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Author Topic: another part of my lunacy in building stuff  (Read 2943 times)

Posts: 85

« on: October 19, 2007, 05:54:59 PM »

I'm building a screwdriver antenna for the car, can't afford one of the super expensive commercial ones and theres never a second hand one when I'm looking so it's off to B&Q hardware store.  I've got it all together just one slight problem of controlling the motor. I'm thinking 433MHz AM RF control unit with a small receiver in the base, Ideally I wanted a rocker switch to control the up down of the coil inside this might not be possible with the design of the RF unit. Any ideas ? the unit will generate it's own noise but I'm guessing it will be filtered at manufacture as the RF unit is a commercial grade one and not a Maplin special (bit like Radio Shack) Am I just spiraling into lunacy on this one?

Charlotte 2E0BSS (Oh Ive found plans for a full size Multi band SSB transceiver so that is the next project)

Posts: 21764

« Reply #1 on: October 19, 2007, 07:19:50 PM »

I don't understand why anyone would want to use RF to control a screwdriver when it's so easy to do with reversible DC, as was intended.

Take a bit of time, go to the pub, watch David Beckham of the Los Angeles Galaxy play in his last game of the season, injured.  He's a bit of a neighbor to me, now, although I've never seen him.  Wifey Posh Spice gets around a bit, though -- we spotted her and the kids having dinner at the local T.G.I.F. a few weeks ago.

Back to screwdriver antennas: Use DC control.  It's simple, it's cheap, it works.  Ameritron makes a slick new (brand new) controller for "any" screwdriver with a lot of memories.  It parks the motor at the bottom every time before it tunes, and then counts turns on the coil as it goes up.  Very resettable, I'm pretty impressed...although it does cost $119 (USD)...

Posts: 85

« Reply #2 on: October 20, 2007, 12:44:43 PM »

$119 isn't that much around £53 ok theres duty on top of that but im sure i could get the controller over here. I only thought about RF controller to cut down the amount of wires.

I'll have to look at the controller too

Charlotte 2E0BSS

Posts: 5688

« Reply #3 on: October 21, 2007, 12:53:54 AM »

Well, think about this:  The number of wires would be aprox. the same.  

You would have to have Pos and Neg coming from the battery of a vehicle or power supply of a shack going to the motor in the antenna.

On top of that, you would have to devise a relay circuit to put between the RF receiver and the motor connections, the receiver is not likely to be able to handle the current of the motor, much less the inrush current when the motor is started.  

On top of that, there would have to be more than one relay, in order to get the double pole isolated reversing DC needed for the motor, this can get complicated and would require some sort of solid state control logic circuit board to control those relays -- or 4 power transistors in an H config and a way to heatsink them.  

A simple double pole, double throw switch at the other end of the power wires would be far less trouble, much less expensive.  

And I'd bet that since the motor is run while tuning the transmitter, the high RF would swamp that little bugger, 450MHz or no, it would be dead in the nearfield.  Solid state junctions, well, any dissimilar junction, which would include the solder joints, would be lightin' up, even at 5-10W tuning power.  That would lock the motor control out if not destroy it.  

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