This document summarizes the design of the Magnetag V3.0 system, which aims to advance gameplay for a magnetic tag vest system by adding hit strength and location tracking capabilities. It describes the design of the coil matrix sensor, analog conditioning circuitry, power supply, analog-to-digital conversion system, and Bluetooth Low Energy transmitter. The coil matrix was designed with 8 coils to provide 75% torso coverage. Testing showed the system can differentiate hit strengths and locations. The BLE transmitter was shown to reliably transmit 8-bit and 16-bit packets with adequate power up to 2 meters. Next steps include refining the hit detection algorithm and migrating components to the nRF51422 chip.

1. MAGNETAG
The Kurt Cobain Conspiracy Club is:
John-Paul Petersen, Andrew Reich, Christopher Lembke, & Kush Gupta
Team Advisor: Dr. Cory Prust

2. MAGNETAG V1.0
COIL MODULE AMPLIFIER ARDUINO
HIT/NO HIT
DETECT
UI
1 RGB LED
1 BUZZER
POWER
3.0V DC
4 ANALOG
PULSE
4 “DIGITAL”
TRIGGER
• Worn as a sash
• 2 front, 2 back
• Hit/No hit
• 3 hits you’re out

3. MAGNETAG V2.0
• Worn poncho style
• 4 front, 4 back
• Hit/No hit
• 3 hits you’re out
• Player stats
tracked via BLE

4. 2x 4-CHANNEL ADC
nRF51422 BLE EVALKIT
8x ANALOG
CONDITIONER
COIL MATRIX
MAGNETAG V3.0

5. MAGNETAG V3.0
BLOCK DIAGRAM

6. MAGNETAG
VERSION SPEC COMPARISON
V3.0 Objective: To advance gameplay via larger sensors with
hit strength and location tracking.
Specification V3.0 V2.0 V1.0
Torso Sensor Coverage 75% 25% 15%
Differentiated Hit Strengths Yes No No
Differentiated Hit Locations Yes No No
Coil Output Variance (+/-) 5% Untested Untested
Bluetooth Ready Yes Yes No
Missed Hits < 0.5% < 2% < 7%
Interference Protection RF Filtering Near EM Sources Near EM Sources
Battery Life 2 hrs 2 hrs 2 hrs
Control Box Size 2.5" x 3.5" 2.5" x 3.5" 2" x 3"
Version Comparison Table

7. COIL MATRIX
3.3V
Analog
Conditioner
REQUIREMENTS:
• Covers most of the torso
• Outputs indicate strike location
• Outputs reflect strike impact
• DC shift for single-ended
amplification and sampling

8. COIL MATRIX
DESIGN SELECTION
1. Circular
• Readily available from sponsor
• Uniform geometry
• Large dead center zone
2. Lemniscate
• Eliminates central dead zone
• Greater surface area coverage
• Peak flux linkage in center
3. Long Coils
• Simple construction
• Efficient coverage
• Peak flux linkage at ends

9. COIL MATRIX
DESIGN
Specification Target
Number of Coils 8
Coil Magnet Wire Gauge 38 AWG
Coil Spacing 5.75 cm
Coil Width 8.25 cm
Coil Length - Type A 56 cm
Coil Length - Type B 38 cm
Coil Length - Type C 104 cm
Coil Length - Type D 41 cm
Coil Matrix Specifications Table

10. COIL MATRIX
DETERMINING STRIKE LOCATION

11. COIL MATRIX
STRIKE LOCATION RESULTS

12. COIL MATRIX
DETERMINING STRIKE IMPACT
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0 20 40 60 80 100 120 140 160 180 200
SampledVoltage[V]
Sample Index [unitless]
Example Coil Pulse Samples With Absolute Peaks Marked
Idle Average
Ch0 Upper Pk
Ch0 Lower Pk
Ch1 Upper Pk
Ch1 Lower Pk
Ch0 Coil Output
Ch1 Coil Output

13. COIL MATRIX
SUBSYSTEM PERFORMANCE TEST DATA

14. COIL MATRIX
SUBSYSTEM PERFORMANCE TEST DATA

15. COIL MATRIX
PROCESSING STRIKES
GAME MAIN
NEW PEAK IN
CHANNEL
BUFFER (n)
?
STORE PEAK w/
CH# IN HIT
BUFFER
RESET CHANNEL
CHECK
(n = 0)
UPDATE
HEALTH LED
ARRAY
ASSIGN VEST
LOCATIONS TO
PEAKS WITH
L.U.T.
STORE STRIKE
FINGERPRINT
IN BLE BUFFER
CHANNEL
NUMBER
INCREMENT
n = (n+1)
Y
ALL CHANNEL
BUFFERS CHECKED
FOR PEAKS
?
N
Y
N
ASSIGN TOTAL
DAMAGE FOR
STRIKE
FINGERPRINT

16. ANALOG CONDITIONER REQUIREMENTS:
• Prepare the signal for sampling
• Boost 80mV peaks to 800mV
• Filter unnecessary frequency
content3.3V
ADCCoil
Matrix

17. ANALOG CONDITIONER
DESIGN SELECTION
1. Pre-amp filtering, inverting input
• Quickly discovered problems with coil
isolation
2. Post-amp filtering, non-inverting input
• Both alterations help isolate the coils

18. ANALOG CONDITIONER
DESIGN
Specification Target
Supply Voltage 3.3 V
Current Consumption 5 mA max
Gain 20.6 dB
Cutoff Frequency 2 kHz
Output Voltage 0 - 2 V
Analog Conditioner Specifications Table

19. ANALOG CONDITIONER
FREQUENCY RESPONSE
-20
-15
-10
-5
0
5
10
15
20
25
0.1 1 10 100 1000
AmplifierGain[dB]
Frequency [kHz]
Analog Conditioner Frequency Response

20. ANALOG CONDITIONER
GAIN PREDICTABILITY
0.02
0.12
0.22
0.32
0.42
0.52
0.62
0.72
19.0 20.0 21.0 22.0
Probabilityz-Value
Signal Gain [dB]
Gain Variance of 50 Random Coil Strikes

21. POWER SUPPLY REQUIREMENTS:
• Run off a 3.7V rechargeable
• Provide stable 3.3V supply
• Provide 40mA for 2 hours
nRF51422
BLE SoC
10 kHz ADC
Coil
Matrix
Signal Conditioner

22. POWER SUPPLY
DESIGN
Specification Target
Miniumum Input Voltage 3.65 V
Output Voltage 3.3 V
Required Output Current 40 mA
Expected Battery Life 2 hours
Power Supply Specifications Table
nRF51422
BLE SoC
10 kHz ADC
Coil
Matrix
Signal Conditioner

23. POWER SUPPLY
VOLTAGE REGULATION
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
3.6
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
OutputVoltage[V]
Input Voltage [V]
Voltage Regulation (Output vs Input)

24. POWER SUPPLY
LOAD RESPONSE
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
VoltageOut[V]
Runtime Hours [hrs]
Voltage Supply Output for a 36mA Load

25. ANALOG TO DIGITAL REQUIREMENTS:
• Sample 8 coils at 10kHz each
• Interface via dual SPI bridges
• Dual chip solution
3.3V
nRF51422
BLE SoC
Analog
Conditioner

26. ANALOG TO DIGITAL
DESIGN SELECTION
1. nRF51422 onboard ADC
• 500 Hz, single channel max
• (We need 8x 10 kHz)
2. Single channel with external multiplexing
• Initially considered the cheapest solution
• High power consumption
• Added complexity
3. 4-channel with internal multiplexing
• Reduced power consumption
• Simplified interfacing
• Reduced cost

27. ANALOG TO DIGITAL
DESIGN
Specification Target
Sampling Rate 40 kHz/chip (10 kHz/channel)
Sampling Bit Depth 12-bit
Data Interface SPI
Data Rate 4 Mbps
Data Clock 4 MHz
Supply Voltage 3.3 V
Reference Voltage 3.3 V
Current Consumption 400 uA max
4-Channel ADC Specifications Table

28. ANALOG TO DIGITAL
SAMPLING RATE TEST
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30
SampleVoltage[V]
Channel (0) Sample Index
4MHz Slave Clock – 13.7 kHz/Channel

29. ANALOG TO DIGITAL
LOGIC ANALYSIS
NOTES:
• 54.6 kHz overall sampling
• 18.3 us between samples
• 8.0 us between chip
shutdown and startup
• Delay caused by unseen
microcontroller routines
4MHz Slave Clock – 54.6kHz Overall Sampling Speed
13.7kHz/channel

30. ANALOG TO DIGITAL
OVER-CLOCKING TESTS
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30
SampleVoltage[V]
Sample Index
2MHz Slave Clock – 10.3 kHz/Channel
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30SampleVoltage[V]
Sample Index
8MHz Slave Clock – 16.4 kHz/Channel
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30
SampleVoltage[V]
Sample Index
1MHz Slave Clock – 6.9 kHz/Channel
0.0
0.5
1.0
1.5
2.0
0 5 10 15 20 25 30
SampleVoltage[V]
Sample Index
4MHz Slave Clock – 13.7 kHz/Channel

31. ANALOG TO DIGITAL
DIMINISHING RETURNS
1MHz Slave Clock – 27.5kHz Sampling 4MHz Slave Clock – 54.6kHz Sampling

32. BLE SOFT DEVICE REQUIREMENTS:
• Transmit 8-bit/16-bit packets
• Identify unique player IDs
• Adequate transmission power
3.3V
Wireless
to Player
Mobile
Devices
4-Channel
ADC
Player UI

33. BLE SOFT DEVICE
DESIGN
Specification Target
Min/Max Packet Size 8-bit/16-bit
UUID Per Player
Transmission Power <15dBm within 2 meters
Modulation GFSK
Supply Voltage 3.3 V
NRF51422 Current Consumption 25 mA max
BLE Specifications Table • Receives coil strike data via dual
4MHz SPI ports
• Processes strike fingerprints for
BLE buffer
• BLE transmits when buffer
updates

34. BLE SOFT DEVICE
WHAT’S A SOFT DEVICE?
• Enables simplified BLE functionality
• Hides complicated BLE communication protocol stacks
• Accessed and controlled via manufacturer SDKs
• Requires licensing for production

35. BLE SOFT DEVICE
SIG PROFILES, PROTOCOLS, AND SERVICES
GATT Based:
HRP:
Heart Rate Profile
HRS:
Blood Pressure Profile
PASP:
Phone Alert Status
Profile
BR/EDR Profiles:
A2DP:
Advanced Audio
Distribution Profile
HSP:
Headset Profile
FTP:
File Transfer Profile
BR/EDR Protocols:
AVCTP:
Audio/Video Control
Transport Protocol
AVDTP:
Audio/Video Distribution
Transport Protocol
IrDA:
IrDA Interoperability

36. BLE SOFT DEVICE
HRP SERVICES
GATT Based:
HRP:
Heart Rate Profile
HRS:
Blood Pressure Profile
PASP:
Phone Alert Status
Profile
HRP provides three services:
• Heart Rate Service
• Battery Service
• Device Information Services

37. BLE SOFT DEVICE
HRP SERVICES
HRP provides three services:
• Heart Rate Service
• Battery Service
• Device Information Services
Idel
HEAR RATE:
timer-time-out?
BATTERY:
timer-time-out?
DEVICE
INFO:timer-time-
out?
GET hear rate
value
(INTEGER)
GET battery
value
(INTEGER)
UPDATE device
info (CHAR)
SEND using
BLE to APP
TRUE TRUE TRUE

38. BLE SOFT DEVICE
HRP SERVICES
HRP provides three services:
• Heart Rate Service
• Battery Service
• Device Information Services
Idel
HEAR RATE:
timer-time-out?
BATTERY:
timer-time-out?
DEVICE
INFO:timer-time-
out?
GET hear rate
value
(INTEGER)
GET battery
value
(INTEGER)
UPDATE device
info (CHAR)
SEND using
BLE to APP
TRUE TRUE TRUE

39. BLE SOFT DEVICE
HRP SERVICES
HRP provides three services:
• Heart Rate Service
• Battery Service
• Device Information Services
Idel
HEAR RATE:
timer-time-out?
BATTERY:
timer-time-out?
DEVICE
INFO:timer-time-
out?
GET hear rate
value
(INTEGER)
GET battery
value
(INTEGER)
UPDATE device
info (CHAR)
SEND using
BLE to APP
TRUE TRUE TRUE

40. BLE SOFT DEVICE
HEART RATE SERVICE DATA STRUCTURES
Data Structures:
struct ble_hrs_evt_t
Heart Rate Service Event
struct ble_hrs_init_t
Heart Rate Service init structure
struct ble_hrs_s
Heart Rate Service structure
One service of interest to us:
• Heart Rate Service

41. BLE SOFT DEVICE
HRP SERVICE FUNCTIONS
Functions:
uint32_t ble_hrs_init(ble_hrs_t *p_hrs, ble_evt_t *p_ble_evt)
Function for initializing the Heart Rate Service
void ble_hrs_on_ble_evt(ble_hrs_t *p_hrs, ble_evt_t *p_ble_evt)
Function for handling the Application’s BLE Stack events
uint32_t ble_hrs_heart_rate_measurement_send(ble_hrs_t *p_hrs, uint16_t heart_rate)
Function for sending heart rate measurement if notification has been enabled
One service of interest to us:
• Heart Rate Service

42. MAGNETAG V3.0
BLUETOOTH TRANSMITTER TEST RESULTS
ButtonID Buffer Size Arbitrary Packet DataBuffered SniffedPacket DataSent
PB1 8-bit 0x3F,0x7B,0xAF,0xF4 0xF4
PB2 16-bit 0x01BC,0x02F1,0x3039,0xF618 0x02F1
8-bit/16-bitBuffer Transmission
Unique Players Adequate Power
16-bit UUID Data Tx Power
0x180D 1m: -60dBm to -43dBm
0x180D 2m: -80dBm to -61dBm

43. MAGNETAG V3.0
SYSTEM STATE DIAGRAM
SYSTEM STATE CHANGES:
• Pulse Triggered ADC Buffer
• Strike Fingerprint Processed
• BLE Buffer Updated
• BLE Transmission Completed

44. MAGNETAG V3.0
CONCLUSION - NEXT STEPS
• Configure the strike fingerprinting algorithm (Andy & Chris)
• Migrate the SPI bridge to the nRF chip (Kush & John-Paul)
• Optimize the ADC sampling rate (John-Paul & Team)
• Ruggedize the system for live play (Team & Sponsor)

45. MAGNETAG V3.0
CONCLUSION - NEXT STEPS
• QUESTIONS?