Breadboard is an iOS app which models a breadboard, with wires, LEDs, and resistors. It's pretty inaccurate, but things which are possible include:
- Components can be placed, and then moved (or removed by dragging an end to an invalid position).
- Plugs light up to indicate if they're connected to ground or power.
- LEDs light up if you connect them correctly.
- LEDs are dimmer if you connect them through more resistors, and die if you connect them through none.
- LEDs must be connected with the right polarity to work (drag from ground to power).
To illustrate these features, this is a screenshot of a setup with dim, bright, dead, and unconnected LEDs: