Logisim is a logic simulator which can be used to build logic circuits. We'll use it to build and experiment with digital logic.
To use Logisim, we connect various components together and observe how they operate.
You can find transistors, power, and ground tools under the "Wiring" menu on the left hand side of Logisim:
To place a component on a circuit, click on it, and then click on the area on the right. The placed transistor looks like this:
The transistor has three pins which are places wires can be connected. On the left is the "collector" which is usually connected to a power source. On the right is the "emitter" which is the output signal of the transistor. On top is the "base" which controls if the transistor is on or off.
Components such as transistors have various attributes that can be changed on the left. Transistors can be switched from p-type to n-type, and rotated.
Logisim also has components which will allow us to interact with our design. In the "Input/Output" section, you will find Buttons and LEDs:
LEDS have one input pin which indicates if the LED will be lit or not. Buttons have one output pin which indicate if the button is currently pressed or not.
Logisim is somewhat more abstract than building a real circuit. For a switch to produce a current, it would have to be connected to a voltage source. Logisim buttons have a hidden voltage all ready to go. Likewise, the LEDS would have to be grounded to light.
With all of the power/ground wires a large circuit would need it would get quite messy!
In order to connect two components, they must be wired together. Use the Wiring Tool under the "Base" menu:
Click on the starting pin, and then click on the ending pin. This will draw a wire between them. You will often want to make several smaller wire segments to rout wires around other components.
You should use Logisim to recreate the telegraph relay circuit from "Code":
Instead of the telegraph switch for input, you should use a Button. Instead of the relay station, you should use a transistor which serves the same purpose. Instead of the telegraph sounder, you should use an LED.
When you're done, the LED should be unlit until the button is pressed, which should light the LED.
You can press the button by choosing the "Poke Tool" from the Base menu.
Copyright © 2018 Ian Finlayson | Licensed under a Creative Commons Attribution 4.0 International License.