user13355: I'm using an ESP8266 so I'm limited to 3.3v.  I've found a number of inductive proximity sensors but they require 5v or more.  Does a 3.3v sensor exist?
I recently dropped some tiny screws on dark carpeting and would like to find them instead of just vacuuming them up.  Stupid project?  Maybe.  But a good excuse to buy another sensor.

user3917: I don't know what voltages inductive proximity sensors use, but in general it's inexpensive to use  3.3V–5V logic level converter modules.  These typically are bi-directional interfaces that use an N-FET and pullups to the two supply voltages to interface different-voltage logics.
See the "Bi-directional level shifters" section of the TAKING IT TO ANOTHER LEVEL: MAKING 3.3V SPEAK WITH 5V article at hackaday.com for a circuit explanation.  See gearbest.com and sparkfun.com and Adafruit for inexpensive modules.
Note, for finding small screws in carpet, a search coil as used in a metal detector might be a better choice than a proximity sensor.  The coil is part of a tuned LC circuit; metal near the loop detunes the circuit slightly, producing a tone by heterodyning. 

13355: <p>I'm using an ESP8266 so I'm limited to 3.3v.  I've found a number of inductive proximity sensors but they require 5v or more.  Does a 3.3v sensor exist?</p>

<p>I recently dropped some tiny screws on dark carpeting and would like to find them instead of just vacuuming them up.  Stupid project?  Maybe.  But a good excuse to buy another sensor.</p>
 3917: <p>I don't know what voltages inductive proximity sensors use, but in general it's inexpensive to use  3.3V–5V logic level converter modules.  These typically are bi-directional interfaces that use an N-FET and pullups to the two supply voltages to interface different-voltage logics.
See the "Bi-directional level shifters" section of the <a href="http://hackaday.com/2016/12/05/taking-it-to-another-level-making-3-3v-and-5v-logic-communicate-with-level-shifters/" rel="nofollow noreferrer">TAKING IT TO ANOTHER LEVEL: MAKING 3.3V SPEAK WITH 5V</a> article at hackaday.com for a circuit explanation.  See <a href="http://www.gearbest.com/other-accessories/pp_241590.html?currency=USD&amp;vip=760186&amp;gclid=CMPxtdWqoNICFUq5wAodEKwP3g" rel="nofollow noreferrer">gearbest.com</a> and <a href="https://www.sparkfun.com/products/12009" rel="nofollow noreferrer">sparkfun.com</a> and <a href="https://www.adafruit.com/product/757" rel="nofollow noreferrer">Adafruit</a> for inexpensive modules.</p>

<p>Note, for finding small screws in carpet, a search coil as used in a metal detector might be a better choice than a proximity sensor.  The coil is part of a tuned LC circuit; metal near the loop detunes the circuit slightly, producing a tone by heterodyning. </p>
