Five-digit combination lock uses five low-cost ICs operating from 5-V supply that can be derived from 12-V auto battery as shown in inset. Six set/reset bistable circuits are formed by cross-coupling pairs of dual-input NAND gates, so 0-V input is needed to change state of each. Five of bistables serve for combination, and sixth prevents operation by number in incorrect sequence. After S1 is set to one number of code, S2 is pushed to enter that number, with process being repeated for other four numbers of combination. Final correct number sets B5 and turns on Tr1, to operate relay that can be used to open door.-S. Lamb, Simple Code-Operated Switch or Combination Lock, Wireless World, June 1974, p 196.
Required Field 
