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  Truth Wristband  A wristband that dynamically reflects the wearer
Truth Wristband

A wristband that dynamically reflects the wearer’s psycho-emotional response to the world, promoting internal states to be externalized and made into interactive forms of expression. The device measures the galvanic skin response (a marker of emotional arousal commonly used in lie detector tests), measuring micro-changes in sweating on your hands. The device’s lights turn from blue to red as the wearer becomes excited. Telling a lie may make you sweat, but ask the right question and the answer doesn’t matter!
 

Truth TV!
Check out the Truth segment on MakeTV (episode 107) airing on PBS stations around the U.S.
 
Truth for Sale!
Truth Wristband Kit
- $44.95
NEW VERSION!
All easy soldering!
via paypal
also available from Maker Shed
 

Below is a video of my brother, Ian, wearing the Truth Wristband while I ask him some questions to see what sorts of things get his Truth meter going. Note that it takes 1-2 seconds for the psychodynamic response to be expressed in the skin response.
(get the latest flash player)
 

 

 


 

Truth for Sale!
Truth Wristband Kit
- $44.95
NEW VERSION!
All easy soldering!
via paypal
also available from Maker Shed


Kit includes:
All the parts you need... an etched PCB, a finger strap with sterling silver plates, a velcro wristband, all the electronics (inc. a programmed pic), a laser cut TRUTH face plate, instructions, 2 AAA batteries, etc.

Required tools:
Fine tip soldering iron, solder, pliers, wire cutter/stripper, flat head screwdriver, scissors, isopropyl alcohol and a toothbrush (for cleaning circuit board after soldering)


 
How to make a Truth Wristband:
 
Download v5 Assembly Instructions here (see below for older versions)
 

Circuitry Bill of Materials

1 pic - PIC18F25K20
1 op amp - MCP6241
1 switch
24" black wire
3 10K resistor
2 330K resistor
1 10M resistor
3 0.1uF capacitor
2 AAA battery holders
5 5mm 8K mcd diffused
common anode RGB LEDs
2 sterling silver finger contacts
1 velcro/elastic finger strap
 
 

Eagle Layout

   
  Click here for 600 dpi etch image
 
PIC code:
main.c
SetOutputs.c
SetOutputs.h

 
Previous Versions:
 
Version 4 (v4.135)
v4 Assembly instructions
Color Eagle Layout
600 dpi Etch Image
main.c
SetOutputs.c
SetOutputs.h


 
Tutorial: What is the "Truth" and how do you measure it?
Background:
The Truth Wristband measures the Galvanic Skin Response (GSR). GSR is a measure of emotional arousal that is detected as a sharp increase in electrical skin conductance. Physiologically, this increased skin conductance is caused by a specific type of sweat gland (eccrine, also called merocrine) that is tied in with the arousal systems of the body, including adrenaline. When you get embarrassed, angry, anxious or have other strong emotions, your skin conductance shoots up reflecting the change in your emotional state. Due to its tie with arousal as well as anxiety, the galvanic skin response is one of the main components of a lie detector test.
 
Electrodes:
Because sweat is electrically conductive, increases in sweating can be measured as increases in skin conductance (i.e. decreases in skin resistance). This skin resistance can simply be measured using two metal plates against the skin.

The best materials for the electrode surfaces are non-reactive with the skin, including gold, gold-plated copper, nickel-plated metal, platinum, palladium, silver-silver chloride, etc., but any metal, even two pennies, will work.
 
 

  Palms, feet, armpits and the forehead have the highest density of eccrine sweat glands, so for this tutorial we'll use a finger straps as a convenient electrode location. Note that physically moving the electrodes can create spurious changes in the resistance measured across the plates and contaminate our measurement. There are ways to work around this, but it's not completely trivial.
 
Voltage Conversion:
The next step is to convert the skin resistance to a voltage. This is easily done with a voltage divider (right). In this case Vin is the positive terminal of a voltage supply (e.g. a battery), Z1 is the skin resistance across the metal plates, Z2 is a standard resistor connected to the negative terminal of the voltage supply, and Vout is the resulting voltage calculated as the ratio [Z2/(Z1+Z2)]*Vin.

