The AM Forum

I have A pair of 4-400's in my RF deck.  I have a good sized blower running air up through the sockets.

I would like to do two things,

1- Put in some sort of air flow sensor (vane & switch?) that can kill the fils if cooling is lost.

2- Put in a delay shut-off for the blower so that when the fils are shut down, the blower runs for an additional period of time (say 2-3 minutes?).

I have a couple of options for 1, but I am kinda stumped for a off-delay that doesn't involve a pricey relay.  Any suggestions?

I'd do it with an 8 pin PIC.

You can do it with a simple transistor and capacitor circuit and a small $2 relay also.

What popped in my head was the control module from just about ANY oil or gas fired furnace.

You would have the shut down delay component, thermal sensor and an optic / IR sensor based fail safe ( you could aim the eye at the 4-400's ). Most even have line level relays right on the PC board.

The low fuel sw. in the example below could be coupled to whatever beverage & container you have in the shack to prevent dehydration while strapping...

But seriously, I like your idea and I have a file drawer full of used modules. If you can't find a used or cheap one let me know. Any self respecting burner service guy has them stashed all over the place  ;)  
                                                 D

I parted out an old TWT amplifier that had a vane cutout circuit.  It was just a lever microswitch with a 4-5" rod attached, and a thin brass vane soldered to the rod.  All you need to figure out is if the leverage is enough to trip the microswitch when the fan is on.

I have many "pricey time delay relays" that will only cost you the price of shipping from East Hartford, Ct. I have so much treasure in my cellar (my wife calls it junk) that I will never use & I will be more than happy to share it.
You can call me evenings @ 860 568 9415 so we can coordinate this. maybe I could take some digital pics or give you make & model numbers.
Bob

You can also buy AC solid state relays for a song on eBay or from surplus dealers. Most are designed for DC control in the 3 to 32 volt DC range. A simple 555 off delay timer could be used to control the AC relay turn off time. They require very little current operate, since most use optoisolators on the control inputs.

Pete