Apparatus and methods are described, including apparatus that includes an antenna (34), configured to, by drawing energy from a magnetic field, provide a main supply voltage. The apparatus further includes operational circuitry (46, 22) configured to operate only if a derived supply voltage, derived from the main supply voltage and supplied to the operational circuitry, is greater than a threshold value, and modulating circuitry (36, 40, 42), configured to modulate a load of the antenna by alternatingly (i) connecting current-drawing circuitry to the main supply voltage, thus causing the main supply voltage to drop below the threshold value, and (ii) disconnecting the current-drawing circuitry from the main supply voltage without disconnecting the operational circuitry from the main supply voltage. Other embodiments are also described.
