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Is the solenoid a 2-position latching device or spring-biassed so requiring power to be applied to maintain one position? I'm guessing the N52 you refer to is a magnet and you want to switch the solenoid on or off with the magnet - by its presence or its polarity?

Do you have a specific hall effect sensor in mind? Most simple hall effect devices output 2.5v when no field is present and 0.2v for a S-pole and 5v for a N-pole, so some additional logic needed unless you can guarantee magnet polarity and strength.

Edit: Missed the A3144 reference in the title doh!

 

I presume a BJT - NPN would work best for this, or would an N - MOSFET be better?

A latch circuit using dicrete components would need two BJTs or two MOSFETs, not just one.

 

Do you actually have the A3144E's ? When I checked for a datasheet it seems the A3144 is no longer in production (although advertised by online sellers via Amazon), and has been replaced by the A1104.
Googling "latch circuit schematic" will get you plenty of examples of latch circuits.

 

One picture is worth a thousand words ............
What are You trying to accomplish ?
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As the title states; what would be the most effective way to latch the transistor "closed" using the output from another "always on" sensor - then have it turn back on by biasing the transistor the other way again, using yet another sensor?

Is this what you are trying to accomplish?
H1 activates the solenoid and Q1 performs the latch.
H2 is powered through Q2 and resets latch.

 

Also, am I right in saying that were the sensors in the diagram momentary push buttons, it'd operate the other way around, seeing as these sensors provide a "negative" signal, or am I mistaken?

Not sure your meaning but a push button could provide either a negative or positive signal depending on how it's configured.
The hall sensors have an open drain FET transistor in their outputs which would be the same as a momentary switch with one side connected to ground providing a negative output.

 

Any special consideration for the resistor values? Or just calculate using V=IR as usual?

R1 = 10K
R2 = 100K
Q1 = 2N3904

 

The Skinner device has a claimed 1200% energy increase and is a scam. Any 'over-unity' device is a scam. By all means build a latching circuit for your education, but don't expect free energy.

 

Does it have to be a P MOSFET? Or could I use an N-MOSFET instead but wired the other way round?

P- Mosfet required and recommended or PNP transistor. How much current does the solenoid draw?

 

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The whole universe is in perpetual motion.. I think people forget this.

Or people know that this is not true. The universe is not in perpetual motion. Per this article, the energy in the universe has gone down by 50% over the past billion years or so, give or take a few million. Read the article.

 

One more question- were I to use PNP BJT and N-MOSFET how would the connections differ?

These two configurations based on H1 activating and H2 resetting.

Using a N channel mosfet switching the low side of the solenoid.

 

Using a step down buck with I/V control, circuit will be running at 18v / 1A for now.
planning to step it up to some 80v and probably more current though as this will work better (shorter on time)
Prototype need not use such high voltage.

One more question- were I to use PNP BJT and N-MOSFET how would the connections differ?

Same circuit in concept but all devices reversed- but the hall-effect devices can't be reversed so will require additional parts. Better to stick with P-channelMOSFET

 

Were it a "scam" where's the profit to be had??

In selling the idea to gullible investors.

 

Yeah ok what happens then?

You’d have to study physics to find out for yourself. The Big Bang theory says the universe started as a singularity. There are theories that state once the universe loses sufficient energy, then it will collapse unto itself… to a singularity. And the cycle begins again.

Of course, this is with respect to our universe. String Theory has projected that at any point in our time, there are multiple universes co-existing. I found it easy to understand based on other theories that our universe doesn’t exist in four dimensions, but rather as much as eleven.

This isn’t my opinion. It has been stated by many, including Nobel prize physicist Stephen Hawking.

But I’ve co-opted your thread enough…

 

For the second diagram with the N MOSFET what resistor values do I use?

 

Try changing R4 to a 10K.

 

Supply to H2 being on the left, and its output coming out the top?

Supply on top, output on left for H2

 

Surely their turn on requirements are different?

Usually. Change R2 to 10K also. Are you sure you have it connected correctly especially concerning the pins on the transistors?
Do you have a solenoid connected? Do you have any other smaller transistors you can use for Q1 and Q2?