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Have you ever connected two PCs together via modems using a twisted pair cable and nothing happened? That’s because the modems are expecting a phone line with all the signals and voltages supplied by the local telephone exchange. This circuit simulates the DC power and signal isolation but not the "dial tone" or the "ring signal". It suffices to connect two PCs together to communicate and exchange files using HyperTerminal. The circuit is self-explanatory and needs only one power supply for both modem lines. Although 50V DC is the usual exchange line voltage, this circuit should operate down to 20V. A 600O line transformer (eg. Jaycar cat. MM-1900) provides signal isolation, while the resistors provide current limiting and keep the lines as balanced as possible. Circuit diagram: How To Connect Two PCs Using Modems Circuit Diagram When using this set-up with HyperTerminal, you should not select a Windows modem driver in the "Connect To" dialog. Instead, con...

This is the simple Reset Protection For Computers Circuit Diagram. This protection circuit is inserted between the reset switch and the motherboard. The earth connection of the computer must be linked to terminal of the protection circuit. The protection circuit can draw its power from the computer supply. When the circuit has been fitted, operation of the reset switch will not immediately restart the computer. Instead, a buzzer will sound to alert you to the reset operation.    Simple Reset Protection For Computers Circuit Diagram The buzzer is actuated for 4 s by monostable IC1A, which is triggered by the reset switch. During these 4 s, the output, pin 5, of IC1A ensures that the reset function, pin 10, of IC1B is disabled. When the reset switch is operated again, monostable IC IB will be triggered and this starts the reset procedure. Transistor T2 is then switched on for 0.5 s and the buzzer is deactuated via Rll and D4. The circuit around T1 and N4 ensures that IC1A can ac...