74123 (Dual Retriggerable Monostable Multivibrator)

Description

The IC 74123 is a dual retriggerable monostable multivibrator (also known as a one-shot timer) from the 7400 series of TTL (Transistor-Transistor Logic) ICs. It is used to generate precise time delays or pulses in digital circuits. The IC has two independent monostable multivibrator circuits, which means it can produce two separate time-delay outputs, making it useful for applications that require multiple timing functions.

IC 74123 Description:

The 74123 is a retriggerable monostable multivibrator that produces a single pulse of a specified duration when triggered by an input signal. It has two independent multivibrator sections, each capable of generating a pulse with a time period determined by an external resistor and capacitor. The retriggerable feature means that the timer can be triggered multiple times, and it will reset the pulse duration every time it is triggered before the previous pulse has completed.

Key Features:

• Dual Monostable Multivibrator: Two independent monostable multivibrator circuits in one package.
• Retriggerable: The pulse duration is reset and restarted if the trigger input is activated again before the previous pulse finishes.
• Adjustable Pulse Width: The pulse width (duration of the output pulse) can be adjusted using an external resistor and capacitor.
• TTL Logic: Built using TTL technology, ensuring compatibility with other TTL logic devices.
• Triggering: The device can be triggered by a negative edge or level trigger, depending on how it is configured.

Pin Configuration (14-pin package):

Functional Description:

1. Monostable Multivibrator:
   • In a monostable multivibrator, the IC is designed to produce a single output pulse of a specific duration (pulse width) in response to a trigger event. Once the trigger is received, the output will stay high for the duration of the pulse, and then it will return to low. The pulse duration is determined by an external resistor (R) and capacitor (C).
   • The IC has two independent monostable circuits (A and B), each with its own trigger, reset, and output pins, allowing it to generate two separate output pulses.
2. Retriggerable:
   • The retriggereable feature means that if the trigger input is activated again before the previous pulse has finished, the pulse will restart with a new duration. This allows the multivibrator to be used in systems where timing needs to be reset based on external events or signals.
3. Adjustable Pulse Width:
   • The pulse width can be controlled by adjusting the external resistor (R) and capacitor (C). The duration of the pulse is given by the formula:

     T=0.693×R×CT = 0.693 \times R \times Cwhere:

     • T is the pulse duration,
     • R is the resistance in ohms (Ω),
     • C is the capacitance in farads (F).
   • By changing R and C, you can adjust the length of the output pulse to suit the needs of your application.
4. Reset (Active Low):
   • Each monostable section of the 74123 has a reset input. The reset is active low, meaning the output pulse will be immediately terminated and set low if the reset input is activated (pulled low) before the pulse is complete.
5. TTL Logic Compatibility:
   • The 74123 is a TTL device, meaning it operates within the standard TTL voltage levels. It is compatible with other TTL-based logic ICs, making it ideal for use in systems that rely on TTL logic.

Applications:

1. Pulse Generation:
   • The 74123 can be used in circuits where a single pulse needs to be generated in response to an event or input signal. It can be used for timing purposes, such as producing a fixed-width pulse to trigger another circuit.
2. Time Delay Circuits:
   • The 74123 is commonly used in time delay circuits, where it can be used to introduce a delay before an action is performed. By adjusting the resistor and capacitor, you can control how long the delay lasts before the output is triggered.
3. Frequency Division:
   • It can be used to divide the frequency of an input signal by producing a pulse at regular intervals. For example, the 74123 can be used to generate a divided clock signal for use in counters or sequential logic circuits.
4. Signal Shaping:
   • The IC can be used for signal shaping, where it converts a noisy or irregular input signal into a clean, precise pulse of a fixed duration. This is useful in systems that require signal synchronization or clean edges for triggering other devices.
5. Digital Timing and Sequencing:
   • The 74123 is widely used in digital systems that require precise timing, such as in sequencing applications, where it generates timed pulses that control the operation of different parts of a system.
6. Switch Debouncing:
   • The IC can be used for debouncing switches in digital circuits. By using the 74123 as a pulse generator, bouncing or noisy switch signals can be cleaned up, ensuring stable and reliable triggering.
7. Edge Detection:
   • The 74123 can be used in circuits where edge detection is required. By producing a precise output pulse in response to a signal transition, the IC can be used to detect specific events or edges in the input signal.

Electrical Characteristics:

• Supply Voltage (Vcc): Typically 5V (operates from 4.75V to 5.25V).
• Input Voltage (Vih/Vil):
  • Vih (High Input): Typically 2V.
  • Vil (Low Input): Typically 0.8V.
• Output Voltage (Voh/Vol): The output levels are based on TTL logic:
  • Voh (High Output): Typically 2V to 5V.
  • Vol (Low Output): Typically 0V.
• Power Consumption: The IC typically consumes moderate power, as it is designed for use in TTL-based logic circuits.

The 74123 is a versatile and essential IC for generating precise time delays or pulses in digital circuits. Its retriggerable monostable multivibrator configuration allows for flexible timing applications, while its dual output functionality makes it useful in systems where multiple timing signals are needed. Whether you’re generating pulse signals, implementing time delays, or working with digital sequencing, the 74123 provides a reliable solution.