A diagnostic circuit is provided. The diagnostic circuit comprises resistors and a capacitor. The diagnostic circuit is monitored to detect the presence of a load and to confirm the proper operation of a relay circuit. The diagnostic circuit generates an oscillating voltage when the relay circuit is open and the load is connected, generates a logic low when the relay circuit is open and the load is disconnected, and generates a logic high when the relay circuit is closed. The diagnostic circuit can be disposed within a diagnostic circuit system that includes one or more of a sensing circuit, a relay circuit, an alternating-current source having a direct current offset, an input circuit, and a high-voltage microprocessor. The high-voltage microprocessor can be bi-directionally optically coupled to an application microprocessor.