Analog Step Sequencer

Lesson 4: Creating a Sequence with the 4017 IC

Agenda

Part 1: (20 min)

• Looking at the 4017 and generating a sequence

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Part 2: (Rest of the time)

• You build the circuits on a breadboard
• Debugging on prior circuits
• Next lesson is the final lesson so I want to make sure all the subcircuits work before we add the final components to link them
• This makes debugging easier

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The 4017 IC

• The 4017 IC is a decade counter.
• This means that it counts sequentially from 1-10 and outputs a logic HIGH at a specified output pin (0-9).
• We will use this chip for the "sequencing" aspect of the project which ties together the circuits from the last two lessons.

Counting

This gif provides a nice visual of how the counter works. Note that this chip is driven by a clock (we will use the clock circuit we built from lesson 2!)

• As you can see, at each clock pulse, the next output (Q0-Q9) goes high
• This is what is meant by counting, the output pins go high sequentially
• Something that is a bit hard to see here is how the counter goes back to zero which is controlled by internal logic of the chip
• We are going to trigger the counter to go back to zero manually because we only need to count from 0-7 (8 times) because we have 8 outputs

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Pinout

As we did with the 555 IC let's distinguish the significant pins from the insignificant pins so that we can understand how the chip works.

Insignificant Pins

• Pin 16: Vdd. This is power so we just connect this pin to the +9V rail
• Pin 8: GND. This is ground so we just connect this pin to the GND rail
• Pin 13: Clock Enable. This pin controls whether the counter is "on". We always want the counter to be on so we will connect it directly to GND (the pin is active LOW). If this pin is high the counter will ignore the clock and pause so that the current output will hold.
• Pins 11 and 12: Q9 and Carry out. As stated before we only want to count 8 times so we will ignore these pins and not hook them up to anything

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Significant Pins

Pins 1-7 and 10: Output pins. These are the counter outputs and will output sequentially from Q0 - Q7. Note that Qi is not connected to pin i so we need to make sure we are looking at the pin for which a given Qi is connected. For example, Q0, the first output is at pin 3.

Pin 14: Clock. This pin drives the counter and will cause the next output to go high at each rising edge of the clock. Essentially, when are clock goes high, the next output goes high. We connect the output of our 555 clock circuit (pin3) to the clock pin of the 4017 (pin 14). If we speed up or slow down our clock using the potentiometer, the counter will count faster or slower as we will see. This will enable us to control the speed at which our sequence plays

Pin 9 and 15: Q8 and reset. When the reset pin goes high, the counter goes back to its initial state which is Q0 goes high. We will connect the ninth output (Q8) to the reset so that once our counter reaches Q8 it will reset. So what will happen is our count goes 0 --> 1 --> 2 --> 3 --> 4 --> 5 --> 6 --> 7 --> 8 (trigger reset so pin 0 actually goes high) --> 1 ...

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Circuit Diagram

Checkpoint. Check if LED flashes (twist potentiometer to reduce delay)