A PIC based temperature controller for laminator – Part 2.
During my last post I started working on a custom temperature controller for a Scotch TL 901 Laminator. I have finished the control PCB schematic and layout. Here is what the final PCB looks like.
Next, I removed the stock temp control PCB and did a test fit of the new board to make sure the mounting holes lined up, and there was enough clearance. Everything looked good so it was time to install the components on the board. There is a lot of surface mount components used to save space, but the packages are all easy to work with. It just takes a bit of practice.
Now that the main controller board is built , its time to work on the PCB that will hold the three 7segment displays and 3 buttons that will make up the user interface. Here is what I came up with:
On the left is the header to connect to the main control board, along the bottom there are 3 buttons for user input, and finally along the top the 3 seven segment displays.
The three 7 segment displays are a LSD3021 common anode unit. Any common anode with the correct pin out should work. If it is not a red display, the current limiting resistors will need to be changed because of the different voltage drop for each color of led. Make sure to keep the current around 10-15 mA maximum per LED. All the same segments are tied together, then go through a 100 Ohm resistor before going to the header which connects to a PIC Micro IO pin. To control each display separately, a P channel MOSFET is used to the Common Anode connections for each digit. The MOSFETs are required because each digit will draw about 80mA when showing the number 8, this is a lot more than an IO pin can provide. The digits will be multiplexed inside an interrupt routine to provide the final 3 digit output.
The three buttons are normally open momentary push buttons from Electronic Goldmine, but any similar switch will work just fine. The pull up resistors are located on the main PCB instead of the display board. This is done in case the connecting cable gets broken or disconnected so the pull up resistors will still hold the button inputs in the default state.
Overall the display board is pretty simple and designed so it could be used in many other applications that need a very readable numerical display. It will work well with a 5v microcontroller as well, but current limiting resistors might need to be changed.
Here is what the blank display board looks like.
In the next post I will start to write code and try out the functionality of the circuit.