Bio-Feedback (Lügen-Detektor)

The idea itself isn`t new - a similar circuit was published in Elektor, May 1976 -but the simplicity of the circuit described here lends it a special charm ... Only three I Cs and a handful of other components are required to recognise the logic levels of sixteen different signals and dis-play them on an oscilloscope screen. The display consists of two rows of noughts and ones. This is achieved as follows. If a sine-wave is applied to the Y-input of an oscilloscope, the display depends on the signal applied to the X-input. If a sawtooth is applied, the sinewave is traced on the screen; if there is no signal on the X-input, a vertical line will be displayed; and, finally, if a sine-wave of the same frequency as the first but with different phase is applied, a circle or ellipse can be obtained. The vertical I ine or circle can be positioned at any point on the screen by adding a suitable DC offset to the X-and/or Y-input signals. In the circuit de-scribed here, two rows of eight lines or circles are displayed.