The ubiquitous 555 timer IC is a versatile building block for a wide range of electronic circuits. Its ability to generate precise time delays makes it ideal for applications like timers, oscillators, and pulse generators. One common application is the monostable one-shot configuration, where the output pulse width is determined by an external resistor and capacitor. However, in some cases, it may be desirable to have a trigger signal that lasts longer than the output pulse width. This is where the concept of an eliminator for the 555 monostable one-shot's output pulse width comes into play, ensuring that the trigger signal doesn't get cut short.
Understanding the Monostable One-Shot Configuration
The monostable one-shot is a fundamental circuit that produces a single output pulse of a predetermined duration in response to a trigger signal. The pulse width is determined by the time constant of an RC circuit, formed by an external resistor and capacitor. When a trigger signal is applied, the internal circuitry of the 555 timer charges the capacitor through the resistor. Once the voltage across the capacitor reaches a certain threshold, the output transitions from low to high, generating the pulse. The duration of the pulse is directly proportional to the RC time constant.
The Challenge: Trigger Duration vs. Output Pulse Width
In some applications, the trigger signal might have a duration shorter than the output pulse width of the 555 monostable one-shot. This can lead to unintended behavior, especially if the circuit relies on the trigger signal to be active for the entire duration of the output pulse. For example, consider a circuit where the output pulse controls a relay. If the trigger signal is too short, the relay might not fully activate before the 555 monostable one-shot's output goes back to low, leading to unreliable operation.
The Solution: Eliminating the Output Pulse Width Limitation
To address this issue, a 555 monostable one-shot eliminator circuit can be implemented. This circuit effectively extends the trigger signal's duration, ensuring that it remains active for the entire length of the output pulse. Here's how it works:
1. Trigger Detection and Latching:
The trigger signal is applied to a Schmitt trigger inverter, which provides a clean and noise-free transition from low to high. The high output of the inverter sets a flip-flop, effectively latching the trigger signal.
2. Output Pulse Generation:
The output pulse is generated by the 555 monostable one-shot, as usual, with its duration determined by the RC time constant.
3. Trigger Signal Extension:
The output of the flip-flop is used to keep the Schmitt trigger inverter's input high, extending the trigger signal for the duration of the 555 monostable one-shot's output pulse. This ensures that the trigger signal remains active even after the initial short pulse is over.
4. Output Pulse Termination:
The output pulse from the 555 monostable one-shot is used to reset the flip-flop, terminating the extended trigger signal when the output pulse ends.
Implementing the Eliminator Circuit
The eliminator circuit can be implemented using a simple combination of logic gates:
- Schmitt Trigger Inverter: This can be realized using a CMOS inverter with hysteresis.
- Flip-Flop: A D-type flip-flop is commonly used for this purpose.
The following circuit schematic illustrates the basic principle:
+-------+
| |
Trigger| |
| |
+-------+
|
+-------+
| |
| D |
| |
Clock +-------+
|
+-------+
| |
| NOT |
| |
+-------+
|
+-------+-------+
| | |
| | |
| 555 | |
| | |
| MONOSTABLE |
| | |
+-------+-------+
Components:
- 555 Timer IC
- Schmitt Trigger Inverter (e.g., 74HC14)
- D-Type Flip-Flop (e.g., 74HC74)
- Resistors and Capacitors (as required by the 555 one-shot)
Benefits of Using an Eliminator Circuit
- Enhanced Reliability: The eliminator circuit ensures that the trigger signal remains active for the entire duration of the output pulse, improving the reliability of the system.
- Simplified Design: It eliminates the need for complex timing circuitry or additional logic to compensate for the short trigger duration.
- Flexibility: The eliminator circuit can be adapted to various applications by adjusting the time constant of the 555 monostable one-shot and the specific logic gates used.
Conclusion
The 555 monostable one-shot eliminator circuit is a valuable tool for overcoming the limitations of traditional 555 monostable one-shot configurations where the trigger signal might be shorter than the output pulse width. By effectively extending the trigger signal, this circuit ensures reliable operation and simplifies system design. Its versatility allows for diverse applications, from simple timing circuits to more complex systems requiring precise control.