pi is not powered then the output will not be driven low, it will just be high impedance, so any connected relay will not be activated.
Thanks for your hints.
A contactor switch or tyristor with zero crossing activation is used for motors with power.
I noticed that 220AC is not suitable as an example.
Therefore I change the example to DC 24V.
We don't want to lose any Node-Red users.
For very expensive and big motors this electronic motor protection relay is used as example:
You turn the motor on/of over RS485 or Ethernet.
Fair enough, it's just I've seen way too many horrors in wiring here at work, like machines that are working but do not match the wiring diagrams.
Then you start tracing cables and lose a big amount of sanity just by wondering how is it possible that someone would wire anything like that, and even more by wondering how is it possible that the machine is even working while wired like that.
And of course, you end up pulling all the cables out and rewiring the whole thing, because safety.
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We have similar (simpler) motor drivers at work, but the idea is similar. You just tell it if it's on or off, and write some variables like the frequency and the rotation direction, and the driver does the rest. It also has a high impedance feedback resistor in case the motor has to stop quickly, it will use the motor to generate power and dissipate through the resistor, so it will brake quite fast (not as much as an emergency brake, but these are for air and fluid pumps, so instant braking is not strictly necessary).
Just had a few spare moments to think about the guide you are writing.
If you are looking for a standalone solution, then you could use an ATtiny85A and a SSR and some resistors to create an active-high pull-up circuit for the buttons. The ATtiny is about the same (low) price as the ULN2803A (not be needed anymore). An OLED panel could be used to show the status of the motor.
A very quick sketch of a simple (untested) circuit.
Another idea is to use a Wemos D1 Mini (flashed with ESPeasy or Tasmota) and a SSR, again with some resistors. An OLED panel could be added to show the status of the motor. As the Wemos has WiFi capability and supports MQTT it could be programmed to send the push-button actions to Node-RED via MQTT. You could even send MQTT commands from Node-RED to the Wemos to operate the motor.
These suggestions make use of inexpensive microcontrollers which are a lot cheaper than using a RPi.
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not to disagree with others @dynamicdave is correct on using a SSR, however from using this exact ssr in my systems it tends to fail quicker then other SSR's , when it heats up from "proper use" aka within its parameters, and you remove the low voltage to turn off , it may stay on. I switched to a different brand that I have not had issues with. They also come with a heatsink much needed for anything needing more then 10 amps of 110volts.
https://www.amazon.com/gp/product/B09GLRRGLT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Hi Dave @dynamicdave ,
interesting circuit.
Also another solution.
As I mentioned earlier, my goal is to make
explain controls with different solutions.
The solution also includes PLC Programmable logic controller - Wikipedia
Ok many users will not have much to do with PLC.
I think that the simple basics of PLC are interesting for the technically interested.
If you have understood the principle, you can e.g
write a control program in Python on the Raspberry Pi or C++ on the Arduino.
I will give examples of this.
In the examples are also two solutions with Node-Red.
I look forward to reading the "guide" - sounds like a very useful document.
If you watch the video @iiot2k posted above it seems these SSRs are only good for around 10A not 25A
For small motors ok.
10A with inductive load and without zero crossing circuit.
And these motors have the property of drawing a lot of current when starting.
I can already see the smoke rising. 
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when using motors or heating elements. a "contactor" is better used.
I actually use low voltage to control a SSR, the SSR is then powered up controlling the Contactor.
this will be best used, the amperage running through the SSR is minimal.
and keeps the higher amps isolated to the contactor.
I'm not good at drawing schematics, but that is how i control my 3 phase heating element in my HLT kettle at brewery.
Not sure if these would be any use?
We built some pump nodes a long time ago.
They re old and don't depend on any libraries as such.
The documentation and node name could do with some serious updating.
smithtek-node-red-duty-standby
Remember to always use hardware protection and if possible feedback through safety protection relays (contacts) to IO pins, that way you get the best of both worlds.
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