hexadecimal keyboard

7-72 elektor july/august 1980 SI 12 03 0310 13 04 IC4 04 74LS175 9 4 Di 01 5O202 2D217 12D303 5V® HEXA DEC. Keyboard DI mim P.STROBE STROBE 80587 Ci Mon =I= 76 hexadecimal keyboard 5 D2 02 " 0 2 14 76" OSC a 04 X3 X2 3 N1 ... N3 = IC5 = 3/4 4011 This circuit consists of an encoder which scans each key of the keyboard independently and then converts the data into a four bit binary number. The scan frequency is determined by an internal oscillator and a single external capacitor. Contact bounce is eliminated by a time delay of 22 ms. When a key is depressed, the DA output of ICI goes high after this delay (DA = data available). The AND output goes low again when the key is released. The DA output clocks IC2 (via N2) causing the data at the inputs of IC3 to be transferred to its outputs. As soon as the second key is depressed, IC2 is again clocked and its outputs change state so that IC4 now holds the corresponding key data. At the same time, the "FULL" LED will light. The keyboard ...