What would be the most efficient method to take a 120VAC output and step it down to 5VDC or less?

 I am trying to get a sensor to close a switch, but the output off the sensor is 120V AC, and that would blow my switch.  Upgrading the switch is not an option, so I need to step down the volts (and most likely amps) to an acceptable voltage.  I'm thinking a concept similar to a plug in wall transformer. Any ideas?

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lemonie7 years ago
The output off the sensor is 120V? I think the out put from the unit is 120V, the actual sensor inside is switching at a lower voltage. Can you take it apart and photograph it? You should be able to remove the 120V bit and replace it, or maybe not even that much.

spacecase544 (author)  lemonie7 years ago
 Allow me to clarify...I have a motion sensor off a outdoor floodlight, and i'm tying it to a number key on a cell phone programmed for a speed dial.  For this project, i'm just trying to plug the sensor into the wall, so the output from the sensor just has to close the number switch.  The output off the sensor was hardlined directly into the two 120V/150W light sockets, so I have to step down the voltage to close the switch without blowing the system.
Yes that's like what I thought. I'm suggesting that you take it apart and use the sensor to switch low voltage. It's not going to switch 120V directly there will be some kind of step-up to 120V inside it.

Use a relay with a mains coil, switched on by the switch, and wire the contacts on the relay back to your 5V source.  This way you get perfect isolation.

Alternatively, if you need to provide 5V yourself, get a small 120-6V transfomer and a 7805 voltage regulator. Power the transformer 120V side from the switch, and take a feed to your amp from the other side and the regulator.
I second the relay idea.  Simple and effective.
rich_moe7 years ago

First, you need to change the voltage then change it from AC into DC. In other words, transformer, then rectifier. If you need high amperage, You are better off recycling an older used computer power supply (see the multiple instructables on this website) than trying to build it yourself (ease of maintenence, cost, safety, etc.). The output of a computer power supply is already set for +5VDC out (as well as +12VDC, -12VDC, and -5VDC, which I gather from your question is not relavent or necessary).

If you just need a signal voltage, then a 'wall wart' should be just what you need.