A few years ago my school bought a Rapman 3.1 from bfb. After it was constructed they quickly realised that the print wasn't that good and so it was neglected in the corner. When we found it we had to repair many things on it and we also decided to upgrade it with a heated bed. We searched for a good solution but with no avail. This guide will show you step by step the construction of our heated bed and hopefully you can make one too.
Step 1: Optional - Housing for Arduino
What we first did was to create a housing for the micro controller (arduino). We used laser cut acrylic to create this housing and stuck it all together with tensol. This part is just for aesthetics but I highly recommend it to limit the risk of short circuits from open connections as the arduino is quite sensitive. Also it makes everything neater and makes sure that the wires don't interfere with the printer.
Step 2: The Print Bed
Step 3: Supplying the Power
Step 4: Arduino/micro Controller
Now for the programming...
In essence what's happening is that the arduino is turning on the heated bed until the temperature reaches a certain point (for pla:~60c for abs:~110c) then it will switch off the resistors until the temperature goes too low then it will switch the resistors back on.
If temp >110
Turn resistor off
Turn resistor on
The switch on the interface would switch the heating process on and the LEDs one would show if the arduino was on while the other would show whether it was ready to be printed on ( it had reached its target temperature).
Step 5: Final Results
But on a serious note the bed worked really well and it improve the print by a mile.
Finally I would recommend this project for any budding enthusiast as its a fairly simple project only needing a few skills and although we had access to the schools machines e.g a laser cutter, remember this is a guide not a rule book you can do it anyway you like.