# Clock signal generator using 7400 IC

This clock signal generator using 7400 IC, is an astable circuit which uses the TTL 7400 integrated circuit. The 7400 IC has four 2 inputs NAND gates (NOT AND). We only use two gates of this integrated circuit.

This circuit is useful. You can put all together and use it as part of a workshop or electronic laboratory. It can be used as a source of a clock signal. This circuit, as implemented with TTL technology components, can only be connected to other digital circuits on the same technology.

## How the Clock signal generator with 7400 works?

In the circuit the two NAND gates, are connected as NOT gates linking its two input terminals, as shown in the circuit diagram.

Circuit operation sequence:

• With fully discharged capacitor C1, we have the output of gate 1, a logic level ‘0’ (low voltage level)
• This means that at the input of gate 2 we have a logic ‘0’ level. This causes the output of the same gate has a logical ‘1’ level, producing an output pulse ‘1’ (high) at its output terminal.
• The pulse at the output terminal gate 2 is transmitted through capacitor C2 to the input terminals of the gate 1, causing the output of gate 1 pass to logical “0” level (low).
• The capacitor C1 that was charged before, is discharged and establishes a logical ‘1’ at the inputs of gate 2.
• Having a logic level ‘1’ at the inputs of gate 2, its output goes to level ‘0’.
• And the cycle repeats …

LED lights intermittently, showing the correct operation of the circuit.

## Clock signal generator printed circuit Board (PCB)

A suggested Printed Circuit Board (PCB) for the Clock signal generator using 7400 IC, is shown on the following diagram. The approximate result of the final circuit is also included.

## List of components for the Clock signal generator

• 1 TTL 7400 / 74LS400 IC (IC1)
• 1 red LED (D1)
• 1 220 ohm resistor, 1/4 watt (R3)
• 2 1K resistors, 1/4 watt (R1, R2)
• 2 470uF / 16 V or more electrolytic capacitors (C1, C2)
• 2 connectors:
• 1 for the power supply (5 V) (CN2)
• 1 for the square wave output (CN1)
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