A Power-Off Delay Circuit

A Power-Off Delay Circuit

Originally, I had my MP3 box powered straight from the +12V line, and controlled it with the power switch on the inverter. That way, I could leave it running when stepping out of the car for just a few minutes. Of course, eventually I forgot to turn the thing off for the night, and found my battery dead the next day. That wasn't fun at all. So I decided to make a simple circuit to auto-power off the computer for me.

This circuit delivers power to the computer as long as the key is in the ignition. After the key is removed, there is a delay of about 10 minutes before the power is cut. There's also a button in the dash to cut off the power immediately.

The circuit is very simple & cheap, as you can see here (also in PostScript).

IMPORTANT NOTE 1: there's a typo in the diagram - the "S" and "D" pins of the MOSFET are reversed.

IMPORTANT NOTE 2: 50 Ohms is for the first resistor is too small. Use 1 KOhm

All it takes is a standard relay, a diode, a capacitor, two resistors, and a MOSFET transistor. The circuit is connected both to a constant +12V source, and to a control line (Vacc). Vacc can be the "accessory power" line, or, in the case of my car, the underappreciated - but very useful - "+12V when the key is in the ignition" line. Voltage on Vacc charges up the capacitor and opens the transistor; this activates the relay and feeds +12V to the computer. When Vacc is no longer powered, the transistor stays open because the charged-up capacitor supplies voltage. Meanwhile, the capacitor slowly discharges via a resistor. Eventually, its voltage drops low enough to close the transistor and shut off the power relay. Alternatively, the capacitor may be discharged instantly by pressing the "cut power" button.

Some notes:

A regular transistor does not work here because it draws some current, enough to discharge any reasonable-sized capacitor much too quickly. A MOSFET transistor draws practically no current at all. I used the widely available 2N7000 MOSFET.
The length of the delay is determined by the capacitor and the resistor values, as well as the MOSFET's gate cut-off voltage. With the 2N7000, a 40 uF capacitor and 5 MOhm resistor result in about 8-9 minutes of delay. Increasing either value will increase the delay (roughly linearly - I suppose). I have both components mounted on screw-type terminals, so I can replace them with easily.
The diode is in there to prevent the capacitor from discharging "backwards" through the car's electrical system.
The power LED is, of course, optional. I mounted it in the dash to complement the "cut-power" button. (Actually, I just had a cool blue LED lying around which I wanted to put to good use...) Use a variable resistor to adjust the LED brightness.

Note that this circuit serves a different purpose from the various shutdown controllers. Shutdown controllers are meant to halt the system in an orderly manner, by sending signals through the serial port. It's up to the software to perform the actual system halt (although most controllers will cut the power anyway after some delay.) My circuit provides a guaranteed, fail-safe, hard power-off. Incidentally, I never needed a proper shutdown controller because I have Linux configured with a read-only root filesystem, so I can rudely cut power at any time without worrying about data loss.

In fact, my circuit can be combined with a shutdown controller. It could be used to, e.g., provide an independent 10-minute delay, before letting the controller do its thing.

(UPDATED 27/10/00)

Parts list

Relay: any generic SPST (single pole, single throw) or SPDT (single pole, double throw) electromechanical relay with a 12V coil, rated for at least 15 Amps @12V. A standard Bosch relay, as used in just about any car's electrical system, is perfect for the job. If you use the Bosch relay, the terminals marked 85 and 86 are the coil, +12V goes to terminal 30, and terminal 87 provides power to the computer.

MOSFET: 2N7000 or BS170. Any electronics shop should carry either of those. Here's links to some datasheets at Fairchild Semiconductors: 2N7000, BS170. Warning: MOSFETs are extremely sensitive to static discharge, so handle them carefully. Use a grounding strap if you've got one. Better yet, get a transistor socket, build the circuit around the empty socket, and insert the MOSFET only after everything else has been assembled & soldered.

Capacitor, resistors, diode: this is just a 12V, low-current application, so nothing special is required. Use generic components from any electronics shop. If you use a 150 Ohms resistor, make sure it's rated to 1W.

Wiring: use nice, thick, heavy-gauge wire to power the computer - it has to be good for 20 Amps or so. And, most importantly, put a fuse on the +12V wire before the circuit!!! (Unless you take power from some existing line that is already fused, such as the cigarette lighter).
I've indicated the high-current wires with thick lines on my diagram. The rest of the circuit is low-current, so thin flexible wiring will do the job.

Screw terminals are convenient for attaching the thing to your car's electrical system. I also mounted the capacitor and 5 MOhm resistor on screw terminals, for easy replacement.

Other stuff

I've also hooked up a jog-dial volume control to my system.

You can contact me by e-mail.