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I would like to find or create a small low-power proximity detector to tell a device when another device is near. This would be coupled to a small microcontroller like the AT-tiny 45. It doesn't need much resolution as it will only influence right and left turns, so I could just have two directional detectors and the device will turn in the direction of the strongest signal. Ideal range would be 5cm-1meter, but I'd be happy with 20cm-50cm.

The idea of using an IR LED and photodiode could work, but I'd prefer an option that works better in the sun, so am looking for other options. I had initially thought of making a tiny antenna and EMF detector antenna, but the proximity resolution is really poor. I'm wondering if someone can suggest ideas relating to small solid state magnets and pickup coils.

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  • \$\begingroup\$ Ultrasound should work well. \$\endgroup\$ – EM Fields Aug 28 '14 at 0:32
  • \$\begingroup\$ What about IR modulated at 38 kHz, you can get integrated demodulators for that (it is used in TVs). \$\endgroup\$ – venny Aug 28 '14 at 0:36
  • \$\begingroup\$ This sounds interesting! Please add a little more information if you have a minute. Are you suggesting transmitting with something like this and receiving with something like this? I gather that the modulation will make it much less susceptible to noise from the sun while outside? \$\endgroup\$ – sn6uV Aug 28 '14 at 0:43
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Easy way of short-range proximity detection and slow data transmission is using infrared light modulated at carrier frequency of several kHz.

This method is resistant to ambient light, because it depends on difference in irradiance between on-state and off-state.

At transmitter side, an IR LED is driven with constant frequency (often 38 kHz), either from an MCU or 555 in astable mode.

At receiver side, a phototransistor output is sampled at at least twice the carrier frequency and differences in received signal strengths are sought. There are integrated circuits such as this that combine phototransistor with demodulator circuitry and provide simple logic level output (often it is open collector).

When used for data transmission, simple OOK (on off keying) is employed to connect UARTs of two devices.

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