Introduction to the 555 Timer and Oneshot Circuits

The 555 timer is one of the most popular and versatile integrated circuits ever produced. This simple 8-pin chip can be configured in a variety of ways to create useful timing and control circuits. One of the most common applications is the oneshot or monostable mode, where the 555 timer outputs a single pulse of a set duration in response to an input trigger signal.

Oneshot 555 Timer Circuits have a wide range of uses, including:

• Debouncing switches and buttons
• Creating time delays
• Generating reset pulses for digital circuits
• Controlling the duration of events like LEDs flashing or relays switching
• Pulse Width Modulation (PWM) for motor speed control and dimming lights

In this article, we’ll take an in-depth look at how 555 oneshot circuits work, how to design them for different pulse durations, and some practical application examples. Whether you’re an electronics hobbyist, student, or professional, understanding monostable multivibrator designs using the 555 timer is a valuable skill to have.

How a 555 Timer Works in Monostable Mode

The block diagram of a standard 555 timer chip looks like this:

The key components are:

• A comparator that monitors the voltage on the Threshold pin (pin 6)
• A comparator that monitors the voltage on the Trigger pin (pin 2)
• A flip-flop that is set or reset by the comparator outputs
• A discharge transistor connected to pin 7
• An output stage that drives the output pin (pin 3) high or low

In the monostable configuration, the 555 timer works as follows:

1. In the stable state, the trigger comparator output is high, keeping the flip-flop reset. This makes the output low and the discharge transistor on. The timing capacitor is held discharged.

2. When a negative trigger pulse is applied to pin 2, the trigger comparator output goes low, allowing the flip-flop to set. This makes the output high and turns off the discharge transistor.

3. With the discharge transistor off, the timing capacitor begins charging through the timing resistor. The capacitor voltage ramps up exponentially.

4. When the capacitor voltage reaches 2/3 of the supply voltage, the threshold comparator output goes high, resetting the flip-flop. This makes the output low again and turns on the discharge transistor.

5. The discharge transistor rapidly discharges the timing capacitor, bringing its voltage back to near zero. The circuit has returned to its stable state, ready for the next trigger.

The pulse width (time the output stays high) is determined by the values of the timing resistor (R) and capacitor (C). It can be calculated using this formula:

T = 1.1 * R * C

where T is in seconds, R is in ohms, and C is in farads.

For example, with R = 100k and C = 10uF, the pulse width will be:

T = 1.1 * 100,000 * 0.00001 = 1.1 seconds

Designing a 555 Monostable Circuit

To make a practical 555 oneshot circuit, we need to add a few things to the basic topology:

• A decoupling capacitor from pin 5 to ground to help reject power supply noise
• A pull-up resistor on the trigger input to keep it normally high
• A Bypass Capacitor on the control voltage input (pin 5) for additional stability

Here’s a complete circuit diagram:

The component values shown will give a pulse width of approximately 5 seconds. The trigger input is active low, so the output pulse is triggered by momentarily connecting the trigger input to ground, perhaps with a pushbutton.

To change the pulse duration, use the formula from the previous section to select new R and C values. Here are some examples:

A couple things to keep in mind when choosing component values:

• Very large Resistor Values (>1M) can make the circuit susceptible to noise and leakage current issues. It’s best to keep R below 1 megohm.
• Electrolytic capacitors are typically used for values of 1uF and up. These have a large tolerance, usually +80/-20%. The actual pulse width may vary significantly from the calculated value.
• For the most accurate and repeatable timing, use a good quality film or ceramic capacitor with 5% or better tolerance. Polystyrene and polypropylene types are good choices.

555 Oneshot Application Examples

Now that we know how to design a 555 monostable circuit, let’s look at a few practical ways they can be used.

Debouncing a Pushbutton

Mechanical pushbuttons and switches often generate spurious multiple transitions when activated due to contact bounce. This can cause problems if the signal is used to trigger digital logic circuits. A oneshot circuit can be used to debounce the switch, as shown here:

The 555 is configured to generate a short pulse (tens of milliseconds) when the button is pressed. Any additional bounces are ignored until the pulse is complete. The clean debounced output can then be used to reliably trigger other circuits.

Creating a Power-On Reset Pulse

Many digital circuits, especially microcontrollers, require a reset pulse at power-up to initialize them to a known state. A 555 oneshot can be used to generate this pulse:

When power is first applied, the 555 generates a pulse about 100ms long. This gives time for the supply voltage to stabilize and for the microcontroller to complete its internal reset sequence before the pulse ends and normal operation begins. The RC network on the trigger input provides a brief delay before the pulse starts.

Generating a Precise Time Delay

In some applications, you may need to trigger an event after a specific time delay from an initial signal. For example, you might want to turn on a cooling fan 30 seconds after a motor starts running. A 555 oneshot can create this precise delay:

The trigger input is connected to the motor’s power supply. When the motor turns on, the 555 starts its timing cycle. After the preset delay time (set by R1 and C1), the output goes high, energizing the relay and turning on the fan. The fan will stay on until the motor is turned off.

PWM Motor Speed Control

Pulse width modulation is a technique for controlling the speed of DC motors. The motor is rapidly switched on and off, with the ratio of on-time to off-time (the duty cycle) determining the effective voltage applied to the motor and hence its speed. Here’s a 555 circuit that generates a variable duty cycle PWM waveform suitable for motor control:

The 555 is configured in astable mode to generate a square wave. The duty cycle is adjusted by varying the resistance ratio of RV1. This changes the charge and discharge times and hence the on and off times of the waveform. The FET driver stage and power MOSFET Q1 handle the high current required by the motor.

Frequently Asked Questions

What is the difference between a bistable and monostable multivibrator?

A bistable multivibrator has two stable output states and will remain in either state until a trigger pulse causes it to switch to the other state. A monostable (oneshot) has only one stable state. When triggered, it will switch to the unstable state for a set time, then automatically return to the stable state.

Can a 555 timer be triggered by a positive pulse?

The standard 555 timer triggers on a negative-going pulse applied to pin 2. However, there are CMOS versions of the 555 such as the 7555 and TLC555 that can be triggered by either a positive or negative pulse, depending on how the trigger input is connected.

How do I make a 555 oneshot triggerable?

To make the 555 triggerable, pin 2 must be held high in the stable state, usually with a pull-up resistor to V+. The trigger pulse pulls pin 2 low to start the timing cycle. Once triggered, further pulses on pin 2 are ignored until the timing cycle is complete.

What is the maximum pulse width a 555 can generate?

The maximum pulse width is determined by the largest practical values for the timing resistor and capacitor. With R=1M and C=1000uF, the calculated pulse width is around 20 minutes. However, accuracy will be poor with such large component values. For long delays, it’s better to use a microcontroller or a dedicated timer IC.

Can I use a 555 to generate a delayed turn-off pulse?

Yes, you can use a 555 to generate a pulse that starts some time after the trigger input goes away. The circuit is similar to the power-on reset example, but with the trigger input connected to the normally-on signal (such as a motor). When the signal goes away, the oneshot is triggered, creating a turn-off delay.