Soldering, breadboards, and the multimeter

The multimeter first

Before anything is powered, the meter. Three functions cover Phase 1. Continuity (the beep): touch the two probes together and it beeps; touch them to two ends of a wire and it beeps if the wire is intact. Use it to check every breadboard connection you doubt, with the power off. DC voltage: red probe to the point, black to ground; use it to check your battery (a fresh 9 V reads about 9.5, a dead one under 7), your supply rails, and the output of each stage with the input shorted (should sit near the mid-point). Resistance: check a resistor’s value before you trust the colour bands, because you will misread them, and check a 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’s value in the circuit with the power off.

Two habits. Always know which range the meter is on; auto-ranging meters save time. Never measure resistance or continuity on a powered circuit; you will get nonsense and can damage the meter.

Breadboards

A breadboard’s rows of five holes are connected. The long rails down the sides are connected along their length, and on most boards the rail is split in the middle; check with the continuity beep. Put +9 V on one rail, −9 V on the other, and use the two remaining rails for ground, jumpered together.

Rules that save afternoons. Keep the circuit compact; long jumpers are antennas. Use short, colour-coded jumpers: red for positive, black for ground, blue for negative, everything else for signals. Put the bypass capacitors right at the chip’s supply pins, not at the rail. Bend component leads neatly so they lie flat; a resistor standing up will get knocked out. Photograph the board after every working stage.

Breadboards are noisy. Their contacts are springy and their rows have a few picofarads of capacitance between them. A breadboarded 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 has a 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 several times worse than the same circuit soldered, which is why “solder it” is the first item on every build log’s what-I-would-change list.

Soldering

A temperature-controlled iron at about 330 °C for lead-free solder, a damp sponge or brass wool for the tip, and thin solder (0.5 to 0.8 mm) with flux inside it. The technique in one sentence: heat the joint, not the solder. Touch the iron to both the pad and the lead for a second, feed solder into the joint from the other side, let it flow, remove the solder, remove the iron. Two to three seconds total. A good joint is shiny and shaped like a small volcano; a bad one is a dull ball sitting on top.

Order of operations for a board: shortest components first (resistors, diodes), then sockets and capacitors, then anything tall. Use an IC socket for the 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. Related: Differential measurementDifferential measurement Measuring the difference between two electrodes rather than one electrode against ground, so anything common to both cancels. , Common-mode rejection ratio (CMRR)Common-mode rejection ratio (CMRR) How much better an amplifier responds to a difference between its inputs than to a signal shared by both, in decibels; 100 dB means 100,000 times. , Input impedanceInput impedance How hard the amplifier's inputs are to push current into; it must be far higher than the electrode impedance or the signal is lost in the divider. . Glossary entry so a mistake does not cost you the chip. Clip leads after soldering, not before.

Mistakes, in order of frequency. Not enough heat, leaving a grainy joint that fails in a week. Too much solder, bridging two pads; fix with the iron and a wick. A cold joint from moving the part while the solder set. Breathing the fumes; a small fan helps. Touching the tip; it does not feel hot until it does.

Practice before the real thing

Buy a five-dollar practice kit or simply solder resistors in a row on a scrap of perfboard for twenty minutes. Look at each joint with a magnifier. When the last ten in a row are shiny volcanoes, you are ready for the INA128.

Reading resistors and capacitors

Four bands on a resistor: two digits, a multiplier, a tolerance. Brown-black-red is 1-0-×100 = 1 kΩ. Check with the meter anyway. Small ceramic capacitors carry a three-digit code: 104 is 10 × 10⁴ picofarads = 100 nF = 0.1 µF. Electrolytic capacitors have a polarity marked with a stripe on the negative side; reverse one on a 9 V supply and it will eventually vent. Film capacitors have no polarity and are the right choice for the audio and filterFilter An operation that passes some frequencies and attenuates others; every filter also delays or distorts the timing of what it passes. Related: High-pass filterHigh-pass filter Removes slow drifts and DC offsets, passing everything above a cutoff; set it too high and you erase slow brain activity. , Low-pass filterLow-pass filter Removes fast content above a cutoff; the anti-alias filter is one, and so is the smoothing you apply to a noisy trace. , Notch filterNotch filter A very narrow filter that removes one frequency, usually 60 Hz mains hum; it also rings and distorts nearby content, so fix the hum at the source first. , Phase distortionPhase distortion Different frequencies being delayed by different amounts, which smears and shifts waveforms such as an ERP peak. . Glossary entry stages.