Re: control a relay from the pi via GPIO

From	rickman <gnuarm@gmail.com>
Newsgroups	comp.sys.raspberry-pi
Subject	Re: control a relay from the pi via GPIO
Date	2016-07-06 17:03 -0400
Organization	A noiseless patient Spider
Message-ID	<nljrnb$udt$1@dont-email.me> (permalink)
References	<nljpg6$me8$1@dont-email.me> <nljqk9$p7j$1@dont-email.me>

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On 7/6/2016 4:45 PM, rickman wrote:
> On 7/6/2016 4:24 PM, Björn Lundin wrote:
>> Hi!
>> I got a pi model B that I want to control a relay.
>>
>> I've got one just like the ome on the picture in the link here
>>
>> <https://halckemy.s3.amazonaws.com/uploads/image_file/file/76359/PoolSchematicDC.jpg>
>>
>>
>>
>> I connect just as the picture, +5v to vcc and ground to ground.
>> The green diod then ligths up.
>>
>> So I connect a gpio to the middle pin marked Inl.
>> GPIO low = nothing happens
>> GPIO high = nothing happens.
>>
>> I grab a voltmeter and measure between the GPIO and ground,
>> and I get ca 0 V resp 3.3 V. So - correct pin, but no action in the
>> relay.
>>
>> Ok, thinking 3.3 might be to small to drive the relay, testing +5 V on
>> the middle pin. Still nothing.
>> Frustrated - testing ground to middle pin - and then the relay clicks,
>> and the red diod lights up, somewhat brighter than the green one.
>>
>> Surley - this is not supposed to happen - or ?
>>
>> I got 3 of those relays, and all three behave the same - ground the
>> middle pin and then action.
>>
>>
>> What am I doing wrong here?
>
> Mostly you aren't reading the specifications of your components.  Cough
> up a part number and  maker of the relay and I'll see if I can find a
> data sheet that explains how the relay is intended to be used.

I dug on the Internet a bit and it looks like your device is an HL-51 
which is a bit hard to read from the image.  Digging further if I read 
the part number from the relay correctly, SRD-05VDC-SL-C, this is a 5 
volt mechanical relay.

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.

The other is the rated current, 71.4 mA.  This is far higher than what 
the rPi I/Os can drive.

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.

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.

-- 

Rick C

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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