Low Power Circuit for EM Warning System Sensor 

by John Russo, James Brent, and Marc Litz 

ARL-TR-4968 September 2009 

Approved for public release; distribution unlimited. 
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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-4968 September 2009 

Low Power Circuit for EM Warning System Sensor 

John Russo, James Brent, and Marc Litz 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Low Power Circuit for EM Warning System Sensor 

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John Russo, James Brent, and Marc Litz 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Low power sensors are important to the Army for purposes of monitoring infrastructure over the life-cycle of operation. 

Isotope batteries can power and operate compact, low power sensors for decades. A low power 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 results of simulation and measurement compare well. The 

minimum power characteristics are also identified in this report. 

15. SUBJECT TERMS

Low power sensor, EM sensor 

16. SECURITY CLASSIFICATION OF:

17. LIMITATION 

 OF 

 ABSTRACT 

UU 

18. NUMBER

 OF 

 PAGES 

26 

19a. NAME OF RESPONSIBLE PERSON

Marc Litz 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified 

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Unclassified 

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(301) 394-5556 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Background 1

1.1 Circuit Description ..........................................................................................................1

2. PSpice Modeling 2

2.1 Parametric Variation of Components ..............................................................................5

2.1.1 Capacitance (C1) .................................................................................................5

2.1.2 Resistances (R10 and R5) ....................................................................................6

2.1.3 Diode Resistor R3 ...............................................................................................7

2.2 Power Dissipation............................................................................................................7

3. Circuit Description 10

3.1 Components of Circuit ..................................................................................................10

3.1.1 Transistors .........................................................................................................10

3.1.2 LED ...................................................................................................................10

3.2 Circuit Logic..................................................................................................................11

3.3 Experimental Results .....................................................................................................12

4. Comparison of Modeling and Circuit 15

5. Conclusions 16

List Symbols, Abbreviation, and Acronyms 18 

Distribution List 19 
iv 

List of Figures 

Figure 1. Breadboard of circuit simulation of low-power sensor. ..................................................2 

Figure 2. Pspice circuit model of low power sensor simulator. Resistor R3 at the top left 

controls the current in the diode. Capacitor C1 controls the repetition rate and pulse 

width. .........................................................................................................................................3 

Figure 3. A repetition rate of 2 Hz is shown in the measured voltages above. The voltage are 

calculated in the simulation across a) 100 uF capacitor C1 [0, 700 mV] and b) LED [0, 2 

V]. ..............................................................................................................................................4 

Figure 4. The current calculated in the LED is shown for single pulse in the repetitive train. 

The pulse width shown is ~2.3 ms. The peak amplitude is 38.9 mA. ......................................5 

Figure 5. The graph shows the relationship with flash rate resistance (R10 & R5) and 

repetition rate of the diode. ........................................................................................................6 

Figure 6. The repetition rate of the sensor decreases with increasing capacitance. ........................6 

Figure 7. The resistors R10 and R5 were varied in the model. The resulting repetition rate is 

shown with a linear trend line drawn with respect to resistance. ...............................................7 

Figure 8. The graph shows the relationship between power dissipation and resistance value 

in resistor R3. .............................................................................................................................9 

Figure 9. The IV curve for the LED shows the voltage threshold graph and the polarity of 

the red LED. .............................................................................................................................11 

Figure 10. The circuit operation can be divided into two operational modes. The charging 

cycle (a) and the voltage doubled flashing cycle (b). .............................................................12 

Figure 11. Pulse width variation of the output is achieved by varying the capacitor C1. The 

oscilloscope traces above all use a 50 ms/division setting. The capacitors are varied from 

a) 47uF, b) 220 uF, c) 470uF and d) 1000uF. The corresponding pulse widths are 19, 55, 

80, and 152 ms. ........................................................................................................................14 

Figure 12. The graph shows the relationship of capacitance, PW, and Rep. Rate. ......................15 

List of Tables 

Table 1. The diode resistor (R3) can be increased to minimize the power consumption to 

10 mW. .......................................................................................................................................8 

Table 2. Changing the Flash Rate Resistors (R10 & R5) using smaller capacitance. ....................9 

