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Re: control a relay from the pi via GPIO

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On 7/7/2016 1:21 PM, Björn Lundin wrote:
> On 2016-07-06 23:03, rickman wrote:
>
> Thanks rickman, for your replies
>
>>
>> So you have two problems.  One is that the relay coil is rated for 5
>> volts which means it should have 3.75 volts on the coil to work
>> properly.  The rPi I/Os won't survive with this much voltage across them
>> when the output is not driven low.
>
> Hmm, yes, but I did try feeding it 5 V as well. no go
>
>
>> The other is the rated current, 71.4 mA.  This is far higher than what
>> the rPi I/Os can drive.
>
> That might be the problem.
> Still, if I grounded the signal pin, the relay came to life,
> so the coil was fed enough to click/switch the relay
>
>
>
>> To solve both problems you simply need to add a transistor to your
>> circuit.  In reality your relay module should have the transistor on it,
>> but it doesn't look like it does.
>
> Yes, I realize that now. Either I do that or try it with an arduino.
> I could communicate with the arduino via the serial port
>
>
>> There seem to be tons of available references on how to interface this
>> type of relay to your rPi.  If you provide better info on what you have
>> I can give you more info on what you can do to use it.
>
> Yes and I read a good part. Some info indicates it is ok to use directly
> with the pi, while other indicates that a transistor is needed. I just
> thought it was very strange that I got movement when I grounded the
> middle pin. As you can tell, electronics is not my strongest subject.
>
> <https://www.youtube.com/watch?v=OvWOcNiQe1c>
> indicates 3 min into the movie that no transistor i needed.
>
> I try the arduino , and it it works, I'll use that instead
>
> I got a big server in the garage that gathers data at night.
> I don't want to run it during the day, so I'm thinking of
> automatically ssh in to it and issue a sudo halt using expect from the
> pi. This still leaves the fans on, so I'd like to kill the power to it
> via the relay. and when power is restored, it boots automatically.

As Dennis pointed out there is a fuzzy blob on the photo that looks like 
it is three terminal and likely a transistor.  So you would need to 
trace out the circuit to see what they have designed.  The circuit on 
the module Dennis has turned out to be a high side switch which means it 
is handling the higher current for you, but is referenced to 5 volts and 
so will over stress your rPi I/O pin.  If this is what is on your module 
it is not a good idea to drive this relay directly from your rPi I/O. 
It will need another transistor to handle the voltage translation.  Just 
one transistor and two resistors.  I would use a common base arrangement 
with the base biased to about 1 volt.  Or you can use a common emitter 
arrangement which will invert the sense of the signal which you can 
compensate for in the software.

Some Arduinos are 5 volt devices which will drive this I/O directly. 
But first you need to trace out the circuit to see just what you have.

-- 

Rick C

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Thread

control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-06 22:24 +0200
  Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-06 16:45 -0400
    Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-06 17:03 -0400
      Re: control a relay from the pi via GPIO Dennis <dennis@none.none> - 2016-07-06 18:00 -0500
        Re: control a relay from the pi via GPIO Dennis <dennis@none.none> - 2016-07-06 21:10 -0500
          Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-06 23:55 -0400
            Re: control a relay from the pi via GPIO Dennis <dennis@none.none> - 2016-07-07 12:30 -0500
      Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-07 19:21 +0200
        Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-07 14:34 -0400
          Re: control a relay from the pi via GPIO Dennis <dennis@none.none> - 2016-07-07 17:10 -0500
            Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-08 00:29 -0400
        Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-12 10:18 +0200
          Re: control a relay from the pi via GPIO Rob Morley <nospam@ntlworld.com> - 2016-07-12 12:49 +0100
  Re: control a relay from the pi via GPIO Gordon Henderson <gordon+usenet@drogon.net> - 2016-07-07 10:44 +0000
    Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-10 17:40 +0200
      Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-10 12:43 -0400
        Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-12 10:06 +0200
          Re: control a relay from the pi via GPIO The Natural Philosopher <tnp@invalid.invalid> - 2016-07-12 09:20 +0100
          Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-07-12 04:23 -0400
            Re: control a relay from the pi via GPIO The Natural Philosopher <tnp@invalid.invalid> - 2016-07-12 09:58 +0100
    Re: control a relay from the pi via GPIO Graham. <graham-usenet@mail.com> - 2016-07-11 23:10 +0100
  Re: control a relay from the pi via GPIO Graham. <graham-usenet@mail.com> - 2016-07-08 21:45 +0100
    Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-10 17:26 +0200
      Re: control a relay from the pi via GPIO Graham. <graham-usenet@mail.com> - 2016-07-11 22:13 +0100
        Re: control a relay from the pi via GPIO Björn Lundin <b.f.lundin@gmail.com> - 2016-07-12 10:00 +0200
  Re: control a relay from the pi via GPIO Big Bad Bob <BigBadBob-at-mrp3-dot-com@testing.local> - 2016-08-18 10:08 -0700
    Re: control a relay from the pi via GPIO rickman <gnuarm@gmail.com> - 2016-08-18 16:40 -0400

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