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Old 27th May 2008, 10:59 PM   #1
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Default control a latching relay with one switch?

I'd like to control a dpdt 12vdc latching relay with single momentary switch.

Is that possible? I"d like to use one switch. Push to activate latch relay on and switch light is on, push again to activate latch relay off and switch light is off.

I'm thinking some combination of single or double coil latching relay is the answer, but i just can't wrap my head around it. Or maybe it can't be done.
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Old 2nd June 2008, 03:44 PM   #2
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Ok. I've found some 'flip flop' circuits that will accomplish my goal.
..but they have the disadvantage of losing their 'state' upon losing power (turn off the component I'm using them in). I need the previously chosen state to stay the same when the component is returned to power.

I've spent a number of days on this and it's really pretty remarkable that this type of relay/switch does not exist anywhere. "it can't be done" I'm told over and over again. But I can go to radio shack and get a small dpdt locking pushbutton, grab a little 12vdc actuator solenoid, and cobble them together on a pcb. That does what I want, but it's clunky, noisy, and looks like hell. It just blows my mind that I cannot buy anything like this off-the-shelf.

Anybody have any suggestions for me? This must be something that members here have come across before. You want, for instance, a very hi-tech look to the front panel and wish to control an internal relay with a submembrane microminiature momentary (like on your laptop computer, car stereo, cell phone, etc). In my case, I only have one momentary to work with and need it to toggle dpdt on and off. Like I said, I can give a little push solenoid a 12vdc pulse through this momentary and thereby depress an interal dpdt locking pushbutton switch. Next time I press the momentary switch I change the interal switch states.

Is this the only way I'm able to do this without microprocessors and loops of diodes, resistors, and transistors?
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Old 2nd June 2008, 04:14 PM   #3
djQUAN is offline djQUAN  Philippines
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you can use a CMOS flip flop circuit with a couple of diodes and a coin cell battery (ex. CR2032) for backup so it remembers its setting when power is turned off.

power consumption of a typical CMOS IC is so low the battery could last a long time.
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Old 4th June 2008, 06:04 AM   #4
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Predict this Toggle Circuits behavior
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Old 5th June 2008, 06:48 PM   #5
cpemma is offline cpemma  United Kingdom
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I collected a few ways of momentary-switch toggling here, including a method found here at diyAudio using relays. Setting a starting state isn't a problem, but remembering the last state after a power-off is much more complicated.
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Old 5th June 2008, 07:09 PM   #6
muizel is offline muizel  Netherlands
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Default This worked for me

I used this circuits.
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Old 5th June 2008, 07:46 PM   #7
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I read that other thread mentioned above and it opens up an interesting point. Why not make use of the powerful flexibility of microcontrollers and learn a little while you're at it? I have nothing against this proposition at all. Core spirit of DIY and all that. I'm even familiar with, and have used them before. But to use a microcontroller, or even a multielement pcb with a battery for memory, in order to accomplish what is, in my opinion, one very small step in complexity beyond flipping a household wall switch just seems ludicrous.

I can do this with a small pushbutton latching dpdt and a low voltage spring-retracted one-way solenoid to acuate it. With patience and a little time, I could shoehorn this into not much bigger than a square inch. Hit it with a pulse from a momentary switch and the solenoid does what your finger otherwise would have. Push the same switch again, and the solenoid simply pushes the switch into its alternate state. I've just built a small impulse relay. But I can't purchase a better made, tested, approved version of this anywhere on earth it seems.

I must use a spst momentary switch for this function. That is a parameter I must work around. That I can't find a compact pcb sized impulse relay on the market just astonishes me.
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Old 5th June 2008, 09:04 PM   #8
cpemma is offline cpemma  United Kingdom
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Originally posted by bluebeard
That I can't find a compact pcb sized impulse relay on the market just astonishes me.
There are latching relay devices (though not at my usual stockists). One type includes a permanent magnet to remember the last state.
New from Fujitsu Components Europe, the PCB-mounting JSL polarised latching relay is the same size as the company's existing JS series device at 29.3 x 10.3 x 12.8mm.

Typical applications are battery powered equipment like thermostats, power control metering, home automation control requiring a memory function etc.
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Old 5th June 2008, 10:09 PM   #9
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cpemma, there are literally hundreds of 'latching' relays available out there. Here's a hell of a deal on some nicely compact ones, items three and five,

Most of these use magnetic latching like the Fujitsu ones. Fujistu, apparantly is meeting a demand for further size reduction by using more exotic powerful magnets. All latching relays, even the ones that latch mechanically, differ from 'standard' relays in that they don't need constant current (or the absence of it) to maintain the contact arms position. In a standard relay, the coil holds the the contact arm in place, or, unpowered, the coil allows the spring held arm to maintain the opposite state.

In latching relays, the coil armature just moves the contact arm back and forth between two magnets. The current on the coil is only necessary to move the arm and then when the magnet secures it, you can leave the relay unenergized. It just waits for another pulse of current to yank the arm back again. There are two basic flavors. One coil and two coil. One coil uses one polarity of current to move one way, and another current of opposite polarity to reverse the contact arm. Two coils use one pulse of current to move the arm, and another pulse to the other coil to reverse the arm. But every one of them requires two separate switches for control.

There is a special variety of latching relays called 'impulse' relays or 'sequencing' relays. They are higher current 'power switching' relays of 15amp and up used in heavy industry and lighting. They are basically off-the-shelf relays with a ratched cam attached to the armature that allows it to be switched back and forth with the same polarity. Magnecraft 711 series is an example. With one of these you can switch on/off any device from multiple locations without the inputs conflicting with one another.

I keep hearing that there is simply not a market demand for lower rated or signal rated versions of this. It's quixotic to argue against the market, I'm just suprised, that's all. I go to the net and can find dozens of quests simliar to mine from guys that would like to do this, as simply as possible. I'd have to be a bonafide EE inside the industry to understand why there is no need for this beyond the DIY crowd .

Part of it, no doubt is that practically everything that moves anymore has some level of microcomponent control inside. A designer just designs this in with programming. They still haven't totally replaced minature electromechanical relays with ICs, there are stilll many of them inside my laptop. They just don't make one of this type for some reason.

I can't find a compact version of this unless I build one of my own.
I'm just hoping someone will eventually point to an idea or a vendor that I've been missing.
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Old 6th June 2008, 05:17 PM   #10
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Well I feel a little bit better, I guess. I was talking with someone at a place called SRI electronics sourcing and he'd been tracking this stuff for almost 25years. He said he's been asked several hundred times over the years for exactly what I'm looking for. The demand is there, so I'm not actually nuts, it's just not quite enough for manufacturers to see a profit in it. He assured me it just doesn't exist.

Oh well.

Guess I'll have to make one.
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