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USB-Powered PIC Programmer
This simple circuit can be used to program the PIC16F84 and similar “flash memory” type parts. It uses a cheap 555 timer IC to generate the programming voltage from a +5V rail, allowing the circuit to be powered from a computer’s USB port. The 555 timer (IC1) is configured as a free-running oscillator, with a frequency of about 6.5kHz. The output of the timer drives four 100nF capacitors and 1N4148 diodes wir-ed in a Cockroft-Walton voltage multiplier configuration.
The output of the multiplier is switched through to the MCLR/Vpp pin of the PIC during programming via a 4N28 optocoupler. Diodes ZD1 and D5 between the MCLR/Vpp pin and ground clamp the output of the multiplier to about 13.6V, ensuring that the maximum input voltage (Vihh) of the PIC is not exceeded. A 100kΩ resistor pulls the pin down to a valid logic low level (Vil) when the optocoupler is not conducting. The circuit is compatible with the popular “JDM” programmer, so can be used with supporting software such as “ICProg” (see http://www.ic-prog.com).