In electronics, a device is said to be a latching device if it maintains any particular fixed state even after removal of the input signal. The same also applies for electronic/electromechanical relays. Basically the electromechanical relays that are used today are of two types:

1. Non-Latching Relay : These relays are most widely used where energy consumption is basically not an issue.This type relay(s) comes back to its original state once the input signal is removed.

2. Latching Relay : These relays are mostly used in automobiles and are rarely used for basic prototyping needs.

Most of the relays which we use today are monostable relays, that means it has only one stable state and so it is called non-latching relays. You must have heard about N.O (Normally Open) and N.C (Normally Closed) terminals of a basic electromechanical relay. However these terminals which are present over the relay show that the electronic relay which we are using has only one stable state.

In this instructable I will show you how to use a non-latching relay as a latching bistable relay by designing a simple electronic module which is powered by an external power source. One can use this module for many other projects and it's intentionally made portable so that one can easily carry it around. Moreover it has many other useful features which provides it many additional functionalities.

Step 1: Gather the Required Components and Tools.

All the tools and components used to make this module can be easily bought from nearby local electronic shops.

COMPONENTS REQUIRED:

1x Small Size Perforated Circuit Board
1x 3 PIN PCB Mount Screw Terminal
1x 5V-DC 240V, 50-60Hz, 5A SPDT Relay
3x BC547 NPN Transistor
2x Small LEDs (Different colors are recommended.)
2x 330ohms Resistors
2x Momentary Push Button Switches (Different colors are recommended.)
1x 1N4007 General Purpose PN Junction Diode
1x NE555 Timer IC
4x Male Header Pins
3x 10Kohms resistors
2x 15Kohms resistors

TOOLS REQUIRED:

Soldering Iron
Soldering Lead Wire
Soldering Flux
Multimeter with Probes
A strong Adhesive

Step 2: The Circuit Diagram

The above shown picture is the circuit diagram of the relay module. All connection of components have to be done in exact same manner as shown in the picture.

The above circuit is an implementation of the NE555 timer IC as a Latch or 1-Bit Memory Cell. The Set (ON) and Reset (OFF) function of the attached relay can be controlled by either using the momentary push buttons as switches or it can be controlled via 5V logic inputs from the SET or RESET pin sides.
The the type of latch that we are using here is SR latch.
In the circuit diagram the pin jack J1 refers to the power inputs and logic controls of the module, whereas the pin jack J2 refers to the 3-PIN screw terminal outputs from the relay.

The following are the inputs required for functioning of module:

VCC - 5V DC (You can take it from a USB port)

GND - Negetive or GROUND connection.

SET INPUT - Preferably Greater than 0V to turn the relay ON

RESET INPUT - Preferably Greater than 0V to turn the relay OFF

NOTE*: The SET signal and RESET signal should not be supplied at the same time, if done so, it will change the state of the relay to OFF state by default .

Step 3: Placing of Components

One may follow his or her own way to place the components on the perforated board. However the procedure that I followed has been explained as follows:

Start with the screw terminal and the relay. First place the screw terminal at one of the ends of the perforated board and glue it to the board by using the strong adhesive if needed.
Then place the relay carefully. Middle terminal of the relay will mostly not align to the holes of the perforated board. I recommend not to forcibly align relay to the circuit board. Use a screwdriver or some kind of drilling machine to enlarge the hole and then install the relay.
Place the diode right next to the coil side of the relay. This diode acts as a Flyback diode in this circuit.
The gradually step by step place all the other components and arrange it in such a way that only a minimum traces of solder would be required to connect the terminals.

Step 4: Soldering Process and Testing

1. First solder the relay's terminals to the Screw Terminal. Remember to make thick traces of solder for this step because sometimes the relay may need to bear heavy loads. During heavy load applications, the solder joints may melt down due to heat.
2. Then solder rest of the components to the circuit board. If cross over conditions arise, use small insulated wires to join the terminals.
3. Once the soldering processes is completed, check for continuity using the multimeter.
4. After continuity testing go for actual testing of the module with 5V as input and connect any electrical load to the relay terminals.
5. To avoid the risk of electric shock or short circuit, you may put on an insulation tape at the base of the relay terminals. (OPTIONAL)

Step 5: Final Pinout and Layout Diagram

1. SET BUTTON - This switch is blue colored and it is used to turn the relay to ON state. The relay will continue to be in ON state even after the force is removed from the Blue button until and unless the Reset button is pressed or reset signal is provided to the module.
2. RESET BUTTON - This switch is red colored and it is used to turn the relay back to OFF position.
3. VCC - The positive 5V supply must be given to this terminal for working of the module.
4. GND - The ground terminal(s) of any input(s) should be connected to this terminal.
5. SET Input - This pin if applied will a positive voltage will turn the relay ON.
6. RESET Input - If a positive voltage is applied to this pin then it will bring the relay back to it's original OFF state.
7. GREEN LED - This is the power indicator LED which glows when VCC and Ground terminals are connected to a 5V power supply.
8. BLUE LED - This LED shows the status of the relay. It glows when the relay is ON and turns off when the relay is OFF.

Step 6: Using the Module With Board Mounted Switches.

The SET and RESET buttons present on the module provides manual control of the relay.
These buttons can also be used simultaneously with the control input signals that come from the SET and RESET terminals.

Step 7: Implementation of the Module With a Microcontroller.

This module can also be controlled by using any microcontrollers. One has to only connect the SET and RESET input terminals of the module to the digital outputs of the microcontroller with a common ground.
1. In the above shown pictures of this step, I have connected the relay module to a NodeMCU and configured pin numbers D1 and D2 as its outputs.
2. Then by the help of blynk application I controlled the ON and OFF functionality of the relay module wirelessly over Wi-Fi.

Step 8: Using the Module As a Touch Switch.

The relay module can also be used as a touch switch.This is because BC547 transistor easily turns ON even with a small base current. So even a small static charge of our body can be applied to the base terminal of these transistors to control the ON and OFF functionality of the relay module.

Step 9: Advantages & Conclusion

ADVANTAGES:
The advantages of this relay module as compared to the conventional relay module are as follows:
1. Unlike the normal relay modules which mostly work under specific logic input voltage such as 5V or 3.3V, this relay module can accept a wide range of input voltage or current signals to control the switching action.
2. The latch mechanism of switching is usually used in controlling circuits which have unstable (pulsating DC) or time varying input signals.
3. Wide range of operating voltage- One can easily use this module with any relays which have coil voltages starting from 5V to 18V. Depending upon the coil voltage, one can supply the VCC voltage accordingly.
4. The state of the relay is stored even after the control input signals are removed.

CONCLUSION:
This relay module just provides a new way to control the relays. Not only this can be used to control electromechanical relays but also it can be used to control solid state relays.
Basically this relay module is an upgrade over the existing relay modules and provides various features and functionalities.