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Automating PC Power-Up Powering off your PC and associated peripherals from the mains will eliminate standby power usage and save you some money. It also reduces the PC’s exposure to mains-borne surges and may prolong power supply life. However, a recent email from one of our readers points out that when powered up from the mains, all late-model PCs simply enter standby, or “soft power” mode. You then need to push the front-panel power switch to boot up. Not content with having to perform this extra step, he went on to describe a simple means of “pushing” the button electronically, so that his PC powers up as soon as mains power is applied. On all late-model (ACPI-compliant) PCs, the power switch is connected to an I/O controller chip on the motherboard. This chip is powered from the 5V standby supply, so it’s always powered when mains power is present. Pressing the switch closes a circuit to ground, signalling the I/O controller to electronically switch power to all other circuits and thus boot up the machine. As shown in Fig.1, a large value capacitor can be connected across the switch to simulate a switch press. At power up, the capacitor charges slowly via a pull-up resistor internal to the I/O controller chip. If the capacitor is large enough, the I/O controller sees a valid logic low level on the switch input line after the power supply outputs have stabilised. We’ve also included a 100Ω resistor in series with the capacitor to limit discharge current into the I/O controller at power off. Several motherboards we’ve examined have a resistor in series with the ground line, which would achieve the same result. However, as we can’t be sure that they all do, it’s wise to include the resistor anyway. The capacitor and resistor can be soldered directly to the power switch pins. Insulate all leads with tubing as necessary to prevent short circuits. Note that the polarity of the capacitor is important. Use your meter to determine which of the two wires from the motherboard is the ground wire. The ground wire will measure between zero and about 500Ω to case ground, whereas the I/O controller input will measure much higher. The suggested value of 470µF for the capacitor should work in most cases. However, differences in power supply design may mean that you’ll need to increase this value for your particular PC. Silicon Chip Binders REAL VALUE AT $12.95 PLUS P & P H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A5.50 p&p. Available only in Australia. Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Important: always disconnect AC power from your PC before opening the case! Note that old “XT” & “AT” type PCs do not consume standby power; the front-panel power switch physically switches mains power to the power supply input. KALEX • High Speed PCB Drills • PCB Guillotine Laser Labels • PCB Material – Negative or Positive Acting • Light Boxes – Single or Double Sided; Large or Small • Etching Tanks – Bubble • Electronic Components and Equipment for TAFEs, Colleges and Schools • Prompt Delivery We now stock Hawera Carbide Tool Bits Fig.1: a 470µF capacitor and a 100Ω resistor connected to the front-panel power switch are all you need to make your ATX computer boot as soon as mains power is applied. siliconchip.com.au 718 High Street Rd, Glen Waverley 3150 Ph (03) 9802 0788 FAX (03) 9802 0700 Website: www.users.bigpond.net.au/kalex Email: kalexpcb<at>bigpond.net.au ALL MAJOR CREDIT CARDS ACCEPTED June 2004  77