Sensor Switch And Clock

060 SENSOR SWITCH AND CLOCK One Type TL084 lc and an old quartz watch enable the con- struction of a de luxe on-off switch. Two of the four opamps con- tained in the TL084 (Al and A2) are used to amplify the input sig- nals from the sensors hundredfold (with the component values as shown in the diagram). Just touching the sensors with a finger causes a good 50 Hz input signal (hum). Note that the amplifica- tion drops rapidly with rising frequency. Diodes D5 and D6 rectify (single-phase) the 50 Hz signal. Since the diodes are connected in anti-phase, touching the "off" sensor results in a positive potential across C10, whereas touching the "on" sensor gives a negative potential across C10. Opamp A4 is connected as an inverting bistable, so that a nega ELEKTOR ELECTRONICS JULY 1989 SUPPLEMENT GENERAL INTEREST CI SO Cc-)2(.100n C7 05 R12 Al IF-0-1111-1=1 560n S1 1:7 13 R1 ICE C3 1N4148 BzI D7 R14 6 560n1N4148 A4 4/1)BC517 Vh 884014-11 DID2R2R9 0 22n IF 2x "r 1N4148 C2 0 IC1 0II ...