Exercise 2.3: Reset

In Exercise 2.2, you learned how to make your counter tick automatically using timing circuits. Now you have a counter that goes 0, 1, 2, 3... all the way up to 99 before rolling over. But for a real clock, we need the seconds to reset at 60 (not 99), the minutes to reset at 60, and the hours to reset at 12 (not 99).

In this exercise, you'll learn multiple methods for resetting counters at specific values. By the end, you'll understand the techniques needed to build a proper digital clock that displays time in the format we're all familiar with!

1) Review: The Reset Pin

Let's review what you learned about the CD4026's reset functionality in Module 1.

Looking back at the CD4026 pinout:

Pin 15 (Reset):

• When LOW (connected to GND): Counter operates normally
• When HIGH (5V): Counter immediately resets to 0
• Must be kept LOW for normal operation

What happens if you leave the reset pin (pin 15) floating (not connected to anything)?

-- The counter works normally The counter might randomly reset due to electrical noise The counter stays at 0 forever Nothing happens

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Manual Reset Button (Review)

In Module 1's project, you built a reset button for your cascading counter. Let's review how it worked:

- Button connects between 5V and reset pin
- When pressed: reset pin goes HIGH → counter resets to 0
- When released: reset pin returns to LOW → counter can count again

This manual reset is useful for setting the time, but for automatic resets (like at 60 seconds), we need the counter to reset itself based on its displayed value.

2) The Challenge: Automatic Reset at Specific Values

For a digital clock, we need:

• Seconds: Count 00 → 59, then reset to 00 (reset at 60)
• Minutes: Count 00 → 59, then reset to 00 (reset at 60)
• Hours: Count 00 → 11, then reset to 00 (reset at 12)

The key question: How can we detect when the counter reaches a specific value and automatically trigger a reset?

There are two main approaches:

1. Diode reset method – Uses the segment outputs and diodes
2. Logic gate reset method – Uses logic ICs to detect specific counts

Let's explore both!

3) Method 1: Diode Reset Circuit

You've actually already used this method! In Module 1's project, you used Zener diodes to reset the tens digit of seconds at 6. Let's understand this method more deeply.

Understanding the Concept

The CD4026 has a special property: if the reset pin (pin 15) is left open (not connected to LOW), the IC resets to 0 automatically.

We can use this property cleverly:

• Keep the reset pin connected to GND through reverse-biased diodes
• The diodes are connected to specific segment outputs
• When ALL those segments are HIGH simultaneously, the diodes block the path to GND
• The reset pin becomes "open" → automatic reset!

Why This Works for Digit 6

Let's look at which segments light up for each digit:

Look at segments e, f, and g. Which digit is the ONLY one where all three are HIGH?

Which digit is the only one where segments e, f, AND g are all HIGH at the same time?

-- 0 5 6 8 9

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This is why the diode method works perfectly for resetting at 6!

Building a Diode Reset for Digit 6

Let's build this circuit step by step. You'll need:

• 3 × 1N4148 diodes (or similar small signal diodes)
• 1 × 10kΩ resistor

Understanding diodes:

• Diodes allow current to flow in only one direction
• The side with a black band is the cathode (negative)
• The other side is the anode (positive)
• Current flows from anode to cathode

1. With power OFF, locate one of your CD4026 ICs that you want to reset at 6 (this would be the tens digit of seconds or minutes).
2. Currently, pin 15 (Reset) should be connected to GND. Remove that connection.
3. Connect three 1N4148 diodes in parallel:
   - All three cathodes (black band end) connect together to pin 15 (Reset)
   - First diode anode connects to segment e output (pin 14)
   - Second diode anode connects to segment f output (pin 10)
   - Third diode anode connects to segment g output (pin 15)
4. Connect a 10kΩ resistor from pin 15 (Reset) to GND.

How this circuit works:

When the counter displays 0-5:

• At least one of segments e, f, or g is OFF (LOW)
• That LOW segment output pulls current through its diode
• Current flows through the 10kΩ resistor to GND
• The reset pin stays LOW → counter operates normally

When the counter displays 6:

• ALL three segments e, f, and g are HIGH
• All three diodes are reverse-biased (cathode more positive than anode)
• No current flows through the diodes
• The reset pin is no longer pulled to GND → it goes HIGH
• Counter resets to 0!

5. Turn power ON and let your counter count.
6. Watch carefully as it approaches 6. It should reset to 0 just as it tries to display 6!

You might see a very brief flash of the number 6 before it resets – this is normal and happens too quickly to matter.

Why do we need the 10kΩ resistor to GND in this circuit?

-- To limit current through the diodes To pull the reset pin LOW when at least one segment is OFF To protect the CD4026 from damage We don't actually need it

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Limitations of the Diode Method

The diode reset method is clever and uses minimal components, but it has limitations:

It only works for specific numbers:

• Easy for 6 (segments e, f, g)
• Easy for 8 (all segments)
• Hard or impossible for other numbers

For resetting at 60 or 12, we need a different approach!

