Free: Simulate a Circuit from a Photo of Its Schematic
There is a circuit in front of you — in a textbook, on a lecture slide, on a whiteboard after class, on page 14 of a datasheet — and you want to know what it does. Not read about what it does. Watch the voltages.
Between you and that is twenty minutes of redrawing. Every tool that can simulate the circuit first makes you rebuild it: place the source, place each resistor, type each value, wire every node. The circuit already exists; you are re-entering it by hand so that software will look at it.
You can skip that step.
What actually happens
Attach the photo in the chat box. The schematic is read — components, how they connect, and the printed values next to them — and the result is rebuilt on the canvas as a real circuit you can edit and simulate. Press Run and ngspice solves it.
The important part is what comes out: not a picture, and not a description of the circuit. An actual netlist. That is the difference between a tool that recognises a schematic and one that runs it.
* read from a photograph
V1 1 0 5V
R1 1 2 1k
R2 2 0 2k
.endThat netlist is not just an intermediate step you never see. It sits in a panel
below the canvas with a Copy button, in standard SPICE. Paste it into LTspice or
ngspice and it runs there too — which makes photographing a schematic a fast way
to get a netlist for a tool that cannot read images itself. Nothing here is
proprietary, and a circuit you bring in this way is not stuck here.
Once it is on the canvas it behaves like anything else you drew yourself. Change a resistor and re-run. Attach a voltmeter. Ask for a change in plain language — "make R2 10k and tell me the new output" — and the circuit updates.
Why this is not just OCR
Reading text from an image is a solved problem. Reading a circuit is not, and it is worth being precise about the difference.
A schematic encodes three things at once. Symbols: a zigzag is a resistor, two parallel plates are a capacitor, a circle with a sine wave is an AC source and a circle with plus and minus is a DC one. Topology: which components share a node, where wires cross without connecting, where a junction dot means they do. Values: 4.7k next to one resistor, 220 next to another, and which label belongs to which part.
Get the symbols right and the topology wrong and you have a different circuit that happens to use the same parts. Get the topology right and miss a value and it will not simulate. All three have to survive together, and they have to come out in a form a solver accepts.
Where it is reliable, and where it is not
Being honest about this is more useful than a list of features.
It reads clean schematics well. Printed diagrams from books, exported images, screenshots, and tidy whiteboard drawings come through with the components and values intact — including circuits with half a dozen parts.
Symbols that look alike get confused. The most common error is an AC source read as a DC one, since the difference is a small squiggle inside a circle. Check the source type before trusting a transient run.
A value the camera cannot resolve becomes a guess. If "4.7k" is a smudge in the photo, something plausible appears instead. Fill the frame with the schematic and keep it in focus; a sharp photo of just the circuit beats a high-resolution photo of the whole page.
Check the result. It lands on an editable canvas precisely so you can correct it. Two seconds comparing the netlist against the picture is worth more than any accuracy claim on this page.
What it costs
Nothing. It runs in the browser, there is no install, no licence key, and no
account is needed to try it — which matters if you are on a lab machine you
cannot install software on.
That is worth stating plainly because the alternatives are not free. Paying
someone on a freelancing site to trace a schematic runs about $10 per circuit
and comes back hours later. The paid simulators do not read images at all, so
you are buying a schematic editor and still doing the redrawing yourself. The
free simulators also make you redraw it.
Signed-in accounts get a higher daily allowance for images than anonymous ones,
and there is a shared daily ceiling so a busy day cannot run the bill away —
reading a schematic costs real tokens even though it costs you nothing. If you
hit it, describing the circuit in words has no such limit.
What it is actually for
The obvious use is homework, and it is a real one: photograph the circuit in the problem set, simulate it, see whether your calculated answer matches.
The one we did not expect is debugging someone else's design. A datasheet application circuit, a schematic in a forum post, a diagram in a paper — things you want to interrogate rather than build. Simulating a circuit is a much faster way to understand it than reading about it, and until now the cost of entry was retyping it.
For teaching, it changes what a whiteboard is. Draw a circuit in front of the class, photograph it, and simulate it in the same minute — the drawing and the working circuit stop being separate artefacts.
Try it
Open the editor, press 🖼 next to the chat box, and attach a photo of any schematic. Free, in the browser, with nothing to install and no account needed to start.
If you would rather describe your circuit in words, that works too and in any language — see how to simulate SPICE circuits online. If a simulation refuses to solve, why SPICE simulations fail covers the usual causes.