Table 3. Changing capacitance in LED circuit. ............................................................................15 

Table 4. The circuit power can be minimized by changing the capacitor, which modifies the 

signal repetition rate. ................................................................................................................17 
1 

1. Background 

The goal of the project is to build a low power sensor circuit that can be used as a metric to 

evaluate an isotope battery. The significant function of an isotope battery is its usage as a power 

source that can release low level power (~1 mW) for decades. Isotope batteries are safe for low 

power applications. “Safe” is defined here in two ways: (1) quantities of isotopes used for the 

batteries do not require any licensing by the Nuclear Regulatory Commission (NRC), and (2) the 

radiation inherent in these quantities is below background levels. The typical efficiency for 

power output of isotope batteries is ~15%.1 This is important to minimize the amount of isotope 

required and maximize power out per micro-Curie of material. A Curie is the unit of 

radioactivity defined as 3.7 x 1010 events per second. 

An example of an isotope battery we have built is described in section 1.1. Beta particles 

emitted from nickel-63 are collected within a P-N junction. Nickel-63 emits very low energy 

electrons (beta particles) with an average energy of 17 keV. These electrons are absorbed by 

silicon carbide (SiC) in P-N junctions.2 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. Low 

power circuit operation is essential in order to use these sources that can last for decades. This 

circuit will be used as a low-power sensor evaluation standard for isotope batteries under 

consideration. 

Applications for low power sensors will continue to grow in importance. Low power sensors 

that can be buried in infrastructure for their life cycle (e.g., buildings, bridges, and roadways) 

would provide valuable warning of failure modes that could develop. The life-cycle requirement 

is in some ways similar to the need for long shelf-life batteries.3 

1.1 Circuit Description 

In the circuit description, a AAA battery is used instead of the isotope battery, which can have 

the same voltage as the AAA battery. The breadboard model of the circuit as built is shown in 

figure 1. The battery operates at a voltage of 1.5 V, which energizes a circuit. The circuit 

includes four transistors (one PNP and three NPN). The transistors operate as switches and 

current regulators. The transistors in the circuit are bipolar junction transistors (BJT). These 

transistors have three terminals—emitter, base, and collector. There are two P-N junctions that 

exist inside a BJT, which are base/emitter junction and base/collector junction. The NPN 

 1Litz, M.; Blaine, K.; et al. On-Demand High Energy Density Materials. 3rd International Energy Conversion Engineering 

Conference, San Francisco, CA, 15–18 August 2005. 

2Ngu, Y; Litz. M. Study of Beta Radioisotopes Direct Energy Converters; ARL-TR-4969;U.S. Army Research Laboratory: 

Adelphi, MD, September 2009. 

3O'Brien, H. A. Radioisotope Applications. Oak Ridge National Laboratory. U.S. Atomic Energy Commission, 1962. 
2 

transistor is characterized by electrons being injected into the base region (emitter-base junction 

is forward biased). The NPN transistor controls the number of electrons coming in the base 

region. The minimum voltage for the transistor to turn on is 0.7 V. The PNP and NPN 

transistors are limited to ~200 mA max. 

Figure 1. Breadboard of circuit simulation of low-power sensor. 

In this circuit, PNP is turned on by the power source and collects the electrons from the current. 

The PNP signals the other NPN transistors, then signals the 100 μF polarized capacitor. The 

current charges the capacitor parallel to the battery and the light-emitting diode (LED). After the 

capacitor is charged up, it discharges to the LED. The LED starts flashing at 1.634 V. The LED 

signals the PNP that it has passed its threshold and has turned on. The PNP transistor signals the 

other three transistors that the LED has turned on, which repeats the whole process. The NPN 

triggers in the circuit, and the LED is turned off and out of the circuit for milliseconds, which 

explains the flashing light. When the voltage is dropped to 0.5 V, the transistor idles and the 

process is repeated. This process occurs so quickly that the only thing visible is the LED 

flashing. 

