The CD4017 counter is often used for sequential circuits, such as led chaser. In this article you can see how it works, with an example.
Logic integrated circuits
A logic integrated circuit interacts with other components through input and output terminals.
An input terminal takes the high level when it is supplied by a positive signal. If it is supplied by a negative signal it takes the low level. An input signal can be set in different ways. In the image below you can see some example.
An output terminal puts out a positive signal at the high level and a negative signal at the low level. Basically it works as an SPDT switch, but actually there aren’ t mechanical contacts inside electronic components. An output terminal can be used to drive a transistor, a relay or a led (max 20mA), or an input terminal (for example in a logic gates network).
The CD4017 counter
The CD4017 integrated circuit is a decade counter that switches on ten outputs one at a time, due to a clock signal on the input pin 14. The input pin 13 enables the clock signal, while the input pin 15 is supposed to reset the output condition.
Once the CD4017 is supplied the first output switches on. If the clock pin takes for an instant the high level and the input pins 13 and 15 are both low, the first output switches off, while the second one turns on. When the last output turns off the cycle repeats. If the reset pin takes for an instant the high level the CD4017 backs to the start condition. If the input pin 13 features the high level, the counter ignores the clock signal. If not used the input pins 13 and 15 must be connected to the negative power supply.
The CD4017 usually works with a power supply between 5 and 18 Volts. Its outputs can support a small current (up to 20mA). It is very sensitive to static electricity.
Digital lock with CD4017 and logic gates
This circuit switches on an output if four keys are pressed in the correct sequence. It uses a CD4017 and some logic gates. If the correct key is pressed, the gate number 1 of the CD4081 turns on. Then the high signal is sent to the clock gate of the CD4072. The clock relay (K1) turns on and keeps in that condition for about 0,5 seconds. So the CD4017 activates the next output. When all keys are pressed, the output number 4 of the CD4017 turns on the output relay (K3). As you can see in the logic diagram, each stage is connected to an EXOR gate. By pressing a wrong key the AND gate keeps off, so the EXOR gate turns on and activates the reset gate of the CD4072.