Magnetic-Radiation Remote-Controls

Posted on Feb 7, 2014

This unit can be useful as a short-range, single-channel remote-control. When the pushbutton in the transmitter circuit is briefly activated, the LED D1 in the receiver illuminates and an optional beeper or relay can be operated. Circuit operation is based on a non-modulated 35KHz frequency carrier transmitter, and on a high-gain two-stage 35KHz a

Magnetic-Radiation Remote-Controls
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mplifier receiver, followed by a frequency-voltage converter and DC load driver. No outer antenna is required on both transmitter and receiver sections, due to the very low frequency operation. The antennas are 10mm. diameter, 10cm. long ferrite rods supporting the coils. The receiver operates at ultra-low voltage supply (3V) and standing current (100 µA): in this manner it can be left in stand-by mode for years before a battery replacement is needed. Snags are: the short-range operation (about a medium-sized apartment), the high number of windings for the coils and the high current drawn by the transmitter. Luckily, this latter snag is compensated by the fact that only a short pulse from the transmitter is needed to operate the receiver. Therefore, if the transmitter is not operated continuously, its battery should last long. Q1 and Q2 are wired as a Darlington pair to obtain the highest possible output from a Hartley type oscillator. C2 must be trimmed to obtain the highest sinewave output (best viewed on oscilloscope). In the prototype the sinewave amplitude measured at C1 leads reached 800V peak-to-peak at 9V supply and 450mA current. Q1 and Q2 form a two-stage linear amplifier. Therefore, the small 35KHz signal picked-up by L1 is highly amplified by these devices and feds Q3 wired as a pulse-to-DC converter. When the input signal reaches Q3, the collector voltage of this transistor goes low, thus activating the LED D1...

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