This is an updated version of my previous programmer. Refer to the original for more depth.
Schematic
My new programmer combines the basic and ubiquitous JDM design, as used by Olimex Ltd., with a slight modification by BTKelly which disables VDD until it is actually necessary. This has been found to be necessary when attempting to update certain PIC microcontrollers that have been previously programmed and are configured to use an internal RC oscillator as a clock source. The problem in this situation stems from the fact that a PIC will only accept programming if its PC is all zeroes, i.e., if it has not started to execute code yet. A pre-programmed PIC running on an external clock source will not run on its own, but if the PIC has an internal oscillator, it starts running as soon as it gets power. The original JDM programmer likely predates the advent of PICs equipped with an internal oscillator clock option, because it applies VDD to the microcontroller as soon as you plug it into the computer serial port and provides no VDD-control functionality.
The addition of Q2 (2N7000 MOSFET) gates VDD in hardware, turning on chip power only when it is necessary. It does not provide the ability to control VDD directly from software, as do some programmer designs designed around a parallel port. The modified programmer remains compatible with any device supported by the classic JDM design. Note that Q2 need not be a 2N7000; most any N-channel MOSFET should work. The IRF510 is known to work in this design, as that was all that was available from Radio Shack when Marc Orr's programmer was built. Radio Shack really is disappointing lately.
Construction
The device in the pictures deviates slightly from the schematic as it is actually a direct clone of Marc's programmer. I don't have pictures available of one that is true to the above schematic because it broke in half (don't ask) and was subsequently discarded. I'll build another soon.