Although the voltage passes the 1.634 V threshold of the LED, the voltage increases to about 

1.8 V when it is flashing constantly in the circuit. The rest of the components in the circuit are 

resistors. These 10 resistors control and decrease the amount of voltage in the circuit. The 

polarized capacitor, 5.6 k, and 3.3 k resistors control and set the flash rate of the LED, and 

directly control the repetition rate and pulse width of the LED’s pulse. 

2. PSpice Modeling 

The first step in this experiment was to construct a PSpice simulation to model the circuit. The 

circuit is shown in figure 2. The objective was to (1) verify that the circuit performs as expected, 

and (2) vary components in the circuit to understand the limits of operation and the minimumpower operation points. At the time of this effort, an exact model of the LED could not be 
3 

found. We instead used two diodes that modeled the voltage and current load that were similar 

to that of the LED (which we measured on the breadboard). The forward voltage threshold of 

each diode model was 0.7 V in the simulation. The actual LED in circuit has a forward voltage 

threshold of 1.634 V. By inserting two diodes into the simulation, the voltage closely matched 

the breadboard circuit. Measurements were taken of each component in the circuit with a Fluke175 multi-meter for comparison. The voltage of the two diodes approximately matched the LED 

load of the breadboard. The current measured in the LED on the breadboard circuit was 

22.22 mA. This differed from the simulation calculated current in the model of 38 mA. This 

known difference can still be used when comparing parametric studies in the modeling. 

Figure 2. PSpice circuit model of low power sensor simulator. Resistor R3 at the top left controls the current 

in the diode. Capacitor C1 controls the repetition rate and pulse width. 
4 

The calculated response of the circuit is shown in figure 3. The diode’s output frequency is 

2 Hz, reaching peaks of 1.65 V, while the peak-to-peak voltage calculates to 1.4 V, with a pulse 

width of about 3.5 ms. The model calculated the voltage across the capacitor to be 678 mV 

peak, while the peak-to-peak voltage is 578 mV, with a pulse width of 400 ms. 

Figure 3. A repetition rate of 2 Hz is shown in the measured voltages above. The voltage are calculated in the 

simulation across a) 100 uF capacitor C1 [0, 700 mV] and b) LED [0, 2 V]. 

Current probes were placed across the diodes in the model. The calculated current waveform is 

shown in figure 4. The current peak is 38.5 mA at 1.65 V. The pulse width is ~2.3 ms for the 

current pulse. 
5 

Figure 4. The current calculated in the LED is shown for single pulse in the repetitive train. The pulse 

width shown is ~2.3 ms. The peak amplitude is 38.9 mA. 

2.1 Parametric Variation of Components 

Three components were varied in the following investigation—the capacitor C1, the diode 

resistance R3, and the repetition rate resistance (R10 and R5). The circuit parameters that are of 

interest in this study are power dissipated in the circuit, the pulse width, and the repetition rate. 

The results are described in the following subsections. 

2.1.1 Capacitance (C1) 

After verifying the current and voltage of the LED (diodes), a parametric study of capacitors was 

performed to identify the change in pulse width and repetition rate. The capacitance was varied 

with four values: (1) 47 μF, (2) 220 μF, (3) 470 μF, and (4) 1000 μF. The resulting output 

repetition rates are 4, 0.7, 0.3, and 0.2 Hz, respectively. The smaller capacitance modeled using 

PSPICE resulted in an increased repetition rate (see figure 5). However, the pulse width 

decreases with a smaller capacitor. Measurements of the dependence of repetition rate with 

capacitance variations are confirmed and shown in figure 6. The measured reduction in 

repetition rate with increasing capacitance is confirms the simulation trends. The reduction in 

repetition rate has impact on the energy consumed by the sensor circuit. 
6 

Flash Rate Resistance vs. Rep. Rate

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 1 2 3 4 5 6 7 8 9 10

Flash Rate resistance (k ohms)

Rep. Rate (Hz)

10uF capacitor 

1uF capacitor

Resistance vs. Repetition Rate

1

10

100

1000

10000

0 2 4 6 8 10 12 14

Flash Rate resistance (k ohms)

Rep. Rate (Hz)

10uF capacitor 

1uF capacitor

100 uF capacitor

Figure 5. The graph shows the relationship with flash rate resistance (R10 & R5) and repetition rate of the diode 

resulting from numerical calculations. 

