Good morning everyone! I'd like to preface this by saying I am a completely fresh player in this sport, and I am likely going to miss some obvious things. Please bear with me!
After some exploration, I have decided that the Arduino platform appeals to me for trying to solve this little DIY problem I have. Without going into excruciating detail on the whole setup, let me explain my overall goal;
-The arduino will read a sensor (ultrasonic in this case)
-When the sensor is in the correct state, the arduino will signal a relay
-The relay will be connected to a heavy motor load;
-The motor load is a 220VAC water pump, and draws ~8 Amps when running
-The relay must be NC, such that if the logic fails, the motor stays connected to mains voltage
I've not done so well thus far finding a SSD that is capable of what I am looking for. Mainly it seems my load is too high.
Ditto the SSR. I used one in a project that controls a 240VAC 6A dust collector motor. I wasn't getting good results driving both legs of my SSR (basically two SPST in one package) so I had to use a transistor to turn on the SSR, but there are plenty of examples of that around for turning on relays, small motors, etc.
I would probably have used the exact SSR that Oracle points out, but I found a lot of 6 similar devices cheaper on ebay.
Excellent link, thank you! Are there, in your opinion, any intermediate steps I should take to protect the logic from errant voltage, or can the relay be trusted not to cross over? I have never done anything like this.
It is not a good idea to have motor with a heavy load going when the controlling electronics fails.
You can't do it from the relay itself but you could arrange a circuit that actively turns the relay off under control and when the power is removed from the controller it starts the motor.
A more detailed description of my application is in order, now that I have a moment free to give it!
I recently moved into a house my father built in 1980. It's designed to be as self sufficient as possible in terms of water.
The (very large) roof is fully guttered, the rainwater runs through a few gravel-based filters to remove large particulate matter, and then is pumped into a large ~5,500 gallon holding tank that is built into the basement. The water from there makes its way through a sediment filter, an iodizer, and to my tap. The system works very well with pretty minimal maintenance, but you do need to keep your gutters clean
Anywho, I have been running CAT6 all over the house, and I am slowly building up a fully managed switched network for data and media, with hidden Wireless APs and the whole deal. So I have a good network infrastructure in place.
Since the water system operates on it's own, and the level in storage constantly varies with rainfall and usage, I'd really like a better way to know how much water I have at a given time. Right now the solution is as low-tech as you can get; there is a small trap door in the floor that I drop a long stick into with inch marks on it.
The Arduino will be hooked up to an ultrasonic sensor to monitor the water level, with an ethernet shield. The ethernet shield will allow me to get reports on water level and additionally allow it (I hope) to notify me if the level gets unusually low. The pump load I am talking about is the pump that pressurizes the system. It has a pressure actuated switch on it already, and draws water from the tank whenever needed in order to re-pressurize the system when a tap is opened.
The relay in question will prevent the pressure pump from running if the Arduino senses a dangerously low water level, thereby saving the pump from a long dry death. However, until the system is tested well, I want the pump to be able to run even if the Arduino fails (hence the desire for an NC SSR). That said, I could also just wire in a manual bypass switch.
Sounds interesting. You should be able to achieve everything you're asking for here with an Arduino without any major complications. Good luck with it!
Well I think you find that all commercial solid state relays are normally open due to their use of an internal optical isolator as the gate trigger for the triacs used as the switching element.
However if you were to use a small 5vdc regulated power module that has to be powered from the same AC voltage as the pump uses, there is a way to make the SSR appear as normally closed, with the Arduino able to turn off the SSR.
Run the new +5vdc power module output through a 200 ohm resistor to the + DC input terminal of the SSR, hook the - DC input terminal of the SSR to the negative terminals of both the new power module and your Arduino. Finally wire a digital output pin from the Arduino to the + DC input of the SSR ( the same termianl as the 200 ohm resistor is wired to).
Now when the Arduino's output pin is a low it will shunt the current from the resistor causing the SSR to turn off and when the output pin is set high it will allow the SSR to turn on, however if the arduino is disconnected or powered off, the SSR will still be turned on by the new DC power module.
That make sense?
PS: Of course you should breadboard this up using a small 120vac lamp as a test load before wiring into the pump power.
This might not work because once a chip has it's supply removed the ESD protection diodes will kick in for any signal applied to it. This gives the probability that the chip might latch up and not work once the power is restored. But more than this it means the un powered state of a pin might look like just one or two diodes drop to ground. I say might because if the ESD is done with a zennar then no current will flow and it would work.
A better way might be to connect the +ve end of the SSR to the separate +5V, run the -ve end to ground through the 200R resistor. Then connect the arduino pin to the -ve end of the SSR through a diode (possibly Shockley) with the cathode at the SSR end. In that way a high from the Arduino would turn it off and a low or removal of power would turn it on.
I think this would work irrespective of how the ESD (electro static discharge) protection on the chip was implemented. (This is something they don't tell you about in the data sheets)
