VPP Control

Vpp Control Schematic To get the PIC into programming mode, you have to raise the /MCLR pin to +13V in something less than 7us (as per the programming specification). If it is a flash (EEPROM) part, only a few microamps are drawn, but for the real EPROM, according to the specification, it can take up to 50mA at 13V during programming.

This part of the circuit draws on the parts on hand concept. These may not be the ideal transistors for this application, but I had a bunch of them around, so I went with them. I also designed the circuit so that if the Vpp control input was disconnected (or connected to a pin on the PIC set to high impedance), the default state of Vpp would be OFF. Vpp is set via the potentiometer. I used a 10 turn pot to gain some precision. The 2N2905 is saturated, so the actual output voltage of the regulator is about .2V higher than Vpp. Set the voltage at the test point to 13V when Vpp Control is on (high, or +5V).

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