Repetiton Rate vs Capacitance

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 200 400 600 800 1000 1200

Capacitance

RR (Hz)

Figure 6. The repetition rate of the sensor decreases with increasing capacitance 

as measured in the circuit. 

2.1.2 Resistances (R10 and R5) 

A parametric study of resistance was performed on resistors R10 and R5. These components can 

be seen in the circuit diagram at the lower left-hand corner of figure 2. As the resistance 

increased, the repetition rate decreased. The pulse width of the LED/diodes and the voltages 

across the LED/diodes do not change significantly. The two resistors were each replaced four 

times with 500 ohms, 1 k, 3 k, and 7 k resistances. The resulting eight data sets are shown in 

figure 7. The repetition rate over this small resistance change varies only from 0.2 Hz to 5 Hz. 

The capacitance and the diode resistance R3 are held constant at 100 μF and 12.6 , 

respectively. 
7 

Repetiton Rate vs Resistance

y = -0.0004x + 6.0132

R2

 = 0.9239

0

1

2

3

4

5

6

2000 4000 6000 8000 10000 12000 14000

Resistance

RR (Hz)

Figure 7. The resistors R10 and R5 were varied in the model. The resulting repetition rate is shown 

with a linear trend line drawn with respect to resistance. 

Physically combining R10 and R5 had no effect on the circuit. There was no change when the 

two flash rate resistors (R10 and R5) were combined to add up to 8.9 k. Combining these two 

resistors would make it easier to change and control the values in order to see the change in 

repetition rate of the diode, power dissipation of the diode, and the pulse width. 

2.1.3 Diode Resistor R3 

This resistor does not affect the repetition rate. The significance of this component is in the 

overall power of the circuit. Current through the diodes can be controlled by the diode resistor 

(R3 in figure 2) the 12.6  resistor. Diode current decreases with increasing resistance in the 

Diode resistor R3. The effect on circuit power will be discussed in section 2.2. Changing this 

resistor’s values did not change the flash rate of the LED/antenna loads, or the pulse width, only 

the current draw. 

2.2 Power Dissipation 

The power (P) dissipated in the circuit can be determined by measuring both the current (I) and 

the voltage (V), i.e., P = IV. We would like to minimize power consumed in the circuit in order 

to make it last long. The practical aspects of the circuit that will be determined by the 

application are the power requirements, the pulse repetition rate, and the pulse width of the 

LED/antenna load. These parameters are not mutually exclusive; in fact, they are all related to 

each other. The operational scenario of the sensor will determine the best tradeoff of these three 

characteristics: power, pulse width, and pulse repetition rate. 

We wish to identify the components that contribute most to power loss in the circuit. The diodes 

consume 64 mW of power, each diode consuming 32 mW. The capacitor C1 consumes about 

28.1 mW; the 12.6  resistor (see R3 in figure 2) consumes ~19.5 mW. To decrease the power 
8 

dissipation of the resistor, the diode resistance can be increased. When increased to 15 , the 

power dissipation of the resistor decreased to 18 mW. This alteration left the battery drain at 

65 mW and a single diode drawing 27 mW. Decreasing the resistance to 1  decreased the 

power dissipated in the resistor to 12 mW and the total battery drain to 195 mW, while a single 

diode increased to 110 mW. Further measurements concluded that changing that resistor did not 

change the flash rate of the diode and capacitor. 

Another observation was that the two diodes have the same repetition rate, but different power 

dissipation. Results showed that a single diode has a small comparison with the LED in the 

actual circuit. However, changing the resistor’s values did change the current of the diode a 

small amount. The column consumption is a measure of the ratio of power radiated by the 

LED/antenna, divided by the power used by the remainder of the circuit. 

Variation of resistor R3 changes the current in the LED/diodes and resistor. As resistance 

increases in R3 (see table 1 column 2), the power consumed by the diode (see column 3 table 1) 

decreases. This effect is shown graphically in figure 8. 

