Emergency Shut-Down System for a Water Cooled PC

Detailed How-To with parts list – Ron Wlock

This article describes the construction of an emergency shut-down system (ESDS) for a water cooled PC. When the water flow inadvertently stops, the ESDS automatically turns off the PC to prevent overheating. The ESDS is an electronic circuit consisting of two 555 timers and three relays connected to a water flow sensor switch. When the ESDS is used with the Windows hibernation option, any data in memory is saved to the hard disk before the PC is shut-down. The total cost for parts and material to construct the ESDS is under $70.00 CDN.

Recently, I built a water flow sensor switch that uses a simple LED interface circuit to indicate when water is flowing in a water cooled PC. An article about the project can be found HERE. The LED interface circuit works as designed and turns “on” a red LED when the water flow stops. The limitation of this simple circuit is that a human must be present when the water flow stops, in order to take action to prevent the PC from over heating.

The decision was made to replace the simple LED circuit with a more elaborate circuit that would automatically shut-down the PC when the water flow stops. This new circuit is referred to as the ESDS.

The ESDS uses three common features available on most PC’s:

• The first is the Advanced Configuration and Power Interface (ACPI) that enables a PC to be turned “soft off” by sending a signal to the operating system. This can be done by momentarily pressing the PC power switch (aka ATX power switch) which closes a connection between two pins on the mother board. The PC is not running when it is “soft off”, but there is control circuitry on the mother board energized with 5 volts of standby power.

• The second is the Windows 2000/XP hibernation option that allows a PC to save any data in memory to the hard drive before shutting down with “soft off”.

• The third is the ATX12V power supply that provides the 5 volts of standby power when the PC is either running or in “soft off”.

Information on the ACPI and ATX12V power supply specifications can usually be found in the PC’s mother board manual. Web sites containing the full specifications can be found here: ACPI, ATX12V.

The plan was to design an electronic circuit that would close a connection between the “soft off” power switch pins on the mother board when the water flow stops. This action is identical to pressing the PC power switch and causes the PC to shut down. When the Windows hibernation option is setup, the PC saves any data in memory to the hard drive before shutting down. Therefore, not only is the hardware saved from over heating, but any data in the PC memory is saved from loss.

Mother boards that support the ACPI specification have two power switch pins. A diagram illustrating the location of the pins for an Asus P4T-E mother board is shown below.

Hibernation is an option supported by Windows 2000/XP and is enabled in the Control Panel Power Options.

The Windows help file describes hibernation as follows:

“When you put your computer into hibernation, everything in computer memory is saved on your hard disk, and your computer is switched off. When you turn the computer back on, all programs and documents that were open when you turned the computer off are restored on the desktop.”

A common practice is to configure Windows to invoke hibernation when the PC power switch is pressed. For Windows 2000, this is done in the Power Option Advanced tab. See the images below.

The ESDS is comprised of four main components. A schematic is shown below.

A. Magnetic Reed Switch on the Water Flow Sensor Switch
B. Electronic Timer Circuit #1
C. Latching Relay Circuit
D. Electronic Timer Circuit #2

The ESDS has 2 LED’s and a reset button. The green LED indicates normal operation and can be illuminated when the PC is either running or “soft off”.

The red LED is illuminated when the water flow stops, while the PC is running. It stays illuminated even after the PC is turned off by the ESDS, in order to signal to the operator that a fault has occurred. In addition to the red LED warning signal, the PC power switch on the case is disabled to prevent turning on the PC, before the water flow problem is fixed.

After investigation of the water flow problem, the operator presses the reset button on the ESDS. This turns off the red LED, turns on the green LED, and enables the functionality of the PC power switch. An image of the completed ESDS circuit is shown below.

The magnetic reed switch is part of the water flow sensor switch. When the water flow stops, the contacts on the magnetic reed switch “close” and the ESDS is triggered to shut down the PC.

The magnetic reed switch is a “normally closed” model which means the contacts are “closed” in the absence of a magnetic field. When exposed to a magnetic field, the contacts “open” and remain “open” as long as the magnetic field is present.

Note that the magnetic reed switch for the simple LED circuit described in the article “Building a Water Flow Switch for a Water Cooled PC” is a “normally opened” model. The design of the ESDS requires a “normally closed” model. A diagram showing the magnetic reed switch on a water flow sensor switch is shown below.

Overview of Timer Circuit #1

This circuit is connected to a 12 volt drive plug on the PC power supply and is activated immediately when the PC is first turned on. The purpose of this circuit is to create a 5 second delay before connecting the magnetic reed switch to the ESDS. This delay is necessary because the cooling water does not start to flow immediately when the PC is turned on.

The exact time depends on each water cooling system, but on average, approximately 2 seconds is required to start the water flowing with enough force to push the flow sensor paddle near the magnetic reed switch. Without the 5 second delay, the magnetic reed switch would trigger a shut-down when the PC is initially turned on. A diagram showing the power source for the Timer Circuit #1 is shown below.

This circuit is connected to the 5 volt standby power on the ATX power supply. The 5 volt standby power is available whenever the alternating current (AC) power is supplied to the power supply and the PC can be either running or in “soft off”. The 5 volts provides a power source for circuits that must remain operational when the main DC output on the power supply is off. Example uses for the 5 volts include soft power control, wake-on-LAN and wake-on-modem.

The image below shows the main power connector for an ATX power supply and illustrates which wires need to be tapped for the 5 volts. The pin out diagram is the pin-side view and represents version 3.1 of the ATX12V power supply design specification. The 5 volt standby power (+5VSB) is the purple wire # 9. A suitable ground (COM) is the black wire # 7.

It is recommended to disconnect the AC power cord before opening a PC for servicing or to perform modifications.

Relay #3 is a two coil latching relay. The first coil is the “reset” coil and when energized places the relay contacts in the “reset” position. The contacts hold this position even after power is removed from the “reset” coil. The “reset coil” is energized by momentarily pressing the reset switch for the ESDS. Relay #3 is in the “reset” position for normal operation of the ESDS.

The second coil is the “set” coil and when energized, places the relay contacts in the “set” position. The contacts hold this position even after power is removed from the “set” coil.

When the water flow stops and the magnetic reed switch closes, the relay #3 “set coil” is energized. This moves the contacts to energize the red LED and Timer Circuit #2, which shuts-down the PC. Timer Circuit #2 shuts-down the PC by activating relay #2, which closes a connection between the PC power switch pins on the mother board. In addition, the green LED is turned off and the PC power switch is disabled.

If the Windows hibernation option is enabled, a message appears on the screen indicating that the PC is going into hibernation.

Tests indicate that it takes approximately 20 seconds to hibernate a PC with no data in memory. It may take a few seconds longer, if data in memory needs to be saved to the hard drive, but the extra time is not critical.

The time to shut-down a water-cooled PC is not critical if the PC is running at a cool temperature. An experiment was conducted to determine how rapidly the CPU temperature rises when the water flow stops, and results indicate it takes 3 minutes and 24 seconds to raise the CPU temperature by 4 degrees. See table 1.0 for the test data. Results would vary for each computer.

The purpose of this circuit is to energize Relay #2 for one second. When the contacts for Relay #2 close, a connection is made between the PC power switch pins on the mother board. This causes the PC to shut-down.

To simplify construction of the ESDS, the timer circuits are built from kits with a printed circuit board (PCB). It is much easier to solder components to a PCB than to route wires and solder components on a perforated board. The kits used in this project are available from CanaKit or R P Electronics.

A CK002 Compact Electronic Timer kit is used for Timer Circuits #1 and #2

A CK004 Relay Power Board kit, a PNP transistor, and a 10K ohm resistor are also used in the Timer Circuit #1. The CK004 kit is not shown on the CanaKit website but is available. Almost any 12 volt relay could be substituted for the CK004. A schematic for the Timer Circuit #1 is shown below.

An IN4005 diode is added to the CK004 kit as a precaution to protect components from a potential electro magnetic field (emf) voltage surge caused by a sudden interruption of the relay coil current.

An image of the completed Timer Circuit #1 is shown below:

A schematic for the Timer Circuit #2 is shown below:

The CK002 is designed for an adjustable time interval from a few minutes to 20 minutes. The ESDS requires a 5 second time interval for Timer Circuit #1 and a 1 second time interval for Timer Circuit #2. To achieve these time intervals, the CK002 kit was modified by replacing the kit components R1, P1 and C1.

Since the variable timing resistor P1 was not required, it was replaced with a jumper wire as shown below.

The new values for R1 and C1 were determined with a free 555 timer design software program downloaded from Schematica.com. There was no need to replace C1 for Timer #1. See Table 2.0 for the component values:

The latching relay circuit is very simple and consists of two relays, two LED’s and one push button reset switch.

Relay #2 is a Hamlin single pole, single throw, part # HE3621A0510. It has a built in diode for protection from emf voltage surges.

Relay #3 is a Zettler double pole, double throw, 2 coil latching relay, part # AZ832P2-2C-5DSE. IN4005 diodes are soldered to the coil leads as a precaution to protect components from an emf voltage surge.

Both relays can be easily substituted with similar 5 volt relays if they are difficult to obtain.

The LED’s in the circuit are rated for 2.0 volts at 20 mA and therefore require a 150 ohm resistor when powered with 5 volts. The specifications are different for each model of LED and it is helpful to know the formula for calculating the LED resistor value. The formula is shown below.

Below are images of the latching relay circuit board:

All the components are mounted in a 190mm x 110mm x 57mm plastic box with a wire barrier strip for connecting the system to the water flow sensor switch and PC. The completed ESDS circuit is shown below:

The parts needed to construct the ESDS are listed in Table 3.0. A water flow sensor switch is also required.