LM555 Timer Monostable AC Power Phase Control Circuit.
Click for larger image.

LM555 Timer Monostable AC Power Phase Control Demo

More detailed parts schematic using external +5V supply.

You Tube video LM555 Timer Monostable AC Power Phase Control

This article aims to combine several other smaller circuit designs into newer, more complex designs.

In the past, an Arduino used interrupts from a zero-crossing detector and a software routine to control the phase angle of AC power with Triacs and SCRs. This circuit will use an LM555 timer in the monostable mode to do the same thing.

Referring to the above circuit diagram, diode bridge rectifier D1 and U1, an H11L1 digital output optocoupler with transistor Q1, generates a narrow, negative-going zero-crossing pulse. The pulse created by the D1 (full-wave rectification) is 120Hz.

This pulse triggers the LM555 (pin 2), producing a high output (pin 3), keeping the MOC3011 turned off. When the output goes low (0V), the Triac or SCR circuits connected to the MOC3010 turn on because the optocoupler LED turns on.

The optocoupler LED is connected in the "sink" configuration and turns on only with a low output. At 60Hz, the period is 16.7mS with a half-cycle pulse width of 8.35mS. A zero-crossing pulse triggers the LM555 monostable every 8.35mS.

The LM555's on-time is determined with the 100 K potentiometer, 1K resistor, and a 0.1uF capacitor. The formula is 1.1 * R * C. By adjusting the 100K pot, one can vary the output on time from 0.5 mS to 8.3 mS.

The longer the delay, the less power is delivered to the output. This is best illustrated by the following images.

LM555 Timer Monostable Circuit.
Click for larger image.

LM555 Monostable Operation

The LM555 timer, commonly known as the 555 timer, was invented in 1971 by Hans Camenzind while working at Signetics Corporation. The design has been around for over 54 years.

The LM555 is an integrated circuit (IC) used as a timer, pulse generator, and oscillator for various electronics applications. It operates with a wide range of supply voltages from 4.5 to 15 volts. The chip features eight pins for multiple connections: power, ground, trigger, output, reset, control voltage, threshold, and discharge.

The LM555 has been used in timers, pulse generation, LED flashers, PWM (Pulse Width Modulation), tone generation, and other analog and digital circuits.

Due to its effectiveness and robustness, its design has remained largely unchanged, making it a staple in electronics for both beginners and professionals.

Two main operating modes:

Astable (free-running) mode, where it generates a continuous square wave.

Monostable (one-shot) mode, where it produces one output pulse for each trigger signal. That is the mode used in this project.

See LM555-NE555 One-Shot Multivibrator AC Power Control

The image above shows a typical monostable (one-shot) configuration of the LM555 timer IC, which generates a single pulse when triggered.

When the push button (PB1) is pressed, pin 2 (Trigger) momentarily pulls low, triggering the timer.

The output (pin 3) goes high, turning on the LED, and the capacitor C1 begins to charge through R1.

Once the voltage across C1 reaches 2/3 of the supply voltage, the output goes low, turning off the LED, and the capacitor discharges.

Pressing PB1 creates a single pulse at the output, the duration of which is determined by the formula: t = 1.1 × R1 × C1.

Timing Diagram: The diagram on the right shows the behavior of the trigger (pin 2), output (pin 3), and discharge (pin 7) pins:

Pin 2 (Trigger): when low triggers the timer;

Pin 3 (Output): Goes high when triggered and stays high for the time determined by R1 and C1, then goes low;

Pin 7 (Discharge): Initially low, goes high during the timing period, allowing C1 to charge, then goes low again to discharge C1.

This circuit produces a precise time delay or a one-shot pulse after an event, like timers, debouncing switches, or simple one-shot LED blinkers.

H11C4 SCR output optocoupler controlling a high current SCR as a half wave rectifier.

Optocoupler Triggering of SCR

SCR part number BT152-800R, 800V, 13A.

Alternate SCR Half Wave rectifier with MOC3010 Triac output optocoupler.

(Above) H11C4 SCR output optocoupler controlling a high current SCR as a half-wave rectifier. In this application, the SCR acts as a phase-controlled half-wave rectifier producing a pulsating (unfiltered) DC voltage.

Power Supply: The circuit operates with a 120VAC input, typical for main power applications.

H11C4 Optocoupler: This device uses a photo SCR (Silicon Controlled Rectifier) output to isolate the low-voltage control signal from the high-voltage part of the circuit. When the control signal lights the internal LED of the H11C4, it triggers the photo SCR, which then provides the necessary gate current to turn on the larger external SCR.

The 47kΩ resistor is connected to stabilize the operation of the H11C4 optocoupler SCR gate circuit, ensuring proper current flow to the LED inside the H11C4 for reliable triggering of the photo SCR.

The 270Ω resistor limits the gate current of the larger SCR to a safe level and the other 270Ω limits the current in the control circuit of the H11C4 LED.

The SCR (Silicon Controlled Rectifier) acts as a switch for the AC load. When the photo SCR in the H11C4 conducts, it provides the gate current to trigger this larger SCR, allowing current to flow from the Anode (A) to the Cathode (K), thus energizing the load (B1).

B1 represents the load connected across the output, which could be a battery or another device. In this tutorial, B1 is a 120VAC lamp.

When a signal is sent to the H11C4's input (from a microcontroller or similar device), the LED inside the optocoupler lights up.

The light from the LED activates the photo SCR within the H11C4, which then conducts, providing gate current through the 270Ω resistor to the gate of the larger SCR.

This gate current triggers the larger SCR, allowing current to flow from the Anode to the Cathode, thus energizing the load (B1).

The larger SCR will remain on until the current through it drops below the holding current, which typically happens at the zero-crossing point of the AC cycle.

Output Calculation: The output voltage calculated formula is output = RMS * 0.9 / 2 = 54V.

This formula suggests that for a 120VAC RMS input, the output across the load is approximately 54V. The 120VAC RMS voltage is multiplied by 0.9 to account for some voltage drop or efficiency factor, then divided by 2 because the SCR conducts only during one-half of the AC cycle (since it's a simple rectification without smoothing).

This setup is commonly used in applications where high-power AC loads need to be controlled with low-voltage DC signals, ensuring safety through the isolation provided by the optocoupler.

The H11C4's photo SCR output allows for direct triggering of the larger SCR, making it suitable for applications like light dimmers, motor control, or any scenario requiring phase control of AC power.

LM555 Timer Monostable Circuit.
Click for larger image.

The H11L1 as used with diode bridge D1 and associated components to form a zero-crossing detector. This produces (with the pullup resistor) a positive-going spike when the AC waveform crosses zero.

The above image shows a switching test circuit and its corresponding waveforms for the optocoupler models H11L1M, H11L2M, and H11L3M. Here's a breakdown of the circuit and waveforms:

Optocoupler (H11L1M, H11L2M, H11L3M): The Optocoupler consists of an LED and a photodetector. The LED is on the input side, and the photodetector is on the output side. The light emitted by the LED triggers the photodetector.

Output Side (Right Side of the Optocoupler): The photodetector (inside the Optocoupler) detects the light from the LED and controls the output current. RL is a pullup resistor with a value of 270 Ω. The 0.1 µF Capacitor is for filtering or stabilization purposes. Vo is the output voltage referenced to the ground.

The output configuration shown in the circuit diagram for the H11L1M, H11L2M, and H11L3M optocouplers has a Schmitt trigger with an open-drain output.

Schmitt Trigger: The hysteresis behavior seen in the waveforms (the different thresholds for switching on and off) suggests that the Optocoupler is in a configuration that provides Schmitt trigger functionality.

This design means it has two different threshold levels for turning on and off, which helps in noise immunity and provides a clean switching action.

Open Drain: The output side of the circuit shows a connection to a pullup resistor (RL) and a 5V supply. The open-drain configuration means that the output transistor inside the Optocoupler can only sink current (pull the output low) but cannot source current to pull it high.

When the transistor is off, the pullup resistor pulls the output high to 5V. This configuration is typical for open-drain outputs, where an external pullup resistor is necessary to establish a high state.

This setup is widespread in digital logic applications where signal isolation with noise immunity and specific switching characteristics are required.

Vin: The input voltage waveform is a square wave transitioning between 0V and 50% of some reference voltage.

Vo: The output voltage waveform shows the circuit's response to the input signal. It has a rise time (tr) and fall time (tf), which are defined as the time taken for the output to change from 10% to 90% and from 90% to 10% of its final value, respectively.

Similarly, ton and toff represent the turn-on and turn-off times, which are the times taken for the output to reach 50% of its final value from the initial state.

Key Points:
Rise Time (tr): Time for Vo to go from 10% to 90% of its high state;
Fall Time (tf): Time for Vo to go from 90% to 10% of its low state;
Turn-On Time (ton): Time for Vo to reach 50% of its high state from the low state;
Turn-Off Time (toff): Time for Vo to reach 50% of its low state from the high state.

This setup is typically used to test the switching characteristics of optocouplers, which are crucial for applications requiring signal isolation and high-speed switching.

The above material credit GROK the X AI with additions by Lewis Loflin.

View all of my You Tube Videos
Also visit and subscribe to My YouTube Channel

I've been a part-time adjunct professor at a local community college teaching electricity and electronics.

Today I do this for the shear love of electronics.

I have 45 years experience in electronics, from vacuum tubes to modern solid state and industrial controls. I tend to teach from a general science viewpoint.

Some sample projects. New 7/15/2024 on my electronics website.