Belly Tank Build Diary

[Mike Brown]

I designed and built a small circuit for the linear heat detection system.  There is a green LED to signal that everything is working.  If the cable becomes disconnected a yellow trouble LED illuminates.  A photo of the prototype board is attached. 

[Mike Brown]

If a fire is detected the red LED illuminates.  I used a heat gun to test the system on the bench.  My plans are to make this into a very compact system that just requires 12-16Vdc a terminal barrier to attach the cable and LEDs on the end.  It will add some peace of mind to know that there is a fire detection system in the engine compartment. 

[manta22]

Mike, do you have any data on your linear heat sensor?

[wobblywalrus]

That is impressive that you can figure out how to make something like that.  Are you an electrical engineer?

[Stainless1]

That is impressive that you can figure out how to make something like that.  Are you an electrical engineer?

Or maybe stayed in a Holiday Inn recently  :naughty 

[Mike Brown]

That is impressive that you can figure out how to make something like that.  Are you an electrical engineer?

I am not an electrical engineer but have been designing equipment including electronic circuits for almost my entire life.  No college education, just four years in the Marine Corps.   

[Mike Brown]

Mike, do you have any data on your linear heat sensor?

Please find attached information on the linear heat detector cable. 

[Stainless1]

I'm guessing you chose the 356 degree wire for the car. 
Cool idea....  :clap
Always good to hear that people continue to use their military training... Lots of us out there that learned how to do stuff without going to college... 

[manta22]

Thanks for the info, Mike. That is interesting stuff but I wonder why Kidde doesn't specify the min/max specs for its "open" and "shorted" resistance?

[Mike Brown]

Thanks for the info, Mike. That is interesting stuff but I wonder why Kidde doesn't specify the min/max specs for its "open" and "shorted" resistance?

Neil,

The open resistance is almost infinite, my meter goes to 15 meg-ohm and it registered open.  In the shorted condition it is very shorted, like ohms.  Most fire alarm applications use 4 kili-ohm to 50 kili-ohm end of line resistors for supervision.  Loop current is limited to a few milliamps so that the fire detection system can't actually start a fire.  The conductors are tinned but "wetting current" is still a consideration along with the maximum current the shorted cable would be able to handle.  That is why I prefer to use an opamp circuit like most fire alarm panels to detect the change in state of the cable.  It was just easier to build a little circuit rather than try to fit a fire alarm panel inside a belly tank that has no extra space. 

[manta22]

That makes sense, Mike. I wonder why Kidde did not include that spec in their data sheet.

I applaud your use of op amps in your circuit!  (I used to work for Burr-Brown & then Texas Instruments when they bought BB). Let me know if you need any design help with op amp or instrumentation amplifier circuits.

[Jack Gifford]

... If the cable becomes disconnected a yellow trouble LED illuminates...

So- you monitor the far end and recognize an open circuit?
What constitutes "everything" to illuminate the green lamp?

[Mike Brown]

... If the cable becomes disconnected a yellow trouble LED illuminates...

So- you monitor the far end and recognize an open circuit?
What constitutes "everything" to illuminate the green lamp?

The green LED indicates that the circuit is powered up with no "trouble" on the detection circuit.   

[manta22]

Jack;

The 50k resistor is across the end of the twisted pair so that the circuit knows that the wire isn't broken, which would read as an open circuit.

[Jack Gifford]

Thanks Neil.