4) Method 2: Logic Gate Reset Circuit

For more complex reset conditions (like at 60 or 12), we need to use logic gates to detect specific multi-digit values.

What are Logic Gates?

Logic gates are the fundamental building blocks of digital circuits. They perform basic logical operations on binary inputs (HIGH/LOW) and produce binary outputs.

The main logic gates you should know:

AND gate:

• Output is HIGH only if ALL inputs are HIGH
• Truth table: 0·0=0, 0·1=0, 1·0=0, 1·1=1

OR gate:

• Output is HIGH if ANY input is HIGH
• Truth table: 0+0=0, 0+1=1, 1+0=1, 1+1=1

NOT gate (inverter):

• Output is opposite of input
• Truth table: NOT 0 = 1, NOT 1 = 0

NAND gate:

• NOT AND – output is LOW only if ALL inputs are HIGH
• Truth table: Opposite of AND

NOR gate:

• NOT OR – output is HIGH only if ALL inputs are LOW
• Truth table: Opposite of OR

What is the output of an AND gate when one input is HIGH and one input is LOW?

-- HIGH LOW Floating It depends

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The 74LS00 NAND Gate IC

For our reset circuit, we'll use the 74LS00 quad 2-input NAND gate IC.

"Quad 2-input" means:

• Quad: Contains 4 independent gates
• 2-input: Each gate has 2 inputs

Pin connections:

• Pin 14: VCC (5V)
• Pin 7: GND
• Pins 1-6, 8-13: Four separate NAND gates with their inputs/outputs

Each NAND gate takes two inputs and produces one output.

Detecting "12" for Hour Reset

Let's design a circuit to reset at 12 hours. We need to detect when the display shows "12" – that is:

• Tens digit = 1
• Units digit = 2

Step 1: Understand what "1" and "2" mean in segment form

For the tens digit to show "1":

• Segments b and c are HIGH
• All other segments are LOW

For the units digit to show "2":

• Segments a, b, d, e, g are HIGH
• Segments c and f are LOW

Step 2: Detect "1" on tens digit

We need to detect when segments b AND c are HIGH. This is perfect for an AND gate!

But wait – the 74LS00 has NAND gates, not AND gates. No problem! We can convert NAND to AND by adding an inverter:

• NAND(A, B) → NOT gate → gives us AND(A, B)

Or we can use the property that NAND(NAND(A,B), NAND(A,B)) = AND(A,B).

Step 3: Detect "2" on units digit

Detecting "2" is more complex because we need to check multiple segments.

Actually, there's a cleverer approach: Since we're using CD4026 counters, we can look at the segment that's unique for "2" compared to other digits we care about.

Looking at our application:

• We want to reset when tens="1" AND units="2"
• We can detect tens="1" by checking segments b and c
• We can detect units="2" by checking segment a AND segment g (both HIGH only at 2 in this context)

Practical approach for this module's project:

The exact logic circuit gets complex. In the project, you'll see a simplified approach shown in the reference material that uses:

• Segment outputs from the hour digits
• NAND gates to combine the conditions
• A final output that goes HIGH only at "12"

5) Building a Simple Logic Gate Circuit

Let's practice using logic gates with a simpler example first. We'll build a circuit that lights an LED only when TWO buttons are both pressed – a classic AND gate application.

Two-Button AND Circuit

1. With power OFF, place a 74LS00 IC on your breadboard straddling the center gap.
2. Connect power:
   - Pin 14 (VCC) → 5V
   - Pin 7 (GND) → GND
3. Set up two buttons:
   - Button A: one side to 5V, other side to pin 1 (input A)
   - Connect a 10kΩ pull-down resistor from pin 1 to GND
   - Button B: one side to 5V, other side to pin 2 (input B)
   - Connect a 10kΩ pull-down resistor from pin 2 to GND
4. Connect pin 3 (NAND output) to pin 4 (input of second NAND gate)
5. Connect pin 5 to pin 4 as well (both inputs of second NAND are the same)
6. Connect pin 6 (output) to an LED with 220Ω resistor to GND

This circuit implements: AND(A,B) = NOT(NAND(A,B))

7. Turn power ON and test:
   - No buttons pressed: LED OFF
   - Button A only: LED OFF
   - Button B only: LED OFF
   - Both buttons: LED ON

This is an AND gate – output is HIGH only when BOTH inputs are HIGH!

If you wanted an LED that lights up when EITHER button is pressed, which gate would you use?

-- AND gate OR gate NOT gate NAND gate

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Understanding Truth Tables

Let's verify our circuit with a truth table:

NAND gate (pins 1,2 → 3):

NOT gate (pins 4,5 → 6):

Result: LED (pin 6) is HIGH only when both buttons are pressed – perfect AND behavior!

