This site puts 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 on people, starting with you. Two rules cover almost everything, and the rest of the page explains why each one exists, because a rule you understand is one you will keep when nobody is watching.
Rule one. Anything connected to a body runs from batteries, with no conductive path to the wall.
Rule two. You never stimulate your own or anyone else’s nervous system outside an IRB-approved lab with a principal investigator.
Here is why. The wall is 120 volts. Every device plugged into it has a small leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body. Related: IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). , Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient. . Glossary entry to its own chassis and ground, normally harmless, because you are not connected to it with a low-resistance path. 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 with conductive paste are exactly that path. A fault in a cheap charger, a frayed cord, or simply the ordinary leakage of a power supply can put current through you, and current through the chest at the tens-of-milliamp level is how people die. The medical standard for equipment that touches a patient, IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry, limits leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body. Related: IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps)., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry to a few tens of microamps, a thousand times smaller than the fault current a wall supply can deliver.
A battery cannot do this. Its total energy is small and it has no path to the wall. This is also why hospitals use isolated power and why the AD8232 datasheetDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry, like every biopotential chip’s datasheetDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry, tells you the same thing in smaller print.
100 mAacross the chest for a second or two, the heart fibrillates
15 mAmuscles contract and you cannot let go of the wire
1 mAthe threshold of sensation at 60 Hz
100 µAIEC 60601 limit for a part that touches skin, in normal use
23 µAworst case through the 390 kΩ resistor on a bias drive, if the amplifier fails
10 µAthe limit for anything that could touch the heart
The rail is spaced by how many times smaller each step is, so the 4 factors of ten between top and bottom are roughly to scale. 120 V across a hand-to-hand body resistance of 1 to 2 kΩ lands at the top of this rail. That is the whole danger in one line.
Figure 1. The whole of rule one, on one rail. The three red marks are what the wall can deliver through a low-resistance electrode path. The three green ones are what the standards allow and what a battery behind a resistor can physically produce. There is nothing in between to negotiate with.
The reasoning. A milliamp through the scalp is not dangerous in the electrocution sense, which is exactly why people do it at home. The dangers are different: skin burns from electrochemistry at the 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, which are well documented in the self-experimenter forums; unknown effects of stimulating tissue you cannot see, in a brain that is still developing into your mid-twenties; and, for vestibular and magnetic stimulation, falls and seizures. The evidence that these techniques do anything useful at home is weak, so you would be taking a real risk for a speculative benefit. Learn the circuits. Read the literature carefully; the Phase 6 project has you build a stimulator and test it in a saline bath, where it belongs. Then, if you want to run a stimulation study, join a lab that does it.
The smaller rules
The test
If you would not put it on a ten-year-old cousin, do not put it on yourself.
Why this is a credential
Every device company you might work for lives inside ISO 14971ISO 14971 The international standard for medical device risk management that every company follows. Related: FMEAFMEA Failure mode and effects analysis: a table of every way the device could fail, how bad and how likely each is, and what you did about it.. Glossary entry and IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry. An undergraduate who can explain leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body. Related: IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). , Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient. . Glossary entry limits, who has run their prototypes from batteries because they understood why, and who declined to self-stimulate because the risk-benefit did not justify it, sounds in an interview like someone who can be trusted with a patient. That is rarer than technical skill.
Recall
State rule one of the safety charter and the reason for it.
Anything connected to a body runs from batteries with no conductive path to the wall. Wall-powered equipment can leak or fault current through a low-resistance electrode path; IEC 60601 limits patient leakage to tens of microamps, which only isolation or batteries can guarantee.
Recall
Why is the Arduino the most common way rule one gets broken?
The amplifier is on batteries, but the Arduino is plugged into a charging laptop, so its ground, and therefore the amplifier's reference, is tied to the wall.
Recall
Why does the site forbid home tDCS even though the current is not dangerous in the electrocution sense?
The risks are electrochemical skin burns, unknown effects on developing brain tissue, and for other modalities falls and seizures, taken for a benefit the evidence does not support. Stimulation belongs under an IRB with a PI.