Table 1. The diode resistor (R3) can be increased to minimize the power consumption to 10 mW. 

Changing the Value of the Diode Resistor (12.6 ohms)

Resistance 

(ohms) 

Power Dissipation 

Diode 

(mW) 

Power Dissipation 

Battery 

(mW) 

Current 

of Diode 

(mA) 

Consumption 

% 

 1 109 195 117.000 55.8 

 10 38 82.3 45.800 46.1 

Constant in model 12.6 32.1 75 38.500 43 

 100 5.6 23.4 7.820 24 

 1000 0.6 11.9 1.039 5 

 10000 0.063 10.5 0.122 0.6 

 80000 0.0072 10.3 0.017 0.069 

 100000 no pulse no pulse n/a n/a 
9 

Power Dissipation vs. Resistance 

0.001

0.01

0.1

1

10

100

1000

1 10 100 1000 10000 100000

Diode Resistor (ohms)

Power consumed by Diode 

(mW)

Figure 8. The graph shows the relationship between power dissipation and resistance value 

in resistor R3. 

Increasing the capacitance (C1) decreased the repetition rate and the pulse width. Increasing 

resistors R10 and R5 decreased the repetition rate. The diode resistor (R3) stayed constant with 

normal value of 12.6 . Figure 7 shows the relationship between the flash rate resistors (R10 

and R5) and the repetition rate. As the flash rate resistor increases, the repetition rate of the 

diode decreases. The measured data of figure 7, that are tabulated in table 2, identify the 10 

measured points, and serve to identify the noted trends towards reducing the power drain in this 

circuit for optimized use in sensors. 

Table 2. Changing the flash rate resistors (R10 & R5) using smaller capacitance. 

10 μF Polarized Capacitor

Resistance 

(k ohms) 

Rep. Rate 

(Hz) 

Voltage 

(V) 

Current 

(mA) 

Power 

Dissipation 

(mW) 

Pulse 

Width 

(ms) 

Capacitor 

(μF) 

8.9 20 1.63 39.3 32.2 1.3 10 

5 35 1.627 38.4 31.2 0.7 10 

2.5 66.3 1.615 35.4 28.4 0.55 10 

1 100 1.595 30.9 24.5 0.4 10 

0.5 200 1.57 25.7 20.3 0.02 10 

1 μF Polarized Capacitor

8.9 160 1.635 40.5 33 0.6 1 

5 290 1.62 39.5 32.3 .005 1 

2.5 550 1.61 35.3 27.4 .002 1 

1 1,130 1.609 32.3 25.8 .001 1 

.05 1,760 1.576 27.7 22 .0001 1 

The purpose of these measurements was to explore the use of this circuit for high repetition rate 

operation. The LEDs can be replaced with wideband antenna. Under these circumstances, the 

output signal should be configured for kilohertz repetition rate operation and micro-second pulse 
10 

widths. There are several applications that use high frequency warning sensors. By reducing the 

capacitance and resistance, the repetition rate can be increased to 1,760 Hz using very small 

amounts of current. 

3. Circuit Description 

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, then the flasher will be intiated. The LED in 

this circuit could just as easily be replaced by a small teardrop-monopole 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. 

3.1 Components of Circuit 

3.1.1 Transistors 

The four transistors in the circuit are key components. In circuits, transistors are semiconductor 

devices that are used as current regulators and switches. They are used to amplify or switch 

electronic signals. PNP and NPN represent which semiconductors are in the middle of the two 

conductors. Transistors are considered very useful and effective in circuits because they are 

small and have minimal weight, allowing the development of miniaturized electronic devices. 

They also have highly automated manufacturing processes, which allow for mass production 

(low per-unit cost). Another advantage is lower possible operating voltages, making transistors 

suitable for small, battery-powered applications (AAA battery and isotope battery). They have 

no warm-up period for cathode heaters required after power application, and they consume a 

small amount of power and have greater energy efficiency. Transistors can amplify, producing 

an output signal with more power in it than the input signal. The additional power comes from 

an external source of power. Another advantage of a transistor is that it has an extremely long 

life—some transistorized devices produced more than 30 years ago are still in service. 

