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Float Switch Relay Project - Looking for some help

trevorhiller

Jul 1, 2022
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Hello everyone, I am seeking some assistance with a simple (I think, but not for me personally) electronic DIY project. I have a fish tank that has a device that collects liquid filtered waste from the tank in a collection cup. This runs via a AC 120 volt, ~22 watt pump. I want to be able to install a small float switch at the top of the collection cup to shut the device off when it is full to prevent overflows.

I've purchased a cheap little float switch: TL Reefs Float Switches for Auto Topoff (2-Pack) | eBay

However, these are not designed to have AC voltage through them so I can't simply splice this into the one of the cables on the filter. I'm also told this is risky around the fish tank (water splashing, etc.) So I am looking to connect the float switch inline with a relay that will switch the 120 volt pump off when the switch is activated.

The float switch is reversible, so it can be normally open/normally closed depending on how it is configured. Ideally how this will work in my mind is this: when the float switch is in the down position, the power flows through to the filter and it works normally. When the float switch is in the up (full) position, the float switch closes the circuit and provides low DC voltage to the relay which then opens the AC side of the relay, shutting off the filter.

Sounds simply enough in my head. However, I know basically nothing of relays other than that it is essentially a magnetic switch and need some help choosing one. I was looking on Amazon and they have some 5 pin automotive-style relays that I believe MAY work. They say that max switching rating is 20/30A (which I should be well under). Does it matter if what I am switching is AC vs DC?

I understand that I would need to provide 12 volt power to two pins of this relay to activate the switch. Would something like this work: Amazon.com: Bosch 0332209150 Changeover Mini Relays - 5 Pins, 12 V, 20/30 A : Industrial & Scientific

Any other suggestions/tips?
 

Kiwi

Jan 28, 2013
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Jan 28, 2013
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Hi Trevor,
I can't see any specs for those switches stating what voltages(AC or DC) and current they are rated for.
Do you have other information on them?

Your 22watt 120vac pump only draws about 180mA, so you may be able to wire it directly through the float switch without a relay.

If using a relay, I would use the normally closed float switch contacts to power the relay coil.
The pump would go through the normally open relay contacts.
The float switch will open when the cup is full, turning the relay off.
The relay contacts the would open and turn the pump off.

That relay is for automotive DC applications, not rated for AC.
You need a relay that has 120vac rated contacts.
To save having to also supply 12vdc, I would use a relay with an 120vac coil.
Water won't be a problem if you put all the connections in a waterproof plastic box, above the maximum water level of the tank.
 

trevorhiller

Jul 1, 2022
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Jul 1, 2022
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Ok thanks for your input. I’ve had others who have created similar projects tell me to not use 120 vac running through the float switch. Something about DC being safer for the aquarium than AC. Hence supplying a 12 v DC power supply for the float switch.

This is basically what I’m looking to create:
http://autotopoff.com/Longwire/skimmerato.jpg

I wish I knew what relay they had in that box. If the cords weren’t so long and I didn’t have all the pieces except for the relay I would just buy that one, but I think I can save some money and make it custom.
 

Kiwi

Jan 28, 2013
418
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Relay specs;
Coil voltage: 12vdc
Coil suppression: diode or resistor(protects float switch contacts from arcing when switching relay off)
Contact voltage: 120vac or 120/240vac
Contact current: 1amp minimum. Probably use 5A or 10A as they are more readily available.

You only need a simple on/off relay with a single set of contacts. This is referred to as single pole/single throw(SP/ST). It has only 4 terminals, so easy to connect up.
A single pole relay will often have three contacts, Common(C), Normally Open(NO), and Normally Closed(NC). This is referred to as single pole/double throw(SP/DT). It has 5 terminals, so still easy to connect up. You just don't need the NC contact.

eBay and Amazon should have heaps of suitable relays.
Your local electronics shop should have one, and can supply the required terminals if you don't have any.
 

trevorhiller

Jul 1, 2022
3
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Jul 1, 2022
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Relay specs;
Coil voltage: 12vdc
Coil suppression: diode or resistor(protects float switch contacts from arcing when switching relay off)
Contact voltage: 120vac or 120/240vac
Contact current: 1amp minimum. Probably use 5A or 10A as they are more readily available.

You only need a simple on/off relay with a single set of contacts. This is referred to as single pole/single throw(SP/ST). It has only 4 terminals, so easy to connect up.
A single pole relay will often have three contacts, Common(C), Normally Open(NO), and Normally Closed(NC). This is referred to as single pole/double throw(SP/DT). It has 5 terminals, so still easy to connect up. You just don't need the NC contact.

eBay and Amazon should have heaps of suitable relays.
Your local electronics shop should have one, and can supply the required terminals if you don't have any.
Thank you for the detailed explanation. Very helpful.

if I would have been more observant, I would have seen this item in the related items section on Amazon. It seems like this is the one other people use for this type of setup and looking at the specs, it should work for what I need. Although some of the pins will go unused.
https://www.amazon.com/Electromagne...99-abc3-f61c7bb8928b&pd_rd_i=B07T12WLMT&psc=1
 
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