Building a tDCS device: the constant-current part is the safety, not an afterthought

Teaching the build means teaching the current regulation, because the regulation IS the safety. A tDCS device is, electrically, a current source delivering a small, steady DC current through the head. The whole design problem is "steady" and "small" — a plain battery-and-resistor does neither when skin resistance changes. The core: a constant-current source Skin/electrode resistance drifts (sweat, electrode drying, movement). If you set current with a fixed resistor off a 9 V battery, the current wanders as resistance changes — which is exactly what you must not allow near the head. Use an active constant-current source: - Use the LM334 (a dedicated adjustable current-source IC, rated roughly 1 uA to 10 mA). Its set resistor chooses the current: I is about 67.7 mV / R at room temperature, so 1 mA needs about 68 Ohm and 2 mA about 34 Ohm. Check the formula and temperature coefficient against the datasheet for your part. - The LM317 in its constant-current configuration (I = 1.25 V / R) is often suggested, but its datasheet minimum load current is several milliamps (about 3.5 mA typical, up to 10 mA), so at 1-2 mA it is outside its regulated range. Do not use it for tDCS currents. - Compliance voltage: the source can only hold its current while the battery has headroom. At 2 mA through a head-and-electrode load of a few kOhm you already need several volts, plus the regulator's own overhead. If the load resistance rises past what the supply can drive, the current falls; it never rises. That failure direction is the reason to use a current source. - Target current: 1-2 mA. This is the researched range; do not exceed it. - Supply: a 9 V battery. Battery only — never mains-powered anything on the head. The electrodes and the protocol - Saline-soaked sponge electrodes, 25-35 cm2 (bigger area = lower current density = gentler). Soak in saline, not tap water; not dripping. - Ramp the current up over ~30 seconds and down over ~30 seconds — a sudden on/off causes a flash/phosphene and is unpleasant. - Include a current-limit resistor as a hard ceiling and a fuse, so a single fault cannot deliver more than the design maximum. Verify BEFORE it touches skin Put a load resistor (e.g. ~5 kOhm, standing in for the head) across the output and measure the current with a multimeter in series. Confirm it holds your set current as you swap the load resistor for a different value — that is the test that your source is actually constant-current and not just a resistor divider. Only after it passes that does it go near a person. TENS (for muscle/nerve, pulsed AC, different device) shares the "battery only, verify first, ramp, electrode area" discipline. Circuit basics (current, voltage, resistance): https://wiki.soapbox.community/wiki/Electricity_and_Photovoltaics