555 Timer Latch Circuit Tutorial

There are different types of PCB components that are commonly used in electronic projects and circuits and we are familiar with them, 555 timers integrated circuit is one of them. Its common types of operations are monostable, multiple stable, and bistable. With that also used for switching operations. It operates like a switch if we push it then start operating the circuit and if pushed again breaks the circuit like a normal switch. It is also used in digital electronics to control different components like Arduino controllers and other types of sensors. 

Here we will use 555 timers and create the latch circuit and discuss its operation. But before discussing the circuit first of all we learn the 555 timer introduction that helps to get the details of the circuit.

A brief introduction to 555 timer

  • The 555 timer is a standard part of electronic circuits and electronic projects. Its main function is to provide the time delay oscillator and flip-flop operation.
  • This integrated circuit was created by Gignetics in 1971 that is an American company. As it is an older component but is still used in projects due to its stable nature, less cost, and easy-to-use features.  It is created by numerous manufacturers in low-power CMOS and bipolar structures. It has been created in larger numbers than used in electronic devices. On basis of its manufacturers comes in 25 transistors, 15 resistors, and two diodes on a silicon chip mounted on eight-pin dual in-line packaging.
  • It is a less expensive integrated circuit that is used for different values of potential differences circuits like 4.5 to 15 volts DC.
555 timer circuit diagram

The role of the 555 timer pinout

The 555 timer has 8 pinouts and each of them plays a different role. The following FS PCBA will explain them:

  • Pin 1 and 8 are attached to ground and Vcc through a combination of five-kilo ohm three resistors. These three resistors are configured in such a structure that makes a voltage divider combination;
  • Pin 2 is the trigger pinout that is connected with the inverting input or negative point of the comparator;
  • Pin 3 is the output pin that is connected to the output of the flip-flop;
  • Pin 4 is the reset pinout connected to the flip-flop’s reset pin. By making a connection with the ground timer can be reset. It helps to reset the integrated circuit when there is negative signal is given at the reset terminal of the transistors connected;
  • Pin 5 is attached to inverting of the comparator. To change the reference voltage external volts can be provided here. This terminal is normally attached to a capacitor to get stable reference volts. It helps to control threshold and trigger levels. It is also used for control of the pulse width of the output signal when voltage is given at this terminal;
  • Pin 6 is attached with the noninverting input of comparator circuitry that has output attached to the reset pin of flip-flop;
  • Pin 7 is the discharged pin that is configured to the collector of BJT;
  • Pin 8 is used to provide the +5V to +18V values.

555 Timer Operating Modes

  • Astable Mode

In this mode of operation there is no stable level exists at output and output will be between high and low. The signal at the output terminal switches to low and high without any application of an outer trigger. There is a continuous pulse generated according to the values of resistors and capacitors connected in the circuitry.

555 Timer IC Circuit in Unstable Mode
Astable 555 timer circuit waveform diagram
  • Monostable Mode

This mode of operation of 555 timers has stable and unstable conditions. If the output is high at a stable then the output of the timer is high. There is a single pulse generated when the indication is given to the timer from the trigger button input.

555 timer circuit diagram in monostable mode
  • Bistable modes

In this mode, the timer has stable output states. During every interrupt output varies from low to high respectively for high output. If it is low get the interrupt and retain the low state till other interrupts vary the states. This mode of operation is also called Schmitt Trigger mode. It is used for applications where the load is continuously on and off through the use of a push button.

Bistable mode 555 timer IC circuit diagram

Create a 555 Timer Latch Circuit

We have a general understanding of the 555 timer, now we are going to start making the latch circuit shown in the figure above.

Notes: Above we have discussed the pinout of 555 timers. So Pins 2 and 6 are trigger and threshold pins. So during circuit operation where the voltage will be noted. If the voltage value at pin 2 is less than 1/3 of the input volts then pin 3 or the output pin will be on and if the voltage at pin 6 is less than 2/3 of the input volts then pin 3 or the output pin will be off.

The components required for the 555 timer latch circuit are listed here:

  • 555 Timer
  • LED
  • SPDT Relay
  • 1n4007 Diode
  • BC557 PNP transistor
  • Resistors

Circuit Diagram and Operation of 555-Timer latch Circuit

Here we can see the circuit of the 555 timers latch. In the circuit pin, 2 and pin 6 are used. The output of the voltage divider circuit is attached to pin 6. There is a 1uF capacitor used to make the connection of one resistor of the voltage divider to pin 3 through a 100k resistor. Push button configured between pin 2 and the positive terminal of the capacitor. We can see that there is a push-button connected between pin 2 and the capacitor positive pin. Led connected at the output of timer through current limiter resistor.

In the circuit, there are two resistors of 220-kilo ohms making the voltage divider circuitry. This divider circuit provides the volts to pin 6. Let’s suppose that the circuit is in ON state voltage divider and has a balanced state so the output is in an off-state. If we press the push button in the results capacitors get started the charging through R3 resistor and more current passes through this resistor which makes the unbalanced state. It varies the volts value at pin 2 so the output is in the ON state. If we press the button so pin 6 gets volts of charged capacitors and off the output.

Latch circuits have two types: active high and active low. For active high, the input is ground and the latch is triggered through the high signal. While active low has high input and a low signal is used for triggering the latch.

Our designed circuit is operating as a toggle switch scheme. There are two states shown here high and low. This circuit will operate till the point we push the button to reset the setting and will be in an off state till we again adjust the set condition. Its operation is similar to the flip-flop.

Latch Circuit With a combination of Transistor and 555 Timer

There are different combinations through which latch circuits can be designed to perform operations. As its main function is to provide continuous power to load either in case of the absence of an input supply. So its circuit can be designed in different schemes through the use of transistors, IC, etc. Here we have created another latch circuit through the use of transistors and a timer.

There is a 2n3904 transistor is used in this circuit that is NPN configured which means it has two N and one P region. The main operation of transistors is a switch and amplifier. Transistor is the very basic component of electronics and is considered the main part of any electronic project. In this circuit, it is working as a switch and controls the pulse signal connected to its base terminal. Here you can see that at the sixth terminal of the timer, the capacitor is connected which provides a pure signal free from ripples. At input positive pulse signal is given that is latched with the output of 555 timers. In this circuit, we are getting 200 milliampere output current which has the ability to operate the relay switch and can be used to operate any electronic device. The operating volts given to this circuitry are 5 volts to 12 volts dc.  In this circuit at the base terminal of the transistor, there is 2.2-kilo resistance is attached that is working as current limiting and limits the current up to a limited value to avoid the transistor from any damage.

Some Applications of 555 Timer Latch Circuit

  • Latches circuits are used in different types of storage devices;
  • It is used for data storage since it is a single bit storage module;
  • It is also used for encoding binary numbers;
  • It used to control the different devices in digital electronics and machines.

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