Solid-State Switching w/o a common ground
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Solid-State Switching w/o a common ground

by atmakers on Wed May 12, 2021 1:22 am

Hi everyone,

At the ATMakers Facebook ground, recently got a request to help an AT User continue to sail. He uses a sailing setup that has an automated tiller (to steer) and an automated winch (to adjust the sail). He's been successful for year with it, but his SMA has caused his fingers to be unable to activate his switches.

You can see his request here: https://www.facebook.com/groups/ATMaker ... 4391438332

Currently each hand has a setup like this:
2Switches.jpg (91.66 KiB) Viewed 98 times

Where his thumb can activate two contacts. These are 5V signals that go back to the control box which switches on and off the winch and tiller.

Ean has asked if we can replace the two switches on each hand with ONE switch that goes to a control box that looks for patterns ("short tap, long hold" vs "just long hold") to change the direction of the winch & tiller.

I know we can do this with relays. We can essentially replace his contact switches with relays and control them w/a microcontroller that has the logic to detect the different patterns.

However, I would REALLY like to use solid state components here. We have had a horrible time w/relays - they are susceptible to damage due to impact, they've falsely triggered when a fluorescent light activated, and in this case, I worry about the saltwater environment. Even just the salt air makes me worry about relays.

SO, I'd like to solve this with transistors. I've checked the wiring coming from the control box, and unfortunately, it consists of a 5V common line and four signals lines that each switch activates. There is no ground line coming from the box.

If there were a ground line, I would use a circuit like the one on the right from this Bald Engineer page Bald Engineer page. https://www.baldengineer.com/low-side-v ... witch.html


However, without a common ground, I don't know if there's a way to avoid the relays. Are there components that solve this without a common ground?


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Re: Solid-State Switching w/o a common ground

by adafruit_support_bill on Wed May 12, 2021 5:31 am

You could use an opto-isolator on the input. https://en.wikipedia.org/wiki/Opto-isolator

You can also find'solid-state relays' (SSRs) available off-the-shelf. These are essentially a transistor based switching circuit with optical isolation on the input. These come in all sorts of sizes and ratings. Most of which are fully potted and suitable for use in harsh environments. https://en.wikipedia.org/wiki/Solid-state_relay

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Re: Solid-State Switching w/o a common ground

by atmakers on Sat May 15, 2021 7:49 am

Thanks Bill,

Sorry for the delay - lots going on here.

I think I found an appropriate SSR (that doesn't show any common ground or polarity requirements). I've ordered a few from Digikey and they should be here Monday.
https://www.digikey.com/en/products/det ... BF/1928132

If this works, it will solve many other problems we have so I have my hopes up. Perhaps something like this would make a good STEMMA board - maybe a SMD version like this https://www.digikey.com/en/products/det ... TR/3077637

Either way, I'll update you on how it goes

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Joined: Fri Mar 10, 2017 2:16 pm

Please be positive and constructive with your questions and comments.