Using multiple switchs for same device

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Using multiple switchs for same device

Postby troubleScottie » Mon Apr 24, 2017 2:59 am

Looking at issue in design. My intention is to have all the DC circuits controlled by a microcontroller eg Ardunio. Being paranoid, I want to have a manual override if the electronic system fails or the software has a bug so that one or more devices are not activated. (BTW this does not address the issue that something is always on. A battery switch will address this )

So here is a simple version. Switch1(s1) is a simple single throw, single pole mechanical switch and switch 2 (s2) is a electro-mechanical latch controlled by the microcontroller. Closing either or both switches will deliver power to the device. If both switches are open, the device is not powered. So each device with have a pair of switches.

Obviously, there can be other switches in each circuit and multiple devices in place of the device. That is a different discussion.

148046

Question: Are there any issues with this?

Continuing with my overdesign....

The other issue has to do with something like using either the towing vehicle OR the trailer to power the reverse/backup lights on the trailer. The major differences in this case are: two different batteries, conceivably two different grounds (they should be connected together). The towing vehicles 7 pin cable will provide the switched power for the reverse lights. The trailer battery will provide power from the trailer. This circuit has diodes to isolate the two systems so turning on the reverse lights from one system does not power up any other devices in the other vehicle. Without the diodes, it is possible that putting the power to one system could power devices in the other system.

148047

Are the diodes needed/recommended? Other than cost is there any drawback to this design?

Thinking more about this, some people design the trailer charging system that is just directly connected to the towing battery ( with maybe an isolating switch and some fuses hopefully ). I am guessing this could have the same issues if the systems are not isolated.
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Re: Using multiple switchs for same device

Postby edgeau » Mon Apr 24, 2017 5:48 am

On issue 1. As long as you only want the backup manual switch to turn on your device. If the microcontroller is incorrectly on you will not be able to turn the device off. To be able to turn on or off from either you need double pole..

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Re: Using multiple switchs for same device

Postby QueticoBill » Mon Apr 24, 2017 7:01 am

On the two switch question, why not use a basic 3-way circuit, so either switch can turn it on or off regardless of the other switch position?

The two source, seems preferable to again use a double pole center off switch so the switch selects which source powers the load.
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Re: Using multiple switchs for same device

Postby Dale M. » Mon Apr 24, 2017 9:33 am

I know you want the complexity of micro controller, but if it fails you need disconnect of device from controller... And as for multiple locations for switch... Tried and true established world over is 3-way switch...

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IT does everything you want and more where your examples do not...

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Re: Using multiple switchs for same device

Postby QueticoBill » Mon Apr 24, 2017 11:05 am

I didn't search but seems you could get the same three-way switch in an electronic version and control it by the micro controller - though a bit trickier to tell the micro controller if its on or off. Current sensing transformer at every load? But then you have it all ways.
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Re: Using multiple switchs for same device

Postby H.A. » Mon Apr 24, 2017 3:00 pm

Asiad"
Last edited by H.A. on Thu Jul 06, 2017 4:17 am, edited 1 time in total.
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Re: Using multiple switchs for same device

Postby troubleScottie » Tue Apr 25, 2017 2:11 am

Thanks for the input everyone.

Just to complete the design.

This is environment control system. The rationale is to control the various electrical systems (of which there are too many) from a central panel via touch screen or remotely. Lots of parts (Arduino, touch screens, latches, switches, extension boards, wifi), lots of complexity, too much software including rewriting some of the available libraries, and wiring, etc. Yes, everything could be done without most of this. Yes, this is way over designed for a simple TD. But to a large extent, the journey is as important as the result.

The Arduino device : a bistable latch is actually a three way switch/relay (AC or DC, 8+A draw, upto 240VAC or up to 24VDC) controlled by a short low voltage electrical pulses. It has a reset to specific state via one pin. Each pulse drives the switch between the two states. The switch uses power only when being switched. It can be wired as a SPST or SPDT switch.

The question was to address if the latch ended up in a state that could not be switched again either on or off. Most likely this is due to (1) failure of the Arduino or (2) failure of the code supporting the latch or (3) catastrophic failure of the latch eg it is not allowing any electrical flow. The last state is unlikely but finitely possible.

Although one could rewire a specific latch if needed, the initial thought is to pre-wire a "rescue" path. The "rescue" switches will most likely be immediately adjacent to the control panel. They will not be used under normal conditions.

The 3 way switch arrangement has advantages: solves the issue of turning loads on or off, but requires the latch to be present and at least conducting. It would not solve the (3) issue.

Wiring the two devices as single pole, single throw has the advantage of addressing the three above conditions. It also allows for NO arduino or latch, an upgrade path if you like, but does not allow for turning off devices.
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Re: Using multiple switchs for same device

Postby Dale M. » Tue Apr 25, 2017 9:12 am

I see whole trip enjoyment lost having to trouble shoot why touch screen system is not working according to the concepts and being in dark or light on as overrides don't work the way intended and angry wife when the lights will not come on so she can read and lights won't go off so she can sleep.... Technology is great... But simplicity of time honored wring and mechanical switches wins when on vacation or pleasure trip...

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Re: Using multiple switchs for same device

Postby low277 » Tue Apr 25, 2017 7:35 pm

Dale M. wrote:I see whole trip enjoyment lost having to trouble shoot why touch screen system is not working according to the concepts and being in dark or light on as overrides don't work the way intended and angry wife when the lights will not come on so she can read and lights won't go off so she can sleep.... Technology is great... But simplicity of time honored wring and mechanical switches wins when on vacation or pleasure trip...

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