6) Practical Reset Circuits for Clocks

Now let's discuss practical reset circuits for your digital clock:

Reset at 60 (Seconds and Minutes)

To reset at 60, we need to detect:

• Tens digit = 6
• Units digit = 0

Option 1: Detect when it reaches 60 momentarily

• Use diode reset on tens digit (detects 6)
• This automatically happens when units rolls over from 9→0
• The carry pulse triggers tens to go 5→6
• Diodes detect 6 and reset both digits to 00

Option 2: Use the carry-out signal

• When units digit reaches 9 and gets another pulse, it goes to 0 and carry-out goes HIGH
• When tens digit is 5 and receives this carry, it goes to 6
• Detect tens=6 with diodes → reset both digits

Implementation:

1. Use diode reset method (e, f, g segments) on the tens digit
2. Connect the reset output to BOTH the tens digit reset pin AND the units digit reset pin
3. When tens digit reaches 6, both digits reset to 00 simultaneously

Reset at 12 (Hours)

Resetting at 12 requires detecting a two-digit value where neither digit is 6 or 8 (which have unique segment patterns).

Approach from the reference material:

The project guide shows using NAND gates to detect specific segment combinations:

• Segment "a" from tens place (HIGH only at 1)
• Segment "g" from units place (unique pattern for 2)
• Combining these with NAND logic

The detailed circuit is shown in the project document and involves:

1. Using 3 NAND gates from the 74LS00 IC
2. Detecting the specific segment pattern for "12"
3. Outputting a HIGH pulse when "12" is detected
4. Using this pulse to reset both hour digits

7) Alternative Method: Using Paper Clips

Here's a creative, low-tech solution that's sometimes used in student projects:

Paper clip jumpers: Instead of diodes or logic gates, you can manually connect reset pins to specific segments using paper clips or jumper wires to create the exact reset condition you need.

Advantages:

• No additional components needed
• Easy to experiment and adjust
• Good for prototyping

Disadvantages:

• Not reliable for permanent circuits
• Can create shorts if not careful
• Difficult to troubleshoot

This method is mentioned because in the project reference, alternative creative solutions are encouraged!

8) Using Push Pins for IC Reset Control

Another practical technique: Use push pins or small toggle switches to manually control reset pins during testing.

1. Connect a push pin between VCC and each reset pin
2. Press to reset that digit manually
3. Helps with debugging and time-setting

This gives you manual override capability while developing the automatic reset circuits.

9) Comparing Reset Methods

Let's compare all the reset methods we've learned:

For your final clock project, you'll use:

• Diode reset for resetting seconds/minutes at 6 (tens digit)
• Logic gate reset for resetting hours at 12
• Manual buttons for setting the time

10) Testing Your Reset Circuit

Here's how to test a reset circuit properly:

1. Build the reset circuit with power OFF
2. Connect your multimeter to measure the reset pin voltage
3. Turn power ON
4. Manually advance the counter using a button on the clock input
5. Watch the multimeter as the counter approaches your reset value
6. The reset pin should:
   - Stay LOW (near 0V) for all values before reset point
   - Go HIGH (near 5V) at the reset value
   - Return to LOW immediately after reset
7. Verify the display resets correctly and counting continues

Common issues:

If reset happens too early:

• Check diode orientations (cathodes should all connect to reset pin)
• Verify you're detecting the right segments
• Make sure pull-down resistor is connected

If reset doesn't happen:

• Check all diodes are properly connected
• Verify segments are going HIGH when expected
• Test voltage on each segment at the reset value
• Try a different CD4026 IC

If reset is erratic:

• Add a small capacitor (0.01µF) across the reset pin to GND to debounce
• Check for loose connections
• Verify power supply is stable

11) Advanced: Cascading Reset Signals

For a complete clock, you need coordinated resets:

Seconds:

• Units and tens digits both reset at 60

Minutes:

• Units and tens digits both reset at 60
• Only happens when seconds rolls over

Hours:

• Units and tens digits both reset at 12
• Only happens when minutes rolls over

This cascading behavior requires:

1. Detecting the reset condition for each pair
2. Resetting both digits simultaneously
3. Not interfering with carry signals between pairs

In the project, you'll implement this carefully to ensure smooth operation!

12) Recap

Congratulations! You've learned:

✓ How the CD4026 reset pin works

✓ Why we need automatic resets for clock applications

✓ The diode reset method for detecting specific digits (especially 6)

✓ How logic gates work (AND, OR, NOT, NAND)

✓ The 74LS00 NAND gate IC and its pinout

✓ How to detect multi-digit values with logic gates

✓ Building an AND gate circuit with two buttons

✓ Practical approaches for resetting at 60 and 12

✓ Alternative methods using paper clips and push pins

✓ How to test and troubleshoot reset circuits

You now have all the knowledge needed to build the complete digital clock in the project!

Final Submission

Upload a short video (20 seconds) showing a reset circuit in action. This could be: (1) A diode reset circuit resetting a single digit at 6, OR (2) Your two-button AND gate circuit with LED, OR (3) Any counter with a reset mechanism you've built. Demonstrate the reset happening automatically or triggered by buttons.

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In this module's project, you'll put together everything from Module 2 – debouncing, timing circuits, and reset methods – to build a complete compact digital clock that displays hours:minutes:seconds with accurate timing and proper resets at 60 and 12!