ENERGY STORAGE LEVEL INDICATION CIRCUIT

1 . An energy storage level indication circuit, comprising:

a first energy storage device having a first capacity;

a second energy storage device having a second capacity lower than the first capacity, the second energy storage device being charged by the first energy storage device; and

a signal generator operable to transmit a heartbeat signal in response to the second energy storage device being charged to a threshold, wherein the rate of heartbeat signal transmission indicates an amount of energy stored in the first energy storage device.

2 . The system of claim 1 , wherein at least one of the energy storage devices includes a capacitor.

3 . The system of claim 1 , wherein a faster signal transmission rate indicates a larger amount of energy stored in the first energy storage device, and a slower signal transmission indicates a smaller amount of energy stored in the first energy storage device.

4 . The system of claim 1 , wherein the first energy storage device is charged from an energy harvester operable to harvest energy from environmental conditions.

5 . The system of claim 4 , wherein the energy harvester includes at least one of a photovoltaic cell, a thermal energy harvester, a hydroelectric energy harvester, or a mechanical energy harvester.

6 . The system of claim 4 , wherein the heartbeat signal is a wireless signal, and wherein a controller receives the wireless signal and determines an amount of energy available to the energy harvester by analyzing changes in the rate of signal transmission over time.

7 . The system of claim 1 wherein the heartbeat signal is a wireless signal, and wherein a controller receives the wireless signal and determines the rate of signal transmission to determine the amount of energy stored in the first energy storage device.

8 . The system of claim 1 , wherein the first energy storage device is charged from a voltage output of a sensor.

9 . The system of claim 1 , the system further including a comparator and a solid state switch, the comparator comparing a charge of the second energy storage device to a threshold, and commanding the signal generator to transmit the heartbeat signal in response to the charge of the second energy storage device exceeding the threshold, the solid state switch discharging the second energy storage device concurrently with the signal transmission.

10 . An energy storage level indication circuit, comprising:

an energy storage device; and

a radio control module operable to transmit a wireless heartbeat signal at a rate indicating an amount of energy stored in the energy storage device.

11 . The system of claim 10 , including a controller operable to receive the wireless signal and determine the rate of signal transmission to determine the amount of energy stored in the energy storage device.

12 . The system of claim 10 , wherein the energy storage device is charged from an energy harvester operable to harvest energy from environmental conditions.

13 . The system of claim 12 , wherein the controller is also operable to determine an amount of energy available to the energy harvester by analyzing changes in the rate of signal transmission over time.

14 . The system of claim 12 , wherein the energy harvester includes at least one of a photovoltaic cell, a thermal energy harvester, a hydroelectric energy harvester, or a mechanical energy harvester.

15 . The system of claim 10 , including:

an analog to digital converter operable to measure an amount of energy stored in the energy storage device and to output a digital value representing the amount of energy stored in the first energy storage device, wherein either the radio control module or a microcontroller compare the digital value to a predefined energy threshold to determine a signal transmission rate.

16 . A method of indicating an amount of energy stored in an energy storage device:

harvesting energy from environmental conditions using an energy harvester;

charging an energy storage device using the harvested energy; and

transmitting a wireless heartbeat signal at a rate indicating an amount of energy stored in the energy storage device.

17 . The method of claim 16 , including:

receiving a plurality of the heartbeat signals;

determining a rate of transmission of the heartbeat signals; and

determining the amount of energy stored in the energy storage device in response to the rate of transmission.

18 . The method of claim 17 , including:

determining an amount of energy available to the energy harvester by analyzing changes in the rate of signal transmission over time.

19 . The method of claim 16 , wherein said charging an energy storage device using harvested energy includes:

charging a first energy storage device having a first capacity using the harvested energy; and

charging a second energy storage device using energy from the first energy storage device, the second energy storage device having a second capacity lower than the first capacity.

20 . The method of claim 19 , the method including:

comparing a charge of the second energy storage device to a threshold, wherein said step of transmitting a wireless heartbeat signal is performed in response to the charge of the second energy storage device exceeding the threshold.

21 . The method of claim 20 , wherein said comparing a charge of the second energy storage device to a threshold includes:

A) using a comparator to compare a voltage of the second energy storage device to the threshold;

B) commanding a signal generator to transmit the heartbeat signal in response to the voltage of the second energy storage device exceeding the threshold;

C) commanding a solid state switch to discharge the second energy storage device; and

D) selectively repeating steps (A)-(C).

22 . The method of claim 16 , the method including:

receiving an signal indicative of a charge level of the energy storage device;

processing the signal to a threshold to determine a charge level of the energy storage device; and

determining a rate of signal transmission in response to the charge level.