cyeutter
Posts: 1
Joined: Tue Apr 02, 2019 9:45 pm

3.3v Relay Exists?

Tue Apr 02, 2019 10:10 pm

I'm new to this forum. I'm building my second project which is identical to to the first where I use a motion sensor to trigger a relay which switches a 12v power supply to open a water valve.
The first project works like a champ and has been in use for over a year.
Using nearly identical hardware with the second project, I'm having an issue triggering the relay. I've confirmed the 3.3 volt switching signal is making it to the control pin on the relay but not causing it to switch.
Here's the relay I'm using. https://smile.amazon.com/SunFounder-Mod ... 10&sr=8-30

The relay for the first project is nearly identical, ( same manufacturer, model and also 5v) and works fine with the 3.3v trigger. I've read posts about wiring up additional transistors etc to get around the 3.3v vs 5v issue. I'd prefer not to go that route.
Doesn't anyone make a relay that switches with a 3.3v trigger. Rasperry Pis, being as popular as they are, surely someone is feeding that market. I have been unable to find a 3.3v relay.

Andyroo
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Re: 3.3v Relay Exists?

Wed Apr 03, 2019 1:08 am

There are a few HATs around with either built in relays or relay controls.

The other option would be to look for a solid state relay that switches your load.
Need Pi spray - these things are breeding in my house...

LTolledo
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Re: 3.3v Relay Exists?

Wed Apr 03, 2019 1:37 pm

This not suited for your requirements?
https://uk.rs-online.com/web/p/non-latc ... s/6839258/
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

PhatFil
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Re: 3.3v Relay Exists?

Wed Apr 03, 2019 2:39 pm

while you dont want to hear it, a relay isnt the best component to switch a small dc load, your much better off looking at a mosfet solution. a relay is an AC switching component.

relays have certain drawbacks being mechanical devices with a coil, on release there is the risk of back voltage and current flowing back into the gpio hence the advice of using a relay with an optocoupler to isolate the gpio from such risk.

relays also require a significant draw to activate (significant in terms of gpio capacity) so using a relay can limit your capacity to use other input/output devices over gpio. Or use of other devices can limit your use of relays..

However it is more convinient to have a plug in module ready to go..;) in which case i wiould suggest opting for relay board options that activate on a Low signal as in my very limited experience you can supply the board with Vcc from a 5v rail of your pi and use an opto-isolated gpio connection to pull the connection down to ground to activate.

Paul Hutch
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Re: 3.3v Relay Exists?

Wed Apr 03, 2019 2:43 pm

LTolledo wrote:
Wed Apr 03, 2019 1:37 pm
This not suited for your requirements?
https://uk.rs-online.com/web/p/non-latc ... s/6839258/
Unfortunately that particular unit, and every traditional coil based 3V relay I've encountered, requires too much current for direct drive from the 3.3V GPIO of a Raspberry Pi. The linked relay has a 60 ohm coil so it draws 55mA @ 3.3V (I=E/R). AFAIK that is more than the absolute maximum draw allowed on the 3.3V line and is way more than recommended for a single GPIO.

Brandon92
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Re: 3.3v Relay Exists?

Wed Apr 03, 2019 5:53 pm

PhatFil wrote:
Wed Apr 03, 2019 2:39 pm
while you dont want to hear it, a relay isnt the best component to switch a small dc load, your much better off looking at a mosfet solution. a relay is an AC switching component.
I do not fully agree with this statement. A relay is also commonly used in the automotive industrie. For example, there are a lot of relay in a car that switch DC.
But, for a small load a MOSFET is indeed a better solution.

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ptimlin
Posts: 88
Joined: Tue Jan 08, 2019 7:44 pm

Re: 3.3v Relay Exists?

Wed Apr 03, 2019 6:25 pm

Brandon92 wrote:
Wed Apr 03, 2019 5:53 pm
PhatFil wrote:
Wed Apr 03, 2019 2:39 pm
while you dont want to hear it, a relay isnt the best component to switch a small dc load, your much better off looking at a mosfet solution. a relay is an AC switching component.
I do not fully agree with this statement. A relay is also commonly used in the automotive industrie. For example, there are a lot of relay in a car that switch DC.
But, for a small load a MOSFET is indeed a better solution.
I was going to reply the same but you beat me to it. To claim mechanical relays are for AC switching is simply not true. As you point out, most of our cars are big rolling relay boxes with dozens+ relays all over them. Unless you are from Massachusetts :P , your turn signals are used all the time without any issues of that relay switching on/off every second when activated.

There are plenty of small signal mechanical relays for this type of switching. But as mentioned the issue is almost always one of the coils requiring more current than a RPi GPIO pin can source/sink.

So the original poster, on that board you are trying to use (again), on your first project did you hook up +5V to VCC and Ground and then activated the input with the GPIO? If so did you remember to hook it up this time the same way and did not try to use 3.3V for the VCC hook up? While the input might be triggerable with the 3.3V GPIO, the coil is still going to need +5V so you need to drive VCC with +5V.

PhatFil
Posts: 1303
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Location: Oxford UK

Re: 3.3v Relay Exists?

Thu Apr 04, 2019 1:42 am

thanks for correcting my mistake, ptimlin & Brandon92

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