# 555 timer switch help

#### Element1

Joined Feb 6, 2010
8
Hello,
I am working on a Hydrogen fuel cell car project that requires me to have an emergency shutoff switch on the outside of my vehicle. The problem is that the Fuel cell has a button that need to be depressed for 5 seconds to turn off. I need to figure out how i can use a momentary switch o the outside of the vehicle so that it can be pressed once and once hit activate the off circuit for the 5 seconds necessary to shut down the fuel cell.

#### Wendy

Joined Mar 24, 2008
21,941
So why not use a relay connected to a 555 Monostable? Having trouble figuring out how to drive the relay?

#### Element1

Joined Feb 6, 2010
8
I've been thinking about this and would it be eaiser to wire the momentary switch to a flip flop circuit which directs power from the fuel cell to a relay that engages the off switch. and when the power from the fuel cell goes off so does the connection the relay makes?

#### Wendy

Joined Mar 24, 2008
21,941
A flip flop circuit? How does that apply?

Whatever works.

#### Wendy

Joined Mar 24, 2008
21,941
Just curious, is this a for real fuel cell, or an HHO project? We'll help with the circuits in either case, but HHO is not a favorite subject here.

Fuel cells, on the other hand, are pretty cool stuff.

#### Element1

Joined Feb 6, 2010
8
It's for a University of Houston senior design team competing in the 2010 Shell EcoMarathon. We're using a 500 Watt PEM fuel cell to power the car.

Also I thought a flip flop used momentary commands to direct current, so in my instance it would direct power to a relay that engages the off switch until the fuel cell is off and then the relay turns off by it self.

#### Wendy

Joined Mar 24, 2008
21,941
So how about a simple voltage detector (minimum voltage set by your specifications, but let's say 10V for now) that turns the relay off when the voltage drops below a value?

The set delay is also simple, but you can go either route as you choose.

I assume you have some sort of secondary voltage for this and other circuits, otherwise you'll need a small battery to power the time delay or voltage detector.