start up aid for pcs

Description: The older PC in question has a standby state but lacks a corresponding BIOS setting to enable unattended startup. Although a +5 V standby supply voltage is present, the computer requires a brief push of a button to power on. In contrast, modern PCs typically include a BIOS option for automatic startup following a power outage. The circuit designed allows the PC to power on approximately one second after it receives power. The push-button functionality remains unchanged. The circuit utilizes two well-known components: a NE555 timer configured as a single-shot pulse generator and a TL7705 reset generator. The reset generator produces a pulse lasting about one second once the supply voltage is detected. An RC circuit connects the TL7705 to the NE555, generating a small trigger pulse during the falling edge of the one-second pulse. The NE555 responds by producing a pulse of duration 1.1RC. During this pulse, an output transistor activates, simulating the push-button switch on the PC to initiate startup.

The described circuit effectively addresses the limitations of older PCs regarding unattended power-on capabilities. By integrating the NE555 timer and TL7705 reset generator, the circuit leverages their characteristics to create a reliable startup sequence. The NE555 timer, known for its versatility in generating precise timing pulses, operates in a monostable mode, triggered by the output of the TL7705. The TL7705, designed for generating reset signals, ensures that a clean, well-defined pulse is produced when the system voltage stabilizes.

The RC timing network is crucial in determining the timing characteristics of the circuit. The values of the resistor (R) and capacitor (C) in this network dictate the pulse duration. The equation for the pulse width generated by the NE555 timer is given by the formula: Pulse Width = 1.1 * R * C. This allows for fine-tuning the timing to ensure that the output pulse is sufficiently long to bridge the push-button switch effectively.

The output transistor, which connects to the push-button switch, acts as a relay to simulate the manual operation of the button. When the NE555 timer outputs its pulse, the transistor turns on, closing the circuit that corresponds to the push-button, thus allowing the PC to start without manual intervention. This modification is particularly beneficial for users who require their systems to reboot automatically after power failures, enhancing the usability and reliability of older PC systems.

In summary, this circuit design not only maintains the original functionality of the push-button switch but also introduces an automatic startup feature, making it a valuable enhancement for older PCs lacking modern BIOS capabilities.The older PC that concerned did have a standby state, but no matching BIOS set-ting that allows it to start up unattended. Although a +5 V standby supply voltage is available, you always have to push a but-ton for a short time to start the computer up again.

Modern PCs often do have the option in the BIOS which makes an automatic start after a pow er outage possible. After building in the accompanying circuit, the PC starts after about a second. Incidentally, the push-button still functions as before. The circuit is built around two golden oldies: a NE555 as single-shot pulse generator and a TL7705 reset generator. The reset generator will generate a pulse of about 1 second after the supply voltage appears. The RC circuit between the TL7705 and the NE555 provides a small trigger pulse during the falling edge of the 1 second pulse.

The NE555 reacts to this by generating a nice pulse of 1. 1RC. During that time the output transistor bridges the above mentioned push button switch of the PC, so it will start obediently.