The 74LS04N is one of the classic building blocks of TTL digital electronics. It contains six independent NOT gates, each with one input and one output.

When the input is LOW, the output becomes HIGH. When the input is HIGH, the output becomes LOW.

Instead of using the 74LS04N only as a simple logic inverter, this project turns it into a small TTL signal laboratory. An Arduino Nano generates a repeatable digital test signal, the 74LS04N processes that signal, and LEDs provide direct logic-state feedback.

The circuit can also be observed on an oscilloscope so you can compare the input and output waveforms stage by stage.

The 74LS04N belongs to the 74LS TTL family and provides six independent inverters in a DIP-14 package. One chip supports multiple experiments in a compact format.

For this demonstration, three gates are used:

• Gate 1: signal inversion
• Gate 2: second inversion / polarity restoration
• Gate 3: optional additional processing

The remaining gates stay unused in the base build. Always operate the device within datasheet conditions: suitable 5V supply and valid TTL input levels.

The basic signal path is:

Arduino D9
    |
    v
74LS04N Gate 1
    |
    v
Inverted Signal
    |
    v
74LS04N Gate 2
    |
    v
Original Polarity Restored

The first inverter flips polarity:

INPUT     OUTPUT
LOW   ->  HIGH
HIGH  ->  LOW

After the second inverter, polarity is restored:

INPUT     OUTPUT
LOW   ->  LOW
HIGH  ->  HIGH

In practice, this build is easier to debug if you follow a simple validation checklist for wiring, power stability, and expected output behavior.

Standard 74LS04N DIP-14 pinout:

• 1: 1A input
• 2: 1Y output
• 3: 2A input
• 4: 2Y output
• 5: 3A input
• 6: 3Y output
• 7: GND
• 8: 4Y output
• 9: 4A input
• 10: 5Y output
• 11: 5A input
• 12: 6Y output
• 13: 6A input
• 14: VCC

Connect pin 14 to +5V and pin 7 to GND. Place a 100nF ceramic capacitor close to the IC between VCC and GND for local high-frequency decoupling.

This capacitor is not part of logic function itself; it helps supply stability during transitions, especially on breadboards with long jumper wires.

Use two LEDs to visualize stage outputs:

• LED 1 indicates signal after first inverter
• LED 2 indicates signal after second inverter

Use current-limiting resistors. For quick breadboard demos, 330Ω is a practical starting point.

The Arduino Nano generates a square wave on D9 (for example 1kHz). This makes stage comparison repeatable on both LEDs and oscilloscope traces.

Expected relationship:

D9       = original signal
Pin 2    = inverted signal
Pin 4    = original polarity

• TTL logic voltage behavior
• Logic inversion and stage chaining
• Signal buffering workflow basics
• Supply decoupling value in real wiring
• Waveform observation with oscilloscopes
• Breadboard signal-integrity considerations

Because the 74LS04N has six gates, this same chip can be reused for more experiments after the base lab is stable.

Start with a multimeter to verify IC supply near 5V, then scope D9, pin 2, and pin 4 to confirm stage behavior and timing.

At higher frequencies, you can observe propagation delay and waveform degradation through cascaded stages.

For repeatability, record baseline measurements: transition stability, response timing, and behavior after temporary faults.

1) Add frequency measurement

Measure an external TTL input with Arduino and report frequency through Serial Monitor.

2) Compare one vs two inverter stages

Measure input, one-stage output, and two-stage output on the scope to visualize propagation delay effects.

3) Build a compact TTL logic probe

Reuse the remaining inverter gates to create additional digital indication channels.

4) Migrate to a small PCB

Convert the breadboard circuit into a compact board in EasyEDA with DIP socket, Arduino header, 5V input, test points, LED indicators, probe points, and decoupling capacitor.

The 74LS04N is simple but extremely useful for understanding real digital logic behavior. Combining it with Arduino Nano creates a low-cost, reproducible lab for generating, observing, and validating TTL signals.

This is a practical first step toward deeper TTL signal-conditioning and mixed-signal electronics experiments.