LogicSim alternative: logic in the browser, from transistors up
LogicSim is a free, open-source logic simulator written by Andreas Tetzl, who started it on the Amiga in 1995 and released the Java version 2.4 in 2009 under the GPL. It has AND, NAND, OR, NOR, XOR, equivalence and NOT gates, RS, D, T and JK flip-flops, timers, switches and number inputs, LEDs and decimal or hex displays, and you can save a circuit as a module to reuse in another, with manuals in English and German and translations into several more languages. In 2020 Peter Gabriel reprogrammed it as LogicSim3 (on GitHub, GPL-3.0, still Java), with a new XML file format that LogicSim2 files cannot be opened in. Several unrelated projects share the name, such as small gate simulators on itch.io. If you want something like LogicSim without installing Java, digiwleea runs in the browser and starts one level lower, at the transistor.
What LogicSim is good at
LogicSim is light, free and straightforward: drop gates, flip-flops and displays on a sheet, wire them, and watch the LEDs. Modules let you build up from gates to adders and counters (its 2.4 release added BCD adder and counter circuits), and tutorials have been built around it, such as the digital logic tutorial on techref.massmind.org that its author links to. (digiwleea is an independent project, not affiliated with LogicSim, Andreas Tetzl or Peter Gabriel.)
How digiwleea is different
digiwleea needs no Java: it runs in any modern browser, phones included. Instead of starting from ready-made gates, you build the gates themselves from CMOS transistors, then adders, latches, registers, RAM and an 8-bit CPU that you program in assembly, with every part checked against its truth table, a 40+ level course and an AI tutor (Wleea). LogicSim offers more ready-made parts from the start, such as timers and a monoflop; in digiwleea most parts are things you build yourself and save to your library.
| LogicSim | digiwleea | |
|---|---|---|
| Runs on | Java on the desktop (LogicSim 2.4 needs Java 1.4 or later) | Any modern browser, no install |
| Smallest building block | Ready-made gates and flip-flops | CMOS transistors |
| Reusing a circuit | Save it as a module | Save it as a library part |
| Guided course | No; manuals in several languages | Yes, 40+ levels up to an 8-bit CPU |
| Price and license | Free, GPL | Free lab and Act I; Premium for the full course |
Which LogicSim?
This page compares Andreas Tetzl's LogicSim (version 2.4, on tetzl.de) and Peter Gabriel's LogicSim3 (on GitHub), the best-known program with the name. If you were looking for Sebastian Lague's Digital Logic Sim, which many people shorten to "logic sim", see Digital Logic Sim online; for the similarly named Logisim, see Logisim online.
LogicSim parts and where to build them here
- Gates, including equivalence (XNOR). NAND and NOR from transistors, and every gate's truth table live.
- RS, D, T and JK flip-flops. The SR latch, D flip-flop, T flip-flop and JK flip-flop.
- Clock generator and counters. A synchronous counter, a frequency divider and a BCD (decade) counter.
- BCD adder and 7-segment display. The BCD adder and the BCD to 7-segment decoder.
No sign-up is needed to start: open the lab and place your first transistor, or read the free theory course. The account (and the AI tutor) are optional.
See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.
Open the digiwleea lab →Frequently asked
Is LogicSim free?
Yes. Andreas Tetzl's LogicSim 2.4 is free software under the GNU General Public License, and Peter Gabriel's LogicSim3 is GPL-3.0. Both are Java programs you download and run.
Does LogicSim run in a browser?
LogicSim 2.x could run as a Java applet in a web page (that arrived in 2007), but current browsers no longer run Java applets, so today it runs as a desktop Java program. digiwleea runs in the browser with nothing to install.
What is the difference between LogicSim 2 and LogicSim3?
LogicSim3 is a 2020 rewrite by Peter Gabriel of Andreas Tetzl's LogicSim, whose last version was 2.4. Its README says the code is nearly entirely new and circuits are stored in a new XML format, so files from LogicSim2 cannot be used with LogicSim3.
Is there a LogicSim alternative that needs no Java?
Yes: browser-based simulators such as digiwleea run without Java or any install, on desktop and mobile, and digiwleea adds a guided course from transistors to a CPU.