Detector working "backward" Help!

[Seth!]

Detector working "backward" Help!

Hi. I am modifying a simple Radio Shack detector as a table-top demonstration. It now runs off an AC/DC converter instead of batteries. I had it working fine but then had to disconnect and reconnect some wires.

Now when I turn it on, it is working "backward", which is to say it makes a tone when there is no metal near the coil and goes silent when there is metal near the coil. The meter is also pegging backward. It used to jump up from zero when metal was introduced but now the needle gets buried below zero. Another symptom is that the detector makes noise when it is in battery test mode. I don't think it did that before; I think that the needle just pegged. The needle still pegs properly when I have it on battery test.

I have not used this detector in a long time but I seem to remember this happened before I made these changes although lately it was working fine.

By the way, I know that I have the polarity correct on the power because it will not work if I swap + for -.

Any ideas?? I am stumped and frustrated. Thanks.

 

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[Seth!]

Re: Detector working "backward" Help!

Another thing I just noticed: It seems to work okay if the object is REALLY big, like a frying pan or the metal carpet strip. Adjusting the sensitivity has not favorable effect, however. The needle still goes BACKWARD with smaller objects. It was working just fine yesterday morning. I am mystified. I hope someone can help me.

 

[sniffer]

Re: Detector working "backward" Help!

your coil wires are reversed

 

[Seth!]

Re: Detector working "backward" Help!

Sniffer, I'll see if I can check that out. There are five wires and I triple checked that they are connected the same way as they originally were. I wonder if one came loose and if that would have the same effect.

 

[sniffer]

Re: Detector working "backward" Help!

the way it is wired is you have 2 wires for transmit, 2 wires for receive, and 1 wire for a shield wire

 

[jeff of pa]

Re: Detector working "backward" Help!

Are you sure the Disc isn't set to Max ?

 

[Seth!]

Re: Detector working "backward" Help!

Thanks, Sniffer. That may help!
Jeff of PA, I am starting with Tuning and Discrimination both set at the halfway mark. When I turn the unit on, I press the magic red button (zero) and dial in the tuning until the meter hits zero with no metal nearby. So if I understand you correctly, I don't think I have it set to Max.

Thank you. I am going to check my wiring.