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Re: control a relay from the pi via GPIO

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On 07/06/2016 04:03 PM, rickman wrote:
> 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.
>
There appears to be a three terminal blob to the right of the green LED 
which could be a switching transistor. I have a two relay board that 
looks remarkably similar. I haven't tried it on 3.3 volts, but it works 
fine with an Arduino at 5v.

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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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