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So a normal relay is OFF and when powered is ON. Remove the power to the coil and the relay switches OFF. Simple, right?

But during all the time the relay is ON, it is consuming power to keep that electromagnetic coil energized.

That's a waste of energy.

With 1 or 2 relays, it's not a big deal. Say each relay is 10mA. Total of 20mA. Not bad.

If you're using 5 relays, and the user engaged all 5 relays to be on, that's 50mA wasted power... on what? Nothing but powering these relays.

Now, there is a certain kind of relay called a LATCHING relay. Apply a short burst of voltage to the coil and the relay turns ON.... remove the power, and the relay STAYS in the ON position. There's the power savings! To turn the relay off, you can either apply a reverse voltage to the coil to UNLATCH it, or apply power to a secondary coil to LATCH it in the other direction (i.e. the OFF). After the relay switches to OFF, you can remove the power and it stays OFF.



Very nice from a power-saving point of view. Especially if you're looking into using A LOT of relays for a particular project.

The problem is the logic to control these LATCHING RELAYS. One solution involves the use of microprocessors... but connecting/using up all the microchip's pins to just set/unset latching relays is a waste.

Currently looking at relay driver chips that may be simpler/easier to control.

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MCU + shift register is what i'm using for a project with loads of relays.

You can daisy chain a number of shift registers sometimes.

Are you using the MAXIM relay driver IC?

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Naw, i was referring more to this statement:



I was saying that i use shift registers as serial to parallel converters to effectively increase the number of digital outputs of the MCU. That way, you can easily make your own pulse in software, no? Then i just use a simple BJT relay driver circuit.

Or am i missing the whole point...? haha!

Or are you trying to eliminate the use of a microprocessor?

That's part of my plan too... but me wanting to use a latching relay that is giving me a curve ball. That above method will work fine for a normal relay.

I'm using an MCU... but need to save pins. Running out of them. I can't devote too many pins just to controlling latching relays. (i.e. reversing logic for the coils).

I found a circuit on the net several months ago, using a simple switch (or output from a MCU) and some logic ICs to latch/unlatch a latching relay. Basically, turn on the switch (or send a 1 from the MCU) and it sends a pulse to the relay to turn it on. Turn off the switch (or send a 0 from the MCU) and it sends another pulse (reverse polarity) to turn off the relay.

It only uses power during that brief pulse and the rest of the time, there is no more power applied to the relay.

Using the above technique and some MUX chips, I can turn on/off 16 latching relays from just 4 MCU address pins + some control pins.... at very little power consumption. It only consumes power during the change in state of the relay (and few ms at that).

Can't remember where I found it though.

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Hmmm well pardon my ignorance, but i am not getting why you still need that chip if you're using the shift regs?

The end goal is that you want to send pulses to the relays to turn them on/off, right? So what is the relay driver chip buying you? The shift registers turns three digital i/o pins into a bunch more outputs (depending on the number of shift registers you use) that you can use to send the pulses (i.e. i'm controlling 2 seven seg displays and some two dozen relays with just three digital i/o pins on the microcontroller and a few shift regs). And then you can just tell your program to only send a pulse out each output, thus saving power all times when the output is zero.

Or is it that maybe that relay driver chip is saving you some power through the transistor biasing somehow?? I didn't look that closely at the schem.

I'm confuzzled haha! Sorry again.

Here's a circuit of the MAXIM relay driver. You can use it to control 8 relays, or (4) single-coil latching relays.



But to control these (4) relays, the driver chip requires 3 address lines. OK, That's fine if ALL the control signal will be coming from the MCU.

But in my case, the control will be coming from another chip which only outputs a low or high. I want to use that simple LOW/HIGH signal from that chip to drive a latching relay, without sending that signal back to the MCU just so the MCU can control the latching relay.

If I can get away using ordinary relays, this would be simple. But I want to use LATCHING relays.

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Ooooohhhhh i see. I didn't realize you weren't interfacing the relay control directly to the MCU.

Thanks for the clarification.

I'm intrigued about what this mystery project will be...

Nerds..

Hah sadly true