Objective: Tiny metallic foreign bodies may remain in the human body after accidental ingestion, surgery, or ballistic injury, potentially causing inflammation, tissue damage, and other complications. Although X-ray and CT are widely used for detection and localization, intraoperative motion and workflow constraints may reduce localization accuracy and real-time retrieval efficiency. This study aims to develop a portable multi-frequency electromagnetic excitation circuit to assist the detection and localization of tiny metallic foreign bodies in the human body. Methods: A multi-frequency electromagnetic excitation circuit was designed for balanced-coil eddy-current sensing. The proposed transmitter combines Selective Harmonic Elimination Pulse-Width Modulation (SHE-PWM) with a full-bridge Class-D power amplifier to generate synchronous multi-frequency excitation currents at 50 kHz, 150 kHz, 350 kHz, and 850 kHz. The use of multiple excitation frequencies provides complementary depth sensitivity, in which low-frequency excitation improves penetration depth for deeply embedded targets, while high-frequency excitation enhances the response and spatial resolution of small or superficial objects. Circuit simulations and hardware measurements were conducted to evaluate the time-domain and frequency-domain characteristics of the proposed circuit. Results: Simulation and experimental results showed good agreement with theoretical predictions. The proposed circuit successfully generated synchronous multi-frequency excitation currents with controllable spectral components. The results confirmed that the combination of SHE-PWM and a full-bridge Class-D power amplifier can provide spectrally controllable and energy-efficient excitation suitable for balanced-coil eddy-current sensing. Conclusions: The proposed multi-frequency electromagnetic excitation circuit provides a feasible supplementary solution for tiny metallic foreign-body detection and localization. Its low-cost, portable, and energy-efficient characteristics make it potentially suitable for bedside and intraoperative electromagnetic assistance, especially in scenarios where conventional imaging methods are limited by workflow constraints or real-time localization requirements.