3.1.2 LED 

An LED is a semiconductor diode that emits incoherent narrow-spectrum light when electrically 

biased in the forward direction of the P-N function. The diode used in the circuit is a red LED. 

These devices are typically used as small indicator lights in electronics. They are also used in 

high power applications such as flashlights. The potential difference (Vf) of the LED can vary 

from 1.3 V to 2.1 V. The diode does not turn on and open until the voltage passes through a 

threshold, which is shown in figure 9. 
11 

Figure 9. The IV curve for the LED shows the voltage threshold graph and the polarity of the red LED. 

Current flowing in the forward bias direction of an LED generates light. Reversed bias flow 

generates no light. The LED can be operated with alternating or bi-polar current and voltage, but 

the diode will only light with positive voltage. This causes the LED to turn on and off at the 

frequency of the AC supply. The minimum voltage of the LED as modeled in the circuit is 1.3 V 

at 0.024 mA. The maximum voltage tested in the circuit is 1.8 V at 22.22 mA. The minimum 

current for the LED to be lit is 0.03 mA, with a voltage of 1.5 V. This measurement was taken 

outside of the circuit. In the circuit, the LED begins flashing at 1.634 V at .8 mA. When the 

LED starts constantly flashing in the circuit, the voltage is 1.8 V at a current of 22.22 mA. 

3.2 Circuit Logic 

There are two states of operation for the sensor simulator circuit. The two states are identified 

when the circuit is 1) charging and 2) flashing. To get a better understanding of the circuit (see 

figure 10), we must realize that the transistors in the circuit perform as current switches and 

regulators. State 1) is when the switch is open and the capacitor in the circuit is charging. The 

current from the battery flows through the resistors. The capacitor charges in series with the 

battery and parallel with the LED. The capacitor charges to supply the voltage. 
12 

 (a) (b) 

Figure 10. The circuit operation can be divided into two 

operational modes. The charging cycle (a) and 

the voltage doubled flashing cycle (b). 

State 2) is when the switch is closed. The current is now flowing through the LED and the 

circuit is flashing. The capacitor discharges in series with the battery and the LED. This mode 

of the flasher circuit is when the LED flashes. At this point, both the battery voltage and the 

voltage across the capacitor are available to drive the LED. This process enables voltage 

multiplication. The voltage multiplies because the capacitor is charged up to 1.5 V, then it 

discharges across the LED. After being discharged, the battery is still sending current through the 

circuit, but into the charging of the capacitor. This, in turn, adds the 1.5 V from the cap and the 

battery, and the voltage is increased to pass the diode’s threshold. The capacitor does not charge 

to full supply voltage. The oscillator switches before the capacitor can be reversed-biased. 

3.3 Experimental Results 

The circuit was constructed on a breadboard (see figure 1). 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 of the effects of 

varying these components, an understanding of the trade-space could be developed for the 

application of this sensor/circuit. 

The measured characteristics of the LED in the circuit shown in figure 2b are (a) frequency 

output of the LED is 2 Hzm, (b) peak to peak is 1.52 V, and (c) a pulse width 2.4 ms. The 

measured characteristics of the capacitor shown in the circuit in figure 2a are (a) frequency of the 

capacitor charge is 2 Hz, (b) maximum voltage of 700 mV, and (c) a pulse width is 500 ms. 

The current in the LED is small enough that it is difficult to measure directly on the breadboard. 

The current through the LED was deduced from the current measured in the adjacent resistor R3. 

The minimum voltage to barely light the LED was 1.3 V at a small current of .024 mA. When 
13 

the current is at .03 mA, the LED is lit with 1.5 V. The LED begins flashing at 1.634 V at 

.8 mA. In the circuit, the voltage is 1.8 V at 22.22 mA. The resistor R3, closest to the LED, 

which was the 12.6 Ω resistor, was used to measure the voltage. Voltage across the resistor was 

280 mV. So, by dividing the voltage by the resistance, we get the current for the LED 

(280 mV/12.6  = I = 22.22 mA). So, the maximum voltage tested in the experiment was 1.8 V 

at 22.22 mA. 

