Like many other instructables I start with "My story", or "Motivation for project", and next I will present real introduction.
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.What to use? Nmos (pmos) transistors, or usual pnp (or npn) TTL type? Our choice is npn, since nmos and pmos transistors are not implemented (you can put them on breadboard, but can not apply simulation - not implemented yet). Easier way is to find solution with transistor gates circuits to implement AND gate (Url of previous link is: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html).
We shall demonstrate implementation of NOR gate circuit on our breadboard, when implement RS latch (next step).
Step 2: RS latch.
RS latch circuit diagram is presented on first figure. On second figure it is demonstrate how this is implemented on circuit.io breadboard. Here http://123d.circuits.io/circuits/91852-rs-latch you can try functionality. Not working correctly, but obviously this is problem of circuit.io simulator. Really, the circuit is symmetric, and so after release the right button there is no reason circuit to go to previous state (before push right button). Unfortunately in the end of our presentation we can not test functionality.
Step 3: Main circuit idea.
Up to now we talk about functionality of some integrated circuit - RS latch. Now we shall present general idea of circuit.
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.
Step 4: All circuit on breadboard.
On figure it is demonstrated final solution. You can fork this solution in circuits.io here (URL: http://123d.circuits.io/circuits/92775-4-rs-latch-8-led-dis-comp). If you want to do some real test, then you can use this solution. Here RS latches are emulated by additional Arduino module.