OhmLens

At a glance

OhmLens is a free browser course that teaches electronics through live circuit simulations. It runs 19 lessons in seven modules, starting with what a volt is and ending with datasheets and sizing PCB copper tracks. Each lesson is a working circuit. When you move a slider, the voltage, current and heat readouts update, and a part pushed too far burns out and can be swapped for a new one.

Good fit

It suits beginners, Arduino or Raspberry Pi makers, students who want a second explanation alongside a course, and software people curious about hardware. It is most useful if formulas only make sense once you can watch them work. The later modules cover MOSFET switching, op-amps, pull-ups and debounce, I2C and PWM on a scope, and LDO versus buck regulators.

Consider something else when

Skip it if you need accurate models for designing real hardware. The author says the simulations are deliberately simplified textbook formulas, meant to build intuition, and that you should check the real datasheet before building anything. If you already work comfortably with op-amps, switching regulators and datasheets, most of the 19 lessons will cover ground you know.

Start here

Open lesson 0.1, Your First Circuit. Click switch S1 to stop and start the current, drag the resistor slider and hover over any wire to see its live voltage and current. Then try the lab challenge: light the LED from the 9 V battery at 5 to 15 mA without burning anything, and switch it off and on once with S1.

Cost and requirements

The site describes itself as free, running in the browser and needing no sign-up. Progress is stored only in your own browser and can be reset, so it may not follow you to another device. The site has 13 free calculators, including LED resistor, voltage divider, 555 timer and IPC-2221 trace width. They accept shorthand such as 4.7k or 100n and show the working.

Limits to know

OhmLens is a one-person project. Its author is a cloud architect who designed his first PCB this year, and he says he checks the formulas by hand. He also says AI tools helped write the lesson text, which he reviewed himself. According to the about page, the site sets no cookies and shows no ads, and it counts page views with GoatCounter, which records totals but no IP addresses.

Still unclear

• Whether the drag-and-hover lesson controls work well on a phone or tablet touchscreen.