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Edit/tl;dr: What are some alternatives to reed switches?

I'm looking for a switch that can be closed one-time that doesn't require any case penetration. The design is inside a water-proof enclosure. The switch goes between an 18V, 25Ah battery and some electronics. I'm obviously aware of reed switches, which seem like the perfect solution, but I'm concerned about reliability.

The application is a scientific instrument which is deployed in the ocean and is not serviceable. The switch is necessary because the device is in-transit/storage for sometimes more than 1 year. The device operates for approx. 2 years on this battery.

I've done a lot of searching on this topic. I'm convinced I just do not know the right terminology to get me to my goal. This post most widely breaches this subject with lots of great suggestions but no great solution.

What I really want is something like a fuse, only instead of opening when it 'blows' it would close. This can be a complex circuit or a simple device, the only requirements are low power consumption(10's of uA or less) and no case penetration.

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  • \$\begingroup\$ A latching relay triggered by a magnetic reed switch? \$\endgroup\$
    – PlasmaHH
    Oct 19 '17 at 14:26
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    \$\begingroup\$ Without valid reasons why a reed switch would be "unreliable", this question devolves to one about a religious issue, and should be closed. We can only assume your reasons for avoiding a reed switch are silly, because they certainly seem so, unless shown otherwise. The burden of proof is on you. \$\endgroup\$ Oct 19 '17 at 16:17
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    \$\begingroup\$ It now seems this question is about how to avoid a particular technology for arbitrary religious reasons. Such discussions are a waste of volunteer resources. \$\endgroup\$ Oct 19 '17 at 16:24
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    \$\begingroup\$ You really need to lose the attitude and cooperate with the people who are trying to help you. This is an engineering site, so your concerns with reed switches are entirely relevant. And have those "embarrassing failures" ever been analyzed for root cause? We're trying to help you solve a specific problem, not speculate wildly, which would be off-topic here. \$\endgroup\$
    – Dave Tweed
    Oct 19 '17 at 20:58
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    \$\begingroup\$ @SSVT The fact that you've been seeing occasional failures, along with the nature of those failures, should have been in the question, because they are the actual problem you're having. It places your question in context, helps people understand why you want to avoid reed switches (and whether or not that want is justified), and solve your actual problem. The matter of fact is, reed switches are the go-to solution for a scenario like this. If they're not working for you, we need to understand that, and why. If not, people are going to suggest the go-to reeds, and you get what happened here. \$\endgroup\$
    – marcelm
    Oct 19 '17 at 21:42
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You could use a P-MOSFET and a fuse as shown in the circuit below.

schematic

simulate this circuit – Schematic created using CircuitLab

However, that is a one shot deal and open to accidental enable and you would not be able to power it up to test it and shut it down again. The circuit also has a small trickle current when the power is off.

Instead you could use a P-MOSFET in a latching circuit as shown below.

schematic

simulate this circuit

When the on switch is pressed it will pull the output up to the battery voltage which starts charging C1. When this charges up to the minimum Vgs of M2 the latter will turn on pulling the gate of M1 low latching the circuit on. I have shown values that require pressing the button for 5s to latch it on.

Pressing the off switch immediately turns off M2 and M1, then C1 begins to discharge through the load. Once it discharges enough, again around 5s with a 10K load, you can release the off button and the power will stay off indefinitely.

You should play with the charge and discharge paths to get it the way it needs to be for your load.

OR.. you could just bring the power to two insulated bolts that go through some bulkhead and slap a jumper across them when you want to power it up and then slap silicone over it...

