A brain wave signal monitor with a warning device actuated by failure of the signals to be present at a preselected minimal level for a specific period of time. The warning device is actuated by a circuit that remains locked in the actuating condition until reset by an attendant. A delay circuit controls the minimum rate by preventing actuation of the warning device until a predetermined time has elapsed since the last brain wave signal. A circuit tuned to the local power line frequency receives signals from electrodes on the patient and provides a second type of warning signal if an electrode becomes dislodged. A third signal circuit is actuated if the power supply voltage drops too low. The second and third warning signals are generated by selectively pulsing a calibration signal generator. The latter also supplies signals of known amplitude to the input circuit. An electrically controlled switching circuit can switch selected electrodes or the calibration signal to the input amplifier.
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