combination lock

combination lock elektor december 1981 — 12-37 R. de Boer Reset The block diagram in figure 1 shows that the device consists of two basic sections, namely a window discrimi- nator and a so-called "voltage generator". Initially, output "0" of the voltage gener- ator has a high logic level, in contrast to the other nine outputs. By adjusting preset potentiometer P2, any voltage between 0 V ... 12 V can be fed to the first input of the window discriminator via diode D2. The window discriminator then checks to see whether this voltage is the same as that present at the wiper of potentiometer P1 (the code "switch"). If so, the window discriminator transmits a clock pulse to the voltage generator The window discriminator is con- structed around two opamps ( IC2 and IC3) having a high open-loop gain. Therefore, the outputs of these two ICs can only be logic one or logic zero. The voltage at the inverting input of IC3 is approximately 0.6 V less than that at the non-inverting input of I...