The capacitor was varied in order to change the pulse width of the output. The four different 

types of capacitors in the circuit were the same values as modeled in PSpice: 47 μF, 220 μF, 

470 μF, and 1000 μF capacitance. However, the predicted observation was that these 

measurements were going to be different than in the simulation, as different diodes were used in 

simulation from those used in the actual circuit. This one variation is a significant variable in our 

experiment. The final outcome of this test determined that the larger the capacitor, the slower 

the flash rate and the longer the pulse width. 

As the capacitance increases, the pulse width increases and becomes wider. This trend is shown 

the series of oscilloscope traces shown in figure 11. The power consumed in the circuit is 

calculated by P = IV. We then calculated Energy (E = P*t). Figure 12 illustrates the repetition 

rate and the pulse width of the LED. The power drain from the source is between 35 to 40 mW, 

when varying the five different capacitors that were measured in the circuit: 47 μf, 100 μF, 

220 μF, 470 μF, and 1000 μF. Table 3 shows that as capacitance increases, repetition rate and 

pulse width decrease; the current and power, however, gradually increase. The relationship of 

capacitance, pulse width, and repetition rate are shown graphically in figure 12. The flashing of 

the LED starts slowing down because it takes the capacitor longer to discharge, which explains 

the slow blinking of the red LED. 
14 

 (a) (b) 

 (c) (d) 

Figure 11. Pulse width variation of the output is achieved by varying the capacitor C1. The oscilloscope traces 

above all use a 50 ms/division setting. The capacitors are varied from (a) 47 μF, (b) 220 μF, 

(c) 470 μF, and (d) 1000 μF. The corresponding pulse widths are 19, 55, 80, and 152 ms. 
15 

Power level 35-40mW 

0.1

1

10

100

1000

0 200 400 600 800 1000 1200

C (uF)

RR (Hz)

PW (ms)

Figure 12. The graph shows the relationship of capacitance, PW, and repetition rate. 

Table 3. Changing capacitance in LED circuit. 

Capacitor 

(μF) 

Rep. Rate 

(Hz) 

Pulse Width 

(ms) 

Measured 

Current (I) 

(mA) 

LED Power 

(mW) 

Energy 

(J) 

47 4.444 20 21.43 35.02 3.11 

100 2 30 22.22 36.31 2.18 

220 0.7142 55 23.175 37.87 1.49 

470 0.4 80 23.492 38.39 1.23 

1000 0.196 152 23.81 38.91 1.16 

4. Comparison of Modeling and Circuit 

A pair of D1N4148 diodes was used in the circuit to simulate the LED. At the time of the work, 

no LED was available in the library of P-spice components. The diodes used in the model have 

slightly different current loading characteristics than the LED. That is the most significant 

difference between experiment and modeling. The repetition rate compared well between the 

model and experimental LED circuit, as it was very similar when the capacitance was changed in 

both model and the circuit. This is because the transistors and capacitors in both procedures 

signal at the same time. The flash rate is 2 Hz when the original 100 μF capacitor was used. 

Another comparison that worked well was that voltage across the diodes and LED were very 

similar in value. The diode voltage threshold (for both in series) is 1.4 V, and the LED’s voltage 

threshold is about 1.5 V for it to turn on. The maximum peak in the PSpice model is 1.65 V 

(pulse width is 3.5 ms), and the maximum peak in the circuit is about 1.8 V (pulse width is 

2.4 ms). 
16 

The main differences in this experiment are the current and power dissipation. The current 

across the two diodes is 38.5 mA. The current across the LED is about 22.22 mA. The power is 

different because of the different currents running across the diodes and LED (P = IV). The 

main reason for this difference was that the LED model was not found or located in PSpice. 

However, in further evaluations, we will solve that problem by locating the LED model, so that 

the simulation will compare better to the LED circuit. 

5. Conclusions 

In concluding this experiment, we found that the tasks of building this low power sensor 

simulator and measuring certain parts in the circuit were successful. We fulfilled the job of 

building a circuit that works as a LED flasher/oscillator. The main task in this project was to 

find how much power is drawn in the circuit. After finding the power, we were supposed to find 

ways to create less power drawn from the battery so we could create a very low power sensor. 