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  • \$\begingroup\$ Wow, thanks Trevor! I really appreciate your input on the "one-shot" design aspect. There are a number of reasons that this one-shot option is preferable to something that can be reset. I also appreciate your final alternative, might be the best option yet. ;) \$\endgroup\$
    – SSVT
    Oct 19 '17 at 19:41
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    \$\begingroup\$ Why are you showing normally-closed pushbuttons in your schematics? If I understand what you're doing, they should be normally-open pushbuttons. Which, by the way, still leaves open the question of how to activate them without penetrating a case. \$\endgroup\$
    – Dave Tweed
    Oct 19 '17 at 20:55
  • \$\begingroup\$ @DaveTweed they were normally open, no idea why it drew it like that though. Having issues with the editor today... :( Ya I just showed switches.. no idea how he is going to activate them really.. maybe magnets... external wires... \$\endgroup\$
    – Trevor_G
    Oct 19 '17 at 21:09
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    \$\begingroup\$ @SSVT and yet you didn't give it a like... LOL but your welcome :) \$\endgroup\$
    – Trevor_G
    Oct 19 '17 at 21:39
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    \$\begingroup\$ Sorry @Trevor, it seems I don't have enough cred to give you one. As I assume this thread is dead, I've accepted your answer. \$\endgroup\$
    – SSVT
    Oct 20 '17 at 16:59
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You could use a reed switch directly, or have it activate a latching electronic circuit.

If you can permanently arrange to keep a magnet in the right place, then just a reed switch could work. The magnet placed in a special holder or something then becomes the on/off control. This should be fine as long as your current requirement is low enough for a reed switch to handle. Considering you are talking about running for 2 years from batteries, the current should be well within the capability of a reed switch. In fact, you may have the reverse problem of not having enough wetting current.

Having a latching mechanism has a downside. If it ever gets accidentally activated, then the device is irreversibly on. A single reed switch closure seems rather risky for this. That's why I was thinking of keeping the magnet attached for the duration the device is supposed to be on.

Another option is to have a reed switch activate a electronic latching circuit. The pass element would be a FET with low enough RDSON for the voltage drop to be inconsequential. Check the leakage current and make sure that wouldn't cause too much battery discharge during the 1 year storage time.

If using a read switch as a momentary input to latch the device on, then you should require a sequence that is very unlikely to happen by accident. At least require the switch to be closed for some minimum time, like a few seconds. That way vibration and shocks, like accidentally dropping the unit, won't latch it on.

In my KnurdLight project, I required two button presses to turn the light on. Each had to be about ½ seconds with ½ second gap between them. To do something like that, the reed switch powers up the processor, which then turns on the electronic switch. If the right reed switch sequence isn't detected, then the processor turns itself off again.

Yet another totally different option is a pressure switch. The unit doesn't power up until it is some minimum depth below water.

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  • \$\begingroup\$ Thanks Olin, I do like the idea of including some logic to help with early activation concerns(Truth be told, I'm already doing that). If you were going to attempt your design without a reed switch, how might you do it? \$\endgroup\$
    – SSVT
    Oct 19 '17 at 14:58
  • \$\begingroup\$ @SSVT: I don't understand what you've got against reed switches. They are a simple way to get a single bit of information across a barrier, without taking any power at all until activated. I'd use one only to initially power up the micro, then as a signal input. Once properly activated, the electronic switch takes over and the reed switch becomes irrelevant. \$\endgroup\$ Oct 19 '17 at 15:03
  • \$\begingroup\$ Thanks again Olin, I definitely understand the sentiment. But there are some minor issues with reed switches.Just an example of a common issue would be switch magnetization. If the reed switch contacts became magnetized, see this post, there would be no recourse for 'fixing' this issue in the field. The cost associated with this type of failure is extremely high. I'm definitely not against reed switches, but I need to do my DD and search for alternatives. \$\endgroup\$
    – SSVT
    Oct 19 '17 at 15:22
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    \$\begingroup\$ @SSVT You really should include such information in the question itself. Please detail what failure mode you're worried about (early activation, deactivation in the field, or both?), how large the impact of failure is, why a reed switch would suffer those failure modes, etc. As your question is currently phrased, you're gonna get a lot more people saying "use a reed switch". \$\endgroup\$
    – marcelm
    Oct 19 '17 at 15:33
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    \$\begingroup\$ "I don't see much benefit to posting my concerns with reed switches". The benefit is that we might take you seriously. Your aversion of reed switches smells strongly of a religious conviction rather than a solid engineering tradeoff. It is therefore a waste of time, and off topic here, to delve into. \$\endgroup\$ Oct 19 '17 at 16:22

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