Programmable security sensor

1. A programmable barrier alarm for monitoring a barrier, comprising:

a magnet; and

a sensor, comprising:

a magnetic field detector for sensing a magnetic field produced by the magnet and for producing an electronic signal based on the magnetic field;

a processor; and

a memory for storing alarm threshold values and processor-executable instructions that, when executed by the processor, cause the sensor to;

in a calibration mode of operation:

determine a value of a first electronic signal from the magnetic field detector when the barrier is in the closed position;

calculate a first threshold value based on the first electronic signal; and

calculate a second threshold value based on the first electronic signal;

in a normal mode of operation, compare electronic signals from the magnetic field detector to the first and second threshold values; and

generate an alarm signal if any of the electronic signals are less than the first threshold or greater than the second threshold.

2. The barrier alarm of claim 1 , wherein the first threshold value is determined by increasing the value of the first electronic signal by a predetermined percentage and the second threshold value is determined by increasing the value of the electronic signal by a second percentage.

3. The barrier alarm of claim 1 , wherein the barrier alarm further comprises a user interface for providing a signal to the processor indicative of when the barrier is in the closed position.

4. The barrier alarm of claim 1 , wherein the value of the first electronic signal is determined when the magnetic field is stable for a predetermined time period, as determined by the processor.

5. The barrier alarm of claim 1 , further comprising:

an instrumentation amplifier having an adjustable gain;

wherein the processor-executable instructions further comprise instructions that cause the barrier alarm to:

determine, by the processor, when the electronic signal is not within a predetermined voltage range;

increasing the gain of the instrumentation amplifier when the electronic signal is less than a lower limit of the predetermined voltage range;

decreasing the gain of the instrumentation amplifier when the electronic signal is greater than an upper limit of the predetermined voltage range.

6. The barrier alarm of claim 1 , wherein the processor-executable instructions further comprise instructions that cause the barrier alarm to, while in the calibration mode of operation:

generate a second electronic signal representative of a second magnetic field sensed by the magnetic field detector when the barrier is in an open position; and

calculate the first threshold value based on the first and second electronic signals; and

calculate the second threshold value based on the first and second electronic signals.

7. The barrier alarm of claim 6 , wherein the processor-executable instructions that cause the processor to calculate the first threshold comprise instructions that cause the barrier alarm to, while in the calibration mode of operation:

determine a difference between the value of the first electronic signal and the second electronic signal;

multiply the difference by a predetermined percentage to obtain a product;

assign the product to the first threshold.

8. The barrier alarm of claim 6 , wherein the barrier alarm further comprises a user interface for providing a signal to the processor indicative of when the barrier is in the open position.

9. The barrier alarm of claim 6 , wherein the first and second threshold values are determined when the magnetic field, as determined by the magnetic field detector, is stable for a predetermined time period.

10. A method performed by a programmable barrier alarm monitoring a barrier, comprising:

sensing, by a magnetic field detector, a magnetic field produced by a magnet;

generating, by the magnetic field detector, an electronic signal based on the magnetic field;

in a calibration mode of operation:

determining, by a processor coupled to the magnetic field generator, a value of a first electronic signal from the magnetic field detector when the barrier is in a closed position;

calculating a first threshold value based on the first electronic signal; and

calculating a second threshold value based on the first electronic signal; and

in a normal mode of operation:

comparing electronic signals from the magnetic field detector to the first and second threshold values; and

generating an alarm signal if any of the electronic signals are less than the first threshold or greater than the second threshold.

11. The method of claim 10 , wherein the first threshold value is determined by increasing the value of the first electronic signal by a predetermined percentage and the second threshold value is determined by increasing the value of the electronic signal by a second percentage.

12. The method of claim 10 , wherein the barrier alarm further comprises a user interface for providing a signal to the processor indicative of when the barrier is in the closed position.

13. The method of claim 10 , wherein the value of the first electronic signal is determined when the magnetic field is stable for a predetermined time period, as determined by the processor.

14. The method of claim 10 , further comprising:

determining, by the processor, when the electronic signal is not within a predetermined voltage range;

increasing a gain of an instrumentation amplifier when the electronic signal is less than a lower limit of the predetermined voltage range;

decreasing the gain of the instrumentation amplifier when the electronic signal is greater than the predetermined voltage range.

15. The method of claim 10 , further comprising:

while in the calibration mode of operation:

generating a second electronic signal representative of a second magnetic field sensed by the magnetic field detector when the barrier is in an open position; and

calculating the first threshold value based on the first and second electronic signals; and

calculating the second threshold value based on the first and second electronic signals.

16. The method of claim 15 , wherein calculating the first threshold comprises:

determining a difference between the value of the first electronic signal and the second electronic signal;

multiplying the difference by a predetermined percentage to obtain a product;

assigning the product to the first threshold.

17. The method of claim 15 , wherein the barrier alarm further comprises a user interface for providing a signal to the processor indicative of when the barrier is in the open position.

18. The method of claim 15 , further comprising:

determining, by the processor, when the magnetic field is stable for a predetermined time period; and

determining, by the processor, the first and second threshold values when the magnetic field is stable for a predetermined time period.