Low-Power Circuit for an Electromagnetic Warning System Sensor

February 1, 2010

Army Research Laboratory, Adelphi, Maryland

Low-power sensors are important to the Army for monitoring infrastructure of the lifecycle of an operation. Isotope batteries can power and operate compact, low-power sensors for decades. A lowpower circuit has been developed to generate a repetitive radio frequency (RF) impulse, which will be used to indicate that a sensor has detected a target. This sensor circuit has been modeled and built to evaluate several isotope batteries in preparation. A parametric study of components in the circuit has been performed to minimize power consumption as a function of repetition rate and pulse width of the light-emitting diode indicator or RF impulse output.

The Breadboard Model of the circuit simulationof the low-power sensor. An AAA battery isused instead of the isotope battery, which canhave the same voltage as the AAA battery. Thebattery operates at a voltage of 1.5 V, whichenergizes a circuit.

An example of an isotope battery built for this project involves beta particles emitted from nickel-63 that are collected within a P-N junction. Nickel-63 emits very-lowenergy electrons (beta particles) with an average energy of 17 keV. These electrons are absorbed by silicon carbide (SiC) in PN junctions. The lattice structure of SiC is tighter than in Si. This makes SiC a more robust semiconductor — a radiation-hard material more suitable for use as a direct energy converter. It has been shown to last longer in the presence of radiation. Lowpower circuit operation is essential in order to use these sources that can last for decades. This circuit will be used as a lowpower sensor evaluation standard for isotope batteries under consideration.

The circuit used in this sensor simulator is a discrete element replacement for the circuit found in the LM3909 LED flasher oscillator. The application for this circuit is as follows: If a sensor/detector attached to the circuit goes positive, the flasher will be initiated. The LED in this circuit could just as easily be replaced by a small, teardropmonopole antenna. The resulting mW power levels of radio frequency (RF) radiation will identify activity, and would be a good sensor circuit for a wireless array.

The circuit was constructed on a breadboard. Measurements were performed to identify voltage and current across significant components in the circuit that were involved in this parameter study. The components of interest included the diode resistor R3, the repetition rate resistors (R10 and R5), and the capacitor C1. After recording voltage and current, measurements were made to identify how much power and energy are needed to turn on and sustain the circuit for longer than merely a few cycles. By understanding the effects of varying these components, an understanding of the tradespace can be developed for the application of this sensor/circuit.

This work was done by John Russo, James Brent, and Marc Litz of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ARL-0088