Smart-home device switching circuitry with integrated power stealing control

1. A smart-home device comprising:

an energy-storage element that stores energy that is harvested from an environmental system;

a power wire connector and a return wire connector;

a solid state relay (SSR) switching integrated circuit (IC) comprising:

one or more switching elements configured to operate in:

a first operating state in which the one or more switching elements create a connection between the power wire connector and the return wire connector; and

a second operating state in which the one or more switching elements interrupt the connection between the power wire connector and the return wire connector; and

a digital control circuit that controls the one or more switching elements, wherein the digital control circuit is configured to:

cause the one or more switching elements to operate in the first operating state to activate an environmental function of the environmental system;

determine that an electrical property of the energy-storage element has dropped below a first threshold while the one or more switching elements operate in the first operating state;

in response to determining that the electrical property of the energy-storage element has dropped below the first threshold, cause the one or more switching elements to operate in the second operating state;

harvest energy from the environmental system while the one or more switching elements operate in the second operating state;

determine that a first time has elapsed since the one or more switching elements began operating in the second operating state; and

in response to determining that the first time has elapsed since the one or more switching elements began operating in the second operating state, cause the one or more switching elements to operate in the first operating state.

2. The smart-home device of claim 1 , wherein the first time is short enough in duration that execution of the environmental function of the environmental system is not interrupted.

3. The smart-home device of claim 1 , wherein the SSR switching IC further comprises a monitoring circuit that receives a signal indicating an electrical characteristic of the energy-storage element.

4. The smart-home device of claim 3 , wherein the SSR switching IC further comprises one or more references coupled to one or more comparators, wherein the one or more comparators determine how the electrical characteristic of the energy-storage element compares to the one or more references.

5. The smart-home device of claim 3 , wherein the digital control circuit is further configured to:

while harvesting energy from the environmental system when the one or more switching elements operate in the second operating state, monitor the electrical property of the energy-storage element to determine if the electrical property of the energy-storage element has risen above a second threshold.

6. The smart-home device of claim 1 , wherein the one or more switching elements comprises a pair of field-effect transistors (FETs).

7. The smart-home device of claim 1 , wherein the SSR switching IC further comprises a serial bus interface.

8. The smart-home device of claim 7 , wherein causing the one or more switching elements to operate in the first operating state to activate the environmental function of the environmental system occurs in response to receiving a command to activate the environmental function, wherein the command is received from a smart-home-device processor that is not part of the SSR switching IC via the serial bus interface.

9. The smart-home device of claim 1 , wherein the smart-home device comprises a thermostat, and wherein the environmental system comprises a heating, ventilation, and air conditioning (HVAC) system.

10. The smart-home device of claim 1 , wherein the SSR switching IC is packaged in a physical chip that is separate from the energy-storage element.