My question was intended to ask if you set the Pi I/O ports up to source at least 7.5mA, which is the minimum required by the SSR.

My read is 7.5ma @ 12volts. Not minimum.
Minimum turn on even if its 5 volts will be half that. IMO

Just tested one of these http://www.ebay.com/itm/5-Crydom-D2...881?pt=LH_DefaultDomain_0&hash=item54022e6161

Crydom 25amp ssr.

Turns on at 1.5vdc and .5ma..............

Must by huge difference in input requirements.
I've had a bunch on the bench I've been playing with for another project.

 

My read is 7.5ma @ 12volts. Not minimum.
Minimum turn on even if its 5 volts will be half that. IMO

Here's my reference.

 

Thanks, I was going by his data link. Showed "Trigger Current 7.5mA / 12V"

 

Thanks, I was going by his data link. Showed "Trigger Current 7.5mA / 12V"

Yes, that didn't make sense to me; that's why I searched for another source. Not to say that my source is correct. All Chinese datasheets are suspect.

 

As is proven by op.

 

As far as I know, the I/O pins are set up simply as "Output" or "Input" and when it's "Output", each pin can deliver 16mA max but the total of all pins and the 3.3 rail shouldn't go higher than 50mA (otherwise it's cooked pi for dinner). So, I doubt that limits on current would be an issue. The gist I get from the RPi forum is that there is no regulation of current to those pins - it's up to the user to ensure that not too much is drawn.

And, Max:

If you feel ambitious you can make your own fairly easily and cheaply, opto isolator and Triac, you do not even need zero cross switching as it is not a phase angle controlled application.

To me, the whole project was a bit on the ambitious side. I'm an old programmer, so doing things involving solder is a high-wire act for me. I'll leave the making of components to people who know what they are doing.

So, the plan now is to try using the same SSRs with a 5v control circuit, where the 5v is switched using a transistor from the 3.3v pin. Any suggestions on spec for the transistor?

Also, I'll keep my eye open for second-hand Crydom or Opto SSRs and aim to replace the cheap ones later in the year as I don't have much faith in them!

 

R.Pi only offers 3.3 volts on the true I/O pins, though there's also a constant 5 volts available which comes direct from the board's power supply.

By "bias them on", do you mean give them 5v from the start and use a transistor to switch off? Yes, I guess I could do that.

Give the ssr 5 volts thru appropriate resistor. 470-1k

I/o pin pulls ssr input down to turn off. Thru diode or two if needed.
Reverse logic.

 

So, the plan now is to try using the same SSRs with a 5v control circuit, where the 5v is switched using a transistor from the 3.3v pin. Any suggestions on spec for the transistor?

If looking at a transistor, the 2n7000 will offer minimum components solution.
Max,

 

Thanks folks. I'll do a little circuit at home to test and report back next month. You've been very helpful!

 

Done it, and it works. Thanks.

I realised I have a 12v DC supply too (used for a touchscreen). I decided to use that instead of the 5v, to make sure that the switching is really clean.

Also, because the spec on the SSRs mentions the input resistance at 12v, then I can simulate the circuit with some confidence.

                    SSR
12v---------------[+  -]----
                           |
                           v   
  3.3v control     68R     /
       ------------===----|   BC548B
                           \
                  10R      |
            ------===-------
            |
            _
       Common ground

Very basic stuff to you guys, but it's rocket science to me. The thing I was unsure about was linking the negatives of the 12v and 5v power supplies to get a common ground. I did it via a resistor 47R. I hope that's sensible.

 

In most cases the commons can be connected, no need for a resistor?
Max.