The Most Efficient Latching Circuit

I am currently working on a wall mounted lamp, to turn on and off the light, the most efficient way would be to use a toggle switch. To be honest, I don't like them. A simple push of button to turn on is always way more satisfying.

Here comes the latching circuit to helps. There are variety of latching circuit with diff type of components. The relay-based circuit has an advantage over all of these as they consume zero amp when output is low, since there is no physical contact to the output. The major problem is relay consumes much power when turned on as compared to SSR or transistor-based latching.

Since efficiency is top most priority for a solar charged batter operated circuit. Let's dive into the instructable and take care of the relay’s hunger.

• A standard relay (latch relay can also be used but they are less available and priced higher)

• Neodymium magnet

• Two push button (I you want to make the standard latching circuit, you would also need a third non standard push button 'normally close' type, an old mouse would be great diy source)
• Three standard NPN transistor (like BC547,550 or 2N2222)
• Breadboard and connection wires
• A Voltage source

Let’s take a look at the standard relay latching circuit. You can check out
here for more detail. (https://www.youtube.com/watch?v=n7SuHDmuVUk), the only nonstandard component in the circuit is 'normally close' type switch. Which cuts the power to the relay when pressed.

According to my relay datasheet, the coil resistance is around 100ohm. Theoretically the relay would consume around 50mA till the circuit is on. That is too much when the total load current is 100mA. You can see in pics the circuit consumes around 47mA with no load.

Let's hack the relay.

The trick is to use a carefully placed magnet aligned to the armature of the coil. This makes the standard relay to the latching relay. Magnet alignment and polarity is important. To test the relay, use the same circuit from last step but without the NC switch. The relay must stays turn on without power once the NO switched is pressed.

Now new problem arises, relay never turns off once turned on. The solution would be to apply a negative pulse to the relay to turn it off.

This circuit encounter every problem we have in old circuits. The main focus of the circuit is to supply both negative and positive pulse to the relay. The left switch (see circuit diagram for ref), when pressed connect one end to the Vcc and also powers one transistor that connect other end to the ground and the relay turns on and stays on with permanent magnet. To turn of the relay the right push button powers two other transistors that makes a current loop in the opposite direction and right part to the relay takes power from relay NC output. The whole part works in self-destruction mode :).

You can check in video and images the driving circuit consumes power only when the state is changed. Other than that there is literally no power consumption by the driving circuit.

EDIT::

The circuit can also be make to work only with two transistors, you can substitute the first transistor in the right part of the circuit that turns off the relay by directily connecting 5v out from switch to the relay, every other thing remains the same.

Check the full video to get more clear idea.