The skin resistance commonly fluctuates between 50K and 10M Ohms (and even higher if your hands are really cold/dry), and a value in this range will work for Z2. We will use Z2=10M because it serves to linearize the relationship between Z1 and Vout, although at the expense of creating a very high impedance (low current) circuit that could be susceptible to noise.
 
Buffering and Filtering:
Because the voltage resulting from this voltage divider is high impedance, it is important to buffer the signal with an op amp. It is also a good idea to filter the signal to remove any high frequency noise (e.g. 60Hz). Because the GSR is a slow ~1-2Hz signal, we can low-pass filter at 4.8Hz using a 0.1uF capacitor and two 330K Ohm resistors calculated from Freq=1/(2*pi*R1*C) as in the circuit below. The two resistors are the same value, so the circuit has no amplification calculated at Gain=-R1/R2.

See this page for making filters with an op amp.
To accommodate non-linearities of op amps near the voltage rails, it is generally best to set the (+) input of the op amp to the middle of the power supply input, i.e. 1/2*[(V+) - (V-)], which is generated in the above circuit with R1, R2, and C2.
 
Analysis:
Our goal is to quantify the magnitude of the GSRs to a given stimulus. The below figure takes a look at the data to best determine an analysis method.

The top plot shows the voltage recorded off of the above circuit from a nearly 6 minute recording. The sharp downward voltage deflections are the GSRs and the slow creeping back up is likely due to evaporation of sweat from the finger. Notice how hard it is to quantify these responses with something simple like threshold values.
The bottom plot shows the same signal after high-pass filtering at ~0.48Hz (i.e. ~2 seconds). High-pass filtering is essentially subtracting the baseline average skin resistance and revealing only the changes in skin resistance in the time range of the GSR. This permits the system to quickly "auto-calibrate" for different people and for changes in the baseline skin resistance (e.g. due to evaporation). Notice how much easier it is with the filtered signal measure the magnitude of the response with simple thresholds.
 
Using a PIC for Analysis:
Similar to the Truth Wristband Kit, we use a PIC microcontroller to read the GSR signal, perform the high-pass filter described above and light up LEDs to display GSRs.

Bill of Materials (BOM)
1 pic18F25K20 Mouser 579-PIC18F25K20-I/SP
1 MCP6241 Mouser 579-MCP6241-E/P
2 330K resistor Mouser 71-CCF07-G-330K
3 10K resistor Mouser 71-CCF55-10K
1 10M resistor Mouser 291-10M-RC
3 0.1uF capacitor Mouser K104K15X7RF53L2
1 RG LED Mouser 696-SSL-LX5097IGW
2 finger plates    
1 finger strap    
2 AA or AAA batteries Mouser 573-15A
1 battery holder Mouser  12BH321A-GR

 

Here is the full circuit implemented on a breadboard.
 
Below is a pinout of the pic18F25K20 and here is the full datasheet.
 
Here's the PIC code:
main.c


Under the hood, the pic is essentially performing the following operations:
(1) Read the data
(2) Smooth the data to filter out high freq noise
(3) Calculate the average data value over ~2-3 seconds
(4) Subtract the "instantaneous" signal from the average (this is essentially a high-pass filter)
(5) Set thresholds on the difference value to change the RG LED from Green <-> Yellow <-> Red
 
Here are some of the important variables and steps:
INTCONbits.TMR0IF   Timer used to trigger data sampling. Samples at 50Hz, which is plenty of resolution to read and smooth our 1-2Hz GSR.
smoothPeriod   Variable sets the weighting of a smoother to remove high frequency noise (>3Hz) from our signal. This smoothing is done "iteratively" so that each new data point is incorporated with a running average according the weight.
normPeriod   Variable that is used to iteratively calculate the average of the incoming data (over ~2-3 seconds). Note that calculating the average iteratively saves needing to store large arrays of data usually necