This circuit will change a 'momentary / pushbutton' switch into a always on switch. If you would like to turn it off, you will need another switch. Other methods of operating can be used - I used an automotive 'Car remote control central door locking system' (Ebay - Car remote entry) so I could remotely control the light or whatever you would like to keep on. Please read through the whole instructable before starting!
In this Instructable I assume you know how to solder and that you are using 12 volts. Also I assume you have a general knowledge of relays and common sense.
WARNING - If your output device requires a voltage above 24 volts (E.g. 110v / 230v mains) then do not attempt unless you absolutely know what you are doing!
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Step 1: Gather Resources
You will need
1 - x2 5 pin Relays; x3 if you want the output to be a switch. 3rd 'Switching' relay needs to be able to handle the load for your output device!
2 - Wire - Insulated at least 22 AWG
3 - Battery - Use your car battery, jumpstater, anything that will give 12 volts
4 - Fuse Holder (optional) - So it wont blow up if you accidentally short
5 - Solder, Soldering Iron, Insulation tape - For soldering
6 - Light, or other load on the other end of your Relay
7 - 2 momentary push button / switches
Step 2: Build the Circuit
By now you will have needed to decide whether or not you need the output to be a voltage output or a pair of closed (connected) wires as your output.
I am going to assume that you need the closed wires as this requires one extra step.
0. Attach the fuse to a length of wire and connect one end to the positive terminal of your battery. The other end is the 'Positive Always on wire'.
1. Cut two lengths of wire as your input wires and solder them onto one of the two trigger pins (85 or 86) indicated in the picture. Do this for Both relays. These will be the input pins
2. Cut another two lengths of wire and solder then onto the other trigger pin (85 or 86). Do this for Both relays. These will be the ground pins.
3. On the 1st relay, solder a wire to pin 30. Solder the end of the wire to pin 87a of the second relay (refer to picture).
4. Solder a wire onto pin 30 of the second relay. Solder the other end onto the Positive Always on wire.
5. Solder a wire onto pin 87 of the first relay and connect it to the input pin in step 1
6. Solder a wire from pin 87 of the first relay. This is the output wire (positive out).
7. Attach an end of the Momentary switch to the wire connected to the input pin. Do this for both relays.
8. Connect the other ends the the momentary switch to the positive always on wire.
The Pair of Power out wires is the output wire and a wire connected to ground. The next step has a picture that is helpful.
I just connected a 12v light to the power out wires and I didn't use the switching relay.
Step 3: Add Switching Relay
The switching relay is simple -
Take the power out wires and connect them to pins 85 and 86 of the relay and pins 30 and 87 will be you always on or off switch.
Step 4: Use!
To use this Latching relay circuit, just press the button connected to Relay 1 and the load will stay on even if the button is released. To stop, just press the button connected to relay 2 and the load will turn off and stay off until you push the 1st button again.