The NOR:

I need to search some theory about Flip-Flop for the first step:

This is the first model you see on Google/Wiki if you search Flip-Flop SR.

The true table: https://goo.gl/w9dPSh

It's pretty easy to understand, only 2 Nor, and very fast to build, if you want, with IC 7402.

Online simulator: http://tinyurl.com/yabdpq28

The NOR with the transistors:

In this project we want to use only transistors and resistors component, and we need to go a little bit deeper inside the Nor component.

This "clickable" image is the base of what is inside the NOR component:

We can play with this schematic on the simulator: http://tinyurl.com/y7753y8x

Now we could simplified the schematic in this way: https://goo.gl/RPZZFU

Perfect, the last - simplified - schematic is a good point to start and grow up the project.

The next steps for the projects are:

• connect the RAMPS shield to the Flip-Flop
• connect the Relay shield to the Flip-Flop

Notes:

Key note is that: the circuit is symmetric.

• What happen if i change the top side resistors? what is change in the output?
• Moreover, when the transistor is in Saturation region? This condition is define by which resistors?