Non-invasive monitoring of vital signs and traumatic brain injuries

A fixed duty ratio, variable frequency, square wave generator includes an astable multivibrator having a pair of resistors coupled in series between a supply voltage terminal and a capacitor, such as of a type implemented with a 555 timer, and further includes a JFET connected between the capacitor and ground. In a current-controlled version, the anode of a diode is connected to the gate of the JFET, and the cathode of the diode is connected to a sensing resistor for sensing current in a load. The multivibrator generates a ramp voltage across the capacitor which varies between predetermined fractions of the supply voltage with a fixed duty ratio. The capacitor of the multivibrator charges and discharges through the series connection of the resistors and the JFET channel resistance with charging and discharging times that vary with the input voltage to the JFET, thereby varying the generator frequency.