The design and construction of the working prototype of a device to measure the magnetic field induced by a conducting wire as well as the current that induces this field, based on the magneto-optical Faraday effect, is described in this work. The transducer element is a nanocomposite material made up of maghemite nanoparticles embedded in a porous silica matrix, with a high low-field Verdet constant and a figure of merit, relating Faraday specific rotation to the absorption coefficient, of 5. The device has been mostly developed with low-cost parts, using the open-source prototyping platform Arduino plus a compact commercial lock-in amplifier. It is shown that the sensing material shows a linear response up to 30 G, which would allow measuring AC currents of 1300 A at similar to 10 cm from the wire. This setup allows avoiding the use of any magnetic field concentrator. (C) 2016 Elsevier B.V. All rights reserved.