What info should I look for when choosing a solid state relay for arduino?

Question

I want to control a phone using an arduino board by connecting each pin of the arduino to a button on the phone using relays to simulate a button being pressed or release by switching a relay (on/off). I am using relays because from what I have seen (Use Arduino to Switch Power On and Off! ) relays are controlled swiches and I am using SSR because I want to use the less space possible.

I'm trying to buy a relay at mouser.com but I don't know what are the values I should look for, for those parameters (Control Voltage Range, Load Voltage Rating, Load Current Rating, Input Current).

Answer 1

I would tread carefully. A solid-state relay may or may not be suitable for the relatively low-level signals present across the phone buttons. Check the datasheet, buy just one, and try it out.

But do you need to control a physical phone? An Arduino can connect to a standard modem which has inbuilt commands to dial numbers and generally supervise a phone line.

Answer 2

An SSR is not a good choice for your application. The common use of an SSR is for switching on and off an AC load. If the data sheet mentions SCR or Triac, then it is meant for only switching AC and will not function for controlling DC. So keep this in mind when looking for components: if it says "24-220 VAC" on the data sheet that does not mean it will work for DC.

A second common use of SSR and microcontrollers is the opto-coupled input type. That input method provides a safe scheme for a low voltage low current control signal to switch a large load.

For a button on a phone, you are switching a near zero current DC load. You should be able to control that with a transistor, but that would require some reverse engineering of how the phone keypads where being scanned. You would need to trace out what the voltage was, how much voltage drop could be accommodated.

A certain easy method is to use a small reed relay. Parts are available that have a coil that only draws ~10 mA so can be driven directly from an output pin. The relays are packaged in a DIP so easy to prototype on a breadboard. Many list the coil resistance and voltage for specs, for example a 5V and 500 ohm resistance coil would draw 10 mA. You will find many for 1 - 2 $US.

When working with a relay, you need to take care of the back EMF (aka flyback voltage) that occurs when you turn off. That is handled with a single diode which you will see in every schematic for driving a relay. For a beginner, an alternate/sure success route is to use a driver chip such as a ULN2003 that will drive inductive loads such as small relays.

Answer 3

If you're only controlling a button, the load current and load voltage are pretty much irrelevant since I'd be surprised if you even used 5% of it's rating in either category. I can't tell you how much you'll need here since I don't know the voltage that the relay will be operated on and the resistance of the path for the load.

What you do need to look at is the voltage and current of the input/control/logic signal. It should allow a 5V control voltage and the current draw for that logic signal should be no more than 40mA, although it's ideal to stick to a lower number (say, 30mA).

However, manually picking out a relay manually isn't ideal for a beginner since it requires a bit of extra components to properly run. A relay module (Amazon) allows you just hook up your wires to the jumpers and terminals of the relay module without any extra soldering (as far as I know they aren't breadboard compatible, thus requiring a custom PCB or having it "flop") or parts.