The 4017b Decade Counter is part of the 4000 CMOS Logic family of integrated circuits. It is a 16 pin DIL IC that has 10 primary outputs. Each output is triggered in order from 0-9 when a complete digital pulse (0,1) is received at the 'clock' input
The 'Divide by 10' output or carry out is technically the 11th output despite the name 'Decade' counter. This output is HIGH for half the output cycle (0-4) and LOW for half the time (5-9). This can be used to create a further level of timing as the pulse can be connected to another 4017b which would trigger every time the first has made a complete cycle
The decade counter can be interfaced with other IC's or PIC's that can produce a logic signal (0,1). In this circuit a 555 timer has been used in astablemode to produce a continuous pulse to the 'clock' input of the decade counter. A second decade counter has been added which will count each complete output cycle (0-9).
Each 4017b output produces 3.5V (supply 6V) to the connected components which is why a resistor is not required for the LED's. If a transistor is used to interface with a 1K resistor is required to protect the base leg. If a higher supply voltage is used resistors will be required
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Component Theory - Decade Counter