Azhar,
Modern Memory based devices implement a signalture allowing the programmer to implement different voltages, pin configurations, and programming pulse widths.

For instance:

Atmel:
"Reading the Signature Bytes: The signature bytes are read by the same procedure as a normal verification of locations 030H, 031H, and 032H, except that P3.6 and P3.7 must be pulled to a logic low. The values returned are as follows.
(030H) = 1EH indicates manufactured by Atmel
(031H) = 51H indicates 89C51
(032H) = FFH indicates 12V programming
(032H) = 05H indicates 5V programming"

Dallas:
"The Signature bytes identify the product and programming revision to EPROM programmers. This information is at programming addresses 30h, 31h, and 60h.
Address Value Meaning
30h DAh Manufacturer
31h 20h Model
60h 01h Extension"

These are just two examples. If you drive the programming voltage using a DAC it's easy to set the voltage. Pin Configurations require transistor switches as relays are a bad choice except for the most relaxed requirement.

regards,
p