A digital circuit simulator with real 74HC chips
Mokxi is a free online digital circuit simulator that runs in your browser, with nothing to install and no account needed. A 4 Hz clock into two 74HC74 flip-flop chips, wired as a four-bit ripple counter, counting up in binary on four LEDs. Gates, flip-flops, counters, decoders and displays, as the chips you would buy.
- 455parts on the bench
- 43boards running now
- 1.00xreal time, on every board
What you can do here
In the circuit above, or in the editor it opens.
- Build counters, latches, decoders and adders from 74HC and CD4000 chips
- Wire real DIP packages with supply pins and propagation delays
- Use ideal gates and switches when you only want the truth table
- Clock a circuit from a clock part, a 555 or a button
- Watch any line on the logic analyzer and export it as CSV
- Go all the way to an 8-bit computer built from NAND gates
What the running circuit shows
Four flip-flops, each one dividing by two.
Each of the four D flip-flops has its D input tied to its own Q-bar, so every rising clock edge flips it. The clock drives the first stage, and each stage clocks the next one from its Q-bar, which makes the four outputs a binary number counting up. In the engine the LEDs step 0001, 0010, 0011 and on to 1111 a quarter of a second apart, then roll over to 0000: sixteen states, four seconds a lap.
Clocking each stage from Q instead of Q-bar makes it count down; that one wire is the whole difference. The carry ripples through the stages a few nanoseconds apiece, which is why real designs use a synchronous counter like the 74HC161 or 74HC193, both also on the bench.
The digital parts on the bench
Chips, not just symbols.
The 74HC series parts are the 14 and 16 pin packages: 74HC00, 02, 04, 08, 14, 32, 74, 86, the 138 decoder, the 161 and 193 counters, the 165 and 595 shift registers and the 4051 and 4067 multiplexers, plus the CD4017 decade counter and CD4511 display decoder. Each has a VCC and a GND pin that have to be wired, thresholds relative to its supply and a propagation delay from its datasheet, and its outputs float when it is unpowered. For truth tables before chips, ideal gates, bit inputs and outputs and a step clock are there too.
This page is about building whole digital circuits. For the gates on their own, with a truth table you press rather than read, the logic gate simulator is the place to start.
Questions
Is it free?
Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.
How is this different from the logic gate simulator?
The logic gate page is about single gates and their truth tables. This one is about circuits built from many chips: counters, registers, decoders and displays, clocked and running.
Can I simulate a 4-bit counter?
Yes. The circuit on this page is one, from two 74HC74s, and the bench also has the 74HC161 and the up/down 74HC193 counter chips.
Does it work on a Chromebook?
Yes. It runs in Chrome with nothing to install, which is how many school digital electronics classes use it.
More simulators
Each one opens on a circuit that is already running.
Build a digital circuit
Open the circuit, add a chip and press Run. Nothing to install, and no account needed.