REVERSE ENGINEERING ANALYSIS OF THE

KILL-A-WATTJason Sweeney

Ryan Gittens

Sean Kolanowski

History of Power Meters• Became necessary in the

1880s to track the power usage of individual residences.

• Until recently, have been mostly relegated to commercial applications.

• Recently, many consumer-oriented power meters have been brought to market through both companies and Kickstarters.

Introduction• The Kill-A-Watt measures the

power consumption of any device that is plugged into it.

• Useful for alerting users to the peak, average, and standby power consumption of their appliances.

• Helps identify what devices are costing the user the most money during operation, as well as which devices have high “vampire” power consumption while turned off.

Why the KILL-A-WATT• Very little IC integration.• Simple, easy to understand

circuits.• Easy to replicate

functionality in our design.• Easy to connect the circuits

to a microcontroller.• Lots of documentation

available online.

Block Diagram

Part 1• The Kill-A-Watt is comprised of

two circuit boards. The first board contains the line voltage pass-through, a 13.5v DC rectifying circuit (red box), and a 2.1 milliohm resistor in series with the neutral line (yellow box).

• The DC rectifying circuit is used for driving the electronics in the Kill-A-Watt itself.

• The 2.1 milliohm resistor is used so that the current can be detected by measuring the voltage difference across the resistor.

Part 2

• The second board contains the circuitry for amplifying or attenuating the voltage and current measurements, converting the line voltage sine wave to a square wave for frequency measurement, control buttons, the microcontroller, and the LCD.

Part 2 (continued)Legend: Red: 5v, 100mA regulator.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Dark Blue: Stage 2 of current sensing line. Multiplies output of stage 1 by a gain of 10. Accurate for 0-1 amps. Output to microcontroller pin 40.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Dark Blue: Stage 2 of current sensing line. Multiplies output of stage 1 by a gain of 10. Accurate for 0-1 amps. Output to microcontroller pin 40.

Purple: Line voltage attenuator and conversion from sin wave to square wave. Used for detecting exact frequency of line voltage.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Dark Blue: Stage 2 of current sensing line. Multiplies output of stage 1 by a gain of 10. Accurate for 0-1 amps. Output to microcontroller pin 40.

Purple: Line voltage attenuator and conversion from sin wave to square wave. Used for detecting exact frequency of line voltage.

Pink: Control buttons. Communicate with the microcontroller via two analog inputs, pins 44 and 45.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Dark Blue: Stage 2 of current sensing line. Multiplies output of stage 1 by a gain of 10. Accurate for 0-1 amps. Output to microcontroller pin 40.

Purple: Line voltage attenuator and conversion from sin wave to square wave. Used for detecting exact frequency of line voltage.

Pink: Control buttons. Communicate with the microcontroller via two analog inputs, pins 44 and 45.

Lime: EEPROM used by microcontroller.

Part 2 (continued)Legend: Red: 5v, 100mA regulator. Orange: Ripple capacitor C11 and 6.1v power supply for quad

Op-Amp. Yellow: 2.33v power supply for DC offset of the current and

voltage sensing Op-Amps. Green: Line voltage attenuation circuit with DC offset for

voltage sensing line. Output to microcontroller pin 38. Light Blue: Stage 1 of current sensing line. Multiplies voltage

difference across R2 (2.1 milliohm) by a gain of 40. Accurate for 1 to 15 amps. Output to microcontroller pin 39.

Dark Blue: Stage 2 of current sensing line. Multiplies output of stage 1 by a gain of 10. Accurate for 0-1 amps. Output to microcontroller pin 40.

Purple: Line voltage attenuator and conversion from sin wave to square wave. Used for detecting exact frequency of line voltage.

Pink: Control buttons. Communicate with the microcontroller via two analog inputs, pins 44 and 45.

Lime: EEPROM used by microcontroller.

Incorporation into our Design• The sections which we are

interested in for our design include the yellow, light blue, dark blue, green, red, and possibly orange sections.

• We will use a single quad Op-Amp, for power regulation, signal attenuation, and signal amplification.

• The 5 volt regulator will be necessary for driving the microcontroller and the voltage divider in the yellow section.

• The orange section may or may not be necessary. Since we won’t be exceeding an output of 5 volts from the Op-Amps, we may be able to power them with the 5 volt regulator, rather than the 6.1 volt output from that circuit.

Incorporation into our Design (cont.)

• We will likely use an Atmel ATmega328p microcontroller for its ease of programming within the Arduino platform. After programming the microcontroller, we can move it into a socket within our own PCB.

• We only need 3 analog signals, ranging from 0 to 5 volts, sent to the microprocessor. They include the attenuated line voltage, the voltage difference across the 2.1 milliohm resistor with a gain of 40, and the voltage difference across the 2.1 milliohm resistor with a gain of 400. We may or may not omit the LCD, since we will be communicating all information wirelessly, anyways.

Necessary Additions to this Design

• We now need to add a way of turning the outlet on and off remotely.

• We also need a method of wirelessly linking our device with a home Wi-Fi network

• After weighing the advantages and disadvantages of several wireless data transmission systems we decided that the best approach would be to use Wi-Fi.

• The Wi-Fi module that we decided to use for testing is the Adafruit CC3000 shield version. For our final prototype, we may use a more low power module like the Qualcomm RTX4100-EC.

• Finally, once we are able to control and monitor the circuit, we need to adapt our design to work with multiple outlets.

Kill-A-Watt Components and Cost Analysis

Works Cited• [1] Tranchemontagne, Mike. "Hooked on Arduino & Raspberry Pi." :

Kill-A-Watt Circuit Analysis. N.p., 23 Mar. 2013. Web. 29 Sept. 2014. Available: http://www.mikesmicromania.com/2013/03/kill-watt-circuit-analysis.html?m=1

• [2] "Tweet-a-Watt Solder It up." Ladyadanet Blog RSS. N.p., 17 May 2011. Web. 29 Sept. 2014. Available: http://www.ladyada.net/make/tweetawatt/solder.html

• [3] "Inside Kill A Watt." Cafe Electric Llc. N.p., n.d. Web. 29 Sept. 2014. Available: http://cafeelectric.com/killawatt/

• [4] Nate. "Kill A Watt." - News. N.p., 9 Nov. 2008. Web. 29 Sept. 2014. Available: http://hackaday.com/2008/11/10/kill-a-watt-teardown/

• [5] "LowPowerLab." LowPowerLab. N.p., 28 Dec. 2012. Web. 29 Sept. 2014. Available: http://lowpowerlab.com/blog/2012/12/28/wattmote-moteino-based-wireless-killawatt/