We were able to build a circuit on PSpice, so we could observe the parameters of the circuit. We 

built the circuit in PSpice using simple components in a circuit. We made several measurements 

in PSpice and compared them to the actual data later in the experiment. The data was very 

similar to the data recorded in the actual circuit. The LED’s frequency was 2 Hz, mirroring the 

polarized capacitor’s frequency in both the actual circuit and PSpice. The supply voltage 

increased in the circuit from 1.5 V in both the simulation and the actual circuit. The pulse width 

and voltage are different due to the different diodes used in PSpice circuit. The diode model 

used in PSpice differs from the LED in the circuit. The repetition rate is the same because the 

transistors and capacitors signal at the same time in both the simulation and LED circuit. In 

PSpice and the actual circuit, different capacitors were used to change the flash rate and pulse 

width of the LED. Capacitors changed the frequency and wavelength. Changing the frequency 

also changed the power and current going through the LED. Also, in PSpice, we changed the 

3.3 k and 5.6 k resistors, which significantly change the flash rate. We noticed that the 5.6 k 

resistor changes the repetition rate more than the 3.3 k resistor, and that the two flash rate 

resistors act together in the circuit. The pulse width does not change very much when changing 

the resistors in PSpice. Voltage and current did not change at all in the process. 

Finally, in PSpice, we discovered which components in the circuit consume the most power. We 

then tested this power dissipation by removing certain resistors, which would allow us to see a 

change in power dissipation. Current and power, through the diodes, can be controlled by the 

diode resistors. This means that diode current decreases with increasing resistance. These 

results allow us to control power rates in the circuit. Also, this data lets us limit the power used 

in the circuit, and allow us to use a very small amount of energy to turn on the LED flasher. We 

can observe how much power is needed to draw from the battery. We learned a lot about this 

circuit, including its limits, components, and the basic workings of the LED flasher. 
17 

This circuit is intended to be used as a sensor simulator to evaluate the power load from an 

isotope battery source. To this end, we would like to easily reconfigure the circuit to draw a 

variety of power loads that range from 100 μW to 100 mW. We can vary the circuit parameters 

to create this varying load. The circuit elements that have significant control over power 

consumption are the capacitor, the diode resistor (limits current to output diode), and the 

repetition rate resistor (that varies the diode output repetition rate). Unfortunately, variation of 

the capacitor affects both repetition rate and pulse width, making calculation more complex. 

Also, the capacitor and repetition-rate resistor do not change the power dissipation as much as 

the diode resistor (which limits the current). We have calculated the circuit parameters for 

operation of the circuit over three decades of power load. The results are shown in table 4. 

Table 4. The circuit power can be minimized by changing the capacitor, which modifies 

the signal repetition rate. 

Power Dissipation Capacitor 

(uF) 

Diode resistor 

(ohms) 

Flash rate resistor 

(k ohms) 

100uW 100 6100 8.9 

1mW 100 600 8.9 

10mW 100 52 8.9 

100mW 100 1 8.9 

In this table, it shows that the capacitor and flash rate resistor can stay constant for the power 

dissipation to change. The diode resistor controls the amount of current going through the 

diodes, which controls the power consumption of the diodes. The measured results and 

parameter studies resulting from the modeling are expected to be used to evaluate several isotope 

batteries that are being developed for long-lived low-power sensors. 

In the future, we will replace the diode pairs used in the simulation with a better model of the 

LED when modeling the circuit with PSpice. In the experimental circuit, we would combine the 

two resistors that control the flash rate—5.6 k and 3.3 k resistors—in order to simplify the 

packaging. Another possibility to investigate in the future is replacing the LED with an antenna. 

The measurement of radiated RF would follow using spectrum analyzers. 
18 

List Symbols, Abbreviation, and Acronyms 

BJT bipolar junction transistors 

RF radio frequency 

LED light-emitting diode 

NRC Nuclear Regulatory Commission 

SiC silicon carbide 
19 

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