These are Eagle v6.4.0 layout and pdf files for creating a PCB heatbed for the printrbot jr. The heatbed is mounted under a aluminum plate (see here) that acts as the printring platform.
In the PCB heatbed center is a little hole. Through this hole a thermistor is touching the aluminum plate's backside to measure the temperature at the plates center.
The PCB heatbed is cover with "plastik 70" spray for electrical insulation. (For PLA a heatbed temperature of max. 65 °C is needed so the "plastik 70" should not decompose and should therefore be suitable in this application.)
The heatbed and sensor are directly driven by the stock printrbot jr. printrboard. Only a stronger power supply was needed (12V with at least 15A --> P >= 180W).
At the edge and top of the PCB heatbed a power LED with matching series resistor (both SMD) are soldered in place.
*) UPDATE #1: After one week of using the heatbed I can say that it works great. I did not flash the new temperature sensor's resistance values to the printrboard. It is close enough to the expected stock temperature sensor's range. If I set 47 °C [117 °F] in my pronterface interface I reproducibly get a surface temperature of 55 °C [131 °F] (measured with this infrared thermometer).
*) UPDATE #2: Please remember to drill the holes in the PCB with a much bigger diameter as needed and use washers to compensate the bigger hole size. This is needed since the PCB and aluminum plate have different thermal expansion coefficient and if they were thightly fix to each other, without any play in the drill holes, the resulting stress could crack and/or destroy the whole setup.
- PCB one sided (35 µm Cu, min. size 170 x 125mm) covered with photo resist [e.g. this board]
- Thermal grease (the one used with computer CPUs)
- Kapton tape
- Aluminum plate (170 x 155mm) with mounting holes
- Plastik 70 spray (isolation layer) [e.g. this one]
- Thermistor 100kOhm base resistance [e.g. this one]
- SMD LED with matching resistor for 12V usage
- Metal connector pins
- Upgraded power supply (12V and at least 15A) [e.g. this one]