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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...

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closed as too broad by markt, Andy aka, Daniel Grillo, Scott Seidman, Matt Young Mar 12 '15 at 13:59

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Is IR allowed? There are very simple detectors which just detect the presence of an IR carrier signal. \$\endgroup\$ – mkeith Mar 1 '15 at 6:24
  • \$\begingroup\$ @mkeith, yeah sure. \$\endgroup\$ – Sparkler Mar 1 '15 at 6:25
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    \$\begingroup\$ I just re-read your question. Is the switch on the same board as the LED? If so you can use an opto-coupler IC to keep them totally electrically separate. This would basically be a photo-transistor and an LED inside a single package. \$\endgroup\$ – mkeith Mar 1 '15 at 7:03
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    \$\begingroup\$ Google TSOP38238 from Vishay. When it detects the carrier, it sets the output low. When the carrier is absent, the output is high. \$\endgroup\$ – mkeith Mar 1 '15 at 7:12
  • \$\begingroup\$ LED to photo transistor, LDR, light sensor, ... . Sound - audible with transducers (speaker/beeper/mic ...) , ultrasonic. Piezo or other vibration. Capacitive coupling. Small RF modules for maybe $2-$4. SAW oscillator TX, regen/super-egen RX. |Spin a magnet (alternator). Electrostatic. ... \$\endgroup\$ – Russell McMahon Mar 1 '15 at 7:55
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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.

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