Re: control a relay from the pi via GPIO

From	Dennis <dennis@none.none>
Newsgroups	comp.sys.raspberry-pi
Subject	Re: control a relay from the pi via GPIO
Date	2016-07-07 17:10 -0500
Organization	A noiseless patient Spider
Message-ID	<nlmjvp$pq8$1@dont-email.me> (permalink)
References	<nljpg6$me8$1@dont-email.me> <nljqk9$p7j$1@dont-email.me> <nljrnb$udt$1@dont-email.me> <nlm31a$nj7$1@dont-email.me> <nlm7b7$7vo$1@dont-email.me>

Show all headers | View raw

On 07/07/2016 01:34 PM, rickman wrote:
> 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.
>
I am pretty sure mine has the same circuit as his but with 2 relays. It 
looks similar and has "2 Relay Module" and "HL-52" on the board. I think 
they make 1, 2, 4 and 8 relay versions that are just multiple copies of 
the same circuit.

The question is why high switching (PNP) rather than low side (NPN) 
which would work fine with either 3.3 or 5 volts. My guess is for 
safety. The processors I know of come up with the I/O in input mode 
which would look like a high signal (tri-state or pull-up) which would 
leave the relay unenergized. You can then do your set up before changing 
to output mode and selecting the relay. Using an NPN could energize the 
relay immediately on power up which you may not want.

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

csiph-web