The notebook, and the build log

The notebook is the one habit on this site that I would keep if I had to drop everything else. Not because anyone will grade it. Because in six weeks you will not remember which gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry resistor you used, and in six months you will not remember why the noise floorNoise floor What your instrument records when the input is shorted or connected to a perfectly quiet source; everything smaller than this is invisible to you. Related: Thermal (Johnson) noiseThermal (Johnson) noise The random voltage every resistor produces just by being warm; about 13 nanovolts per root hertz for 10 kilohms, which matters at microvolt levels. , CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry dropped between Tuesday and Thursday, and the plot you saved will have no axes on it.

What goes in

Every session, before you touch anything: the date, what you intend to do, and what you expect to see. That last line is the important one. Writing down the prediction is what turns fiddling into an experiment.

During the session: what you actually did, with part numbers and values. “GainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry resistor 500 Ω, so gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry ≈ 100.” “ElectrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry at O1 and left earlobe, Ten20, DRLDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. Related: Common-mode signalCommon-mode signal A voltage that appears equally on both inputs of a differential amplifier, such as 60 Hz hum picked up by the whole body. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry on forehead.” The exact command you ran. A sketch of the wiring, photographed and taped in if it was on paper.

After: what you saw, with numbers and units. “60 Hz peak about 2 V pp at the output before the DRLDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. Related: Common-mode signalCommon-mode signal A voltage that appears equally on both inputs of a differential amplifier, such as 60 Hz hum picked up by the whole body. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry, about 200 mV after.” What went wrong. What you will do next time. A plot, with labelled axes and a title, saved to a folder named by date.

Paper or digital

Either, as long as it is one place. Paper is faster at the bench and impossible to lose to a corrupted drive. A plain-text file or a notes app is searchable. I use paper at the bench and copy the essential lines into a text file at the end of the day. Photograph the paper pages; a phone photo of a page of wiring notes has saved me more than once.

Whatever you choose, never use a tool that requires a subscription to read your own notes later.

From notebook to build log

A build log is a notebook entry cleaned up for another person. It has a fixed shape on this site, and the shape is the point: what I set out to do, what I actually did, what went wrong, what I measured, what I would change, what surprised me. There is a template and an example.

The build logs are the portfolio. A resume says “built an EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry amplifier.” A build log shows the plot, the noise number, the mistake, and the judgment that came out of it. Recruiters and professors read the second kind of thing, and almost no student produces it.

Two rules for plots

Every plot has axes with units and a title that says what it is and when it was taken. Every plot that matters gets the raw data saved next to it, as CSV, so it can be re-plotted. “Look at this trace I saved as a screenshot” is a sentence that ends careers in labs.

A minimum viable entry

If you are tired and want to skip it, write four lines:

2026-09-14  single-channel EEG, breadboard v2
did: moved DRL from forehead to right earlobe
saw: blink artifact dropped from ~200 µV to ~120 µV; alpha unchanged
next: solder it

That is enough. Four lines a session for four years is a record almost nobody has.