My proposal for a final project.

My uncle is making his own smoke house, and to make the smoke he decided on using a water heater element. Over the Thanksgiving holiday we were discussing this, and it gave me the idea for my final project. So, my final project will be creating a controller/display for a water heating element. The goal is to use the DE0 to control the power source to the heating element, so that the wood chips used for smoking, will burn and smoke but not catch fire.

Thermocouples will be connected between a DE0 (on the GPIO header) and the water heating element. The DE0 will read and display (the later on a 16x2 LCD add-on) the temperature of the heating element. The power source for the heating element will be connected to a relay and the control of the relay will be connected to the 5v pin on the DE0’s GPIO header. Upon flipping a switch on the DE0, a LED light will turn on, indicating that power is being sent to the relay and that the element has power. Once the heating element has reached the desired temperature a 15-minute timer will start. This timer will be displayed on the LCD. Once this 15 minutes has passed, the 5-volt signal going to the relay will be cut, the LED turned off, and a new timer will be set for 15 minutes. Once this 15 minutes has passed, the heating element will be given power and upon hitting the desired temperature a 15-minute ON timer will be set. This sequence will be repeated until the original switch is flipped off, or until 4 ON-cycles have passed.

As for reading the temperature of the thermocouples I have found that I will require a digitize of sort. This is what I have picked out, MAX31855. My concern is the relay. I am not sure if I am understanding the terminology correct.

What I need is the 5v GPIO of the DEO to send the 5 volts to the relay when called. I am correct in thinking that this particular relay requires 5 volts to turn on and can pass upto 220volts?


If I am correct, is there anything special I should know about hooking up it at the FPGA?

Lastly, does anyone have a recommendation of relay for my needs?

  • 1
    \$\begingroup\$ You can't directly drive a relay from a FPGA unless its an SSR. There are many resources online on how to do this. \$\endgroup\$
    – Wesley Lee
    Nov 29 '16 at 20:38
  • 1
    \$\begingroup\$ Also bear in mind that the DE0 pins are 3.3V logic level, not 5V. The MAX31855 is also 3.3V only, which is good as it will make interfacing with the DE0 easier (but remember to power it from 3.3V not 5V!). \$\endgroup\$ Nov 29 '16 at 20:40
  • \$\begingroup\$ To make it easier, you should concentrate on your design rather than finding individual components, I would suggest the Relay board like this one: ebay.com/itm/… it has an opto-coupler and you can drive it form your board without any problems since it is designed for 3.3V. \$\endgroup\$
    – FarhadA
    Nov 30 '16 at 16:56

You are correct that you need 5 Volts for the relay however you are going also need to source ~30mA to turn on the coil.

Coil voltage: 5VDC Coil Resistance: 178 Ohms

V=I*R = I=V/R 5vdc/178ohms=0.028Amps

If you have a 5 volt power source you could use it to power the relay; and then control a transistor with your FPGA to open and close the low side of the relay circuit.


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