REVERSE ENGINEERING
ANALYSIS OF THE
KILL-A-WATT
Jason 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 consumeroriented 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-circuitanalysis.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-basedwireless-killawatt/