I have small experience with circuits.io, but already like this service. I decide to start one mine digital electronics project. Unfortunately it seems impossible, since circuits.io is under development. There is no integrated circuits, which can be simulated in breadboard. I decide to no wait implementation of such functionality, since I can give some ideas on development of circuits.io.
Aim of this project is to control 8 LEDs with Arduino, but only use 6 output wires. Using logic integrated circuits we can memorize states of some LEDs. Good solution is shift register like 100341. In this case instead of 6 can use only 2 output wires to controll all LEDs. First wire is for data, second is to apply shift step.
Unfortunately in circuits.io there is no integrated circuits, which can be used in breadboard simulation. That is why we shall implement simple integrated circuits, using transistors. We shall implement a 4 RS latch, and something like 4 AND gates.
Step 1: Implement And/or Gate.
We shall demonstrate implementation of NOR gate circuit on our breadboard, when implement RS latch (next step).
Step 2: RS Latch.
Step 3: Main Circuit Idea.
We plan with one data wire to set state of 2 LEDs. Method is as follow. First memorize (in RS latch) state of one LED, and next charge another. Second LED is connected directly to wire, while the first one is set preliminary.
On figure it is presented the idea. Wires ENABLE and RESET are common for all groups of 2 LED-s (i.e. all 8 LEDs). In the future, when we modify presented here solution (to use only digital circuits) instead of transistor we can use AND gate.