555 timer as schmitt trigger

555 timer as schmitt trigger
555 timer as schmitt trigger

The 555 Timer is one of the most popular and flexible ICs ever. Billion-plus 555 timers are made annually. Its low cost, stability, and usability make it popular. Hobbyists love the 555 Timer because it can do numerous functions depending on how it is wired and is cheap and easy to use. Despite its popularity, the 555 Timer has drawbacks. 555 Timers do not offer clear digital HIGH and LOW voltages. Due to this limitation, a 555 Timer and Schmitt Trigger may be used.

Ever needed a schmitt trigger but didn’t want to design a circuit? The dependable 555 timer chip can be connected as a basic schmitt trigger with resistors and capacitors. Here’s a clever way to use a 555 timer as a schmitt trigger with a few extra pieces. You presumably have a 555 and other simple components, so you can try this on a breadboard quickly. How to determine resistor and capacitor values to set thresholds. You’ll learn how to make a 555 timer schmitt trigger for your next project before the end! Sound intriguing? Discover how it works!

Overview of the 555 Timer

The transistor turns off and the capacitor charges through the resistor, changing its value from 0V to 2/3Vcc when the output is LOW. Trigger and comparator 2 receive this rising voltage. Comparator with reference voltage switches low to high at 1/3Vcc. The flip-flop flips states when the comparator raises the trigger voltage above 1/3Vcc and the threshold voltage exceeds comparator 1’s 2/3Vcc. The transistor powers and connects 0V to the timing capacitor. After the capacitor discharges through the transistor at its rated current, the flip-flop resets when pin 6’s voltage crosses 1/3Vcc, completing a cycle.

The 555 timer, the most popular chip of the early 1970s, needs no introduction. Thus, Swiss inventor Hans Camenzind sold 8 billion pieces annually. Two voltage comparators set trigger and threshold values, an RS flip-flop changes state on the high-to-low transition of the trigger pulse, and a quasi-stable (monostable) multi-vibrator is triggered by it.

Functioning of a Schmitt Trigger

The 555 timer is often called Schmitt Trigger. A complex net of resistors, transistors, and comparators form the Schmitt Trigger, an inverter. The Schmitt Trigger outputs digital values (1 or 0), making signal transmission and analysis easier. Imagine a sensor-output sine wave. This oscillating sine wave crosses the Vthresh+ and Vthresh- thresholds. The Schmitt Trigger replaces this sine wave with a square wave.

What Is a Schmitt Trigger and How Does It Work?

Schmitt triggers are comparator circuits that abruptly flip output when inputs exceed threshold levels. It features higher and lower input voltage thresholds. Input above the upper threshold produces high output, whereas input below the lower threshold produces low output.

The Schmitt trigger hysteresis precludes rapid output switching when the input signal is noisy or varies slowly around the threshold level due to this dual threshold mechanism. Difference between thresholds is hysteresis voltage.

Hysteresis is implemented by positive feedback from the Schmitt trigger. The output gets high when input voltage exceeds the threshold. The input voltage is raised by feeding back this high output. For the output to drop again, this higher input voltage must fall below the lower threshold. The output drops when the input drops below the lower threshold. Low output is sent back to input, lowering input voltage. The input must exceed the upper threshold again for the output to rise.

Due to its positive feedback and hysteresis, a Schmitt trigger output is less likely to switch rapidly than a comparator. It only changes state when the input is far from the hysteresis voltage range. This “triggering” tendency lends the Schmitt trigger its name.

Schmitt triggers are utilized as square wave generators, signal conditioners, and waveform shapers. Using the 555 timer integrated circuit as a Schmitt trigger is a simple alternative to an op amp.

Overall, a Schmitt trigger is a comparator circuit with positive feedback hysteresis. After crossing one of two input voltage thresholds, it switches output abruptly yet dependably. Hysteretic characteristic makes it noise-immune and beneficial for many applications.

The 555 Timer IC Explained

The 555 timer IC is used in many electronics projects. This popular, low-cost timing circuit delivers accurate time delays or oscillation. The 555 timer, named for its three 5kΩ resistors, operates from 4.5 to 18 volts.

The 555 timer has two comparators, an RS flip-flop, a discharge transistor, and a resistive voltage divider. Comparators receive inputs from threshold and trigger pins. Comparator output controls flip-flop. It controls the discharge transistor.

Main modes of the 555 timer are monostable and astable.

Monostable Mode (One-shot)

In monostable mode, the 555 timer generates one-shot pulses. After a trigger input, it emits a single pulse of a defined duration. An external RC network coupled to the threshold and discharge pins controls pulse length. Monostable mode helps create clocks, delay circuits, and missing pulse detectors.

Astable Mode (Oscillator)

Astable mode uses the 555 timer as an oscillator, providing a precise frequency of pulses. Again, external RC network parameters determine frequency. For clock signals, pulse-width modulation, and pulse generators, utilize astable mode.

Due to its low cost and simplicity, the 555 timer is used in many DIY electronics applications. Timers, oscillators, and pulse generators can be made with a few external components. 555 timers are fantastic ICs to understand and experiment with for electronics beginners.

Using the 555 Timer as a Schmitt Trigger

555 timer ICs can be Schmitt triggers, hysteresis-based comparator circuits. This creates a hysteresis loop since the 555 output changes states when the input voltage reaches a threshold, but the inverse change threshold is different.

Connect pins 2 and 6 to configure the 555 timer as a Schmitt trigger. The hysteresis loop switch points depend on these pins’ voltage. Connect a potentiometer between VCC and GND and a wiper to pins 2 and 6. This lets you alter trigger points.

Pin 3 goes high when pins 2 and 6 exceed 2/3 VCC. To lower the output again, the voltage must drop below 1/3 VCC. The hysteresis between the high and low thresholds ensures quick switching and noise rejection.

Benefits of a Schmitt trigger include:

Noise immunity

Schmitt trigger hysteresis prevents noise and voltage spike-induced switching. The input voltage must shift significantly to flip output states.

Fast switching

Schmitt triggers swap states quickly, enabling clean pulses. Many digital circuits use Schmitt triggers due to their noise immunity and high-speed switching.

Squaring up signals

A Schmitt trigger’s quick switching can “square up” analog signals into digital pulses. Analog signals that are slightly asymmetric or noisy become symmetrical digital pulses.

Making a 555 timer a Schmitt trigger is a simple hack that can improve your circuits. Hysteresis can be customized with a few external components. Have more questions? Let me know!

555 Timer Schmitt Trigger Circuit Design

A Schmitt trigger circuit with a 555 timer requires a few simple components:

  • 555 timer IC
  • Two resistors
  • One capacitor

Popular ICs like the 555 timer can be configured as Schmitt triggers. This circuit changes between stable states when an input exceeds a threshold.

Connect one resistor between pins 6 and 7 of the 555 timer, one between 6 and 2, and the capacitor between pins 1 and 8 to form the Schmitt trigger. Choose resistor values based on the threshold voltage you want to trigger the state change.

Pin 3’s output is high when pin 2’s input voltage is below the lower threshold. The output becomes low as input voltage exceeds the upper threshold. This hysteresis effect prevents the output from returning to high until the input drops below the lower threshold again.

The key is choosing resistor values to set your application’s threshold voltages. To find the proper thresholds, try different resistances. Start with 100K ohms for the resistor between pins 2 and 6 and 47K for pins 6 and 7.

This basic Schmitt trigger 555 timer circuit can be used in projects that require an output that changes state when an input signal reaches a threshold. You turned the 555 timer into a Schmitt trigger with a few standard parts.

555 timer as schmitt trigger
555 timer as schmitt trigger

Applications of the 555 Timer Schmitt Trigger

Schmitt trigger 555 timers have several uses. Square wave oscillators are used to create clock signals. The output will fluctuate between high and low states when connected to the trigger input, forming a square wave. The RC time constant of pins 2 and 6’s resistor and capacitor determines oscillation frequency.

A simple voltage detector is another usage for the 555 Schmitt trigger. By adjusting the higher and lower threshold voltages using resistors, the output switches states when the input voltage crosses one. This alerts you whenever the voltage exceeds or falls below a set range.

555 Schmitt triggers make good noise immunity circuits. Because of the hysteresis between the higher and lower thresholds, it ignores minor voltage spikes and glitches that could cause a basic comparator to flip states incorrectly. This helps avoid erroneous triggering in loud conditions.

Using the 555 Schmitt trigger as a touch sensor is ingenious. When you touch a conductive plate or touch pad, pins 2 and 6 detect your body’s capacitance, changing the output state. It’s easy to make touch-activated switches and controls.

Another use is an electronic switch debouncer. Switch bouncing occurs when mechanical switches close or open. A switch connected to a 555 Schmitt trigger’s input ignores transient pulses and only changes output state when the input stabilizes above or below the threshold, debouncing the switch.

As shown, the Schmitt trigger 555 timer is helpful for analog signal processing, timing, and control. With a few external components, this basic IC can execute numerous sophisticated functions. Try different circuit layouts to see what you can make!


It’s easy to make the 555 into a Schmitt trigger with a few more parts. Adjust trigger points to suit your needs by changing resistor values. This 555 hack can clean digital waveforms or reduce signal noise. The best aspect is that the parts are cheap and readily available. Try this circuit with a 555, resistors, and capacitors from your parts drawer. What additional smart tricks can you do with this vintage chip? Experimenting opens unlimited possibilities. The article should inspire your inner tinkerer. Time to hack!

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