Soldering, breadboards, and the multimeter

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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 gain 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 EEG amplifier has a noise floor 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 amplifier 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 filter stages.