On one side I have 5V DC powering an LED, and on the other side I have a button (switch/latch/whatever). What's the simplest circuit for this button to control the state of the LED (hopefully without OpAmps)? Range is not important, just the principle. Not including electromagnetic induction...
closed as too broad by markt, Andy aka, Daniel Grillo, Scott Seidman, Matt Young Mar 12 '15 at 13:59
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Simplest way I know is using light, put a phototransistor into the circuit as a low-side switch and shine a light at it, it will turn your LED on. This is good within line of sight. You can make this a fun laser tagging game my shielding the phototransistor so only a head on laser beam would hit it.
If you have to use radio waves within a short range, you can look into
TA7642-based AM receiver and a single transistor AM transmitter. Feed the output of
TA7642 into a Schmitt trigger and control a MOSFET with the output of it. This is good within up to a few tens of meters, about covering your house and maybe include your neighbor in the range. If you have some elderly living within that range this can be a very Good Samaritan project.
For even wider coverage range, you may be better off using Wi-Fi or 3G and control your trinket using Internet.