Patent ID: 7804199

Claim:
A fail-safe circuit for a gas valve, the fail-safe circuit comprising: at least one input that can be connected to a control device and at least one output that can be connected to the gas valve, where the fail-safe circuit only supplies an output voltage to open the gas valve to the at least one output if an input signal containing at least two different successive frequency signals is provided by the control device at the at least one input of the fail-safe circuit; the fail safe circuit comprising: a charging circuit, which has at least one capacitor, where the charging circuit charges the at least one capacitor of the charging circuit only when a first frequency signal is applied to or is present in the input signal, and wherein the charging circuit does not charge the at least one capacitor in the charging circuit when a second frequency signal is applied to or is present in the input signal, the second frequency signal having a lower frequency than the first frequency signal, and the at least one capacitor in the charging circuit discharges when the second frequency signal is applied to or is present in the input signal; a voltage transformer circuit, the voltage transformer circuit produces an output voltage to open the gas valve from a supply voltage when the second frequency signal is applied to or is present in the input signal, wherein the voltage transformer circuit has at least one capacitor, which charges when the second frequency signal is present in the input signal, and wherein the at least one capacitor of the voltage transformer circuit continues to provide the output voltage to keep the gas valve open for a period of time when the first frequency signal is applied to or is present in the input signal; and wherein the voltage transformer circuit includes a transistor having a control terminal that is connected via a resistor to the capacitor of the charging circuit, where the transistor of the voltage transformer circuit only conducts when the capacitor of the charging circuit is sufficiently charged by the charging circuit, and the capacitor of the charging circuit discharges as the second frequency signal is applied to or is present in the input signal of the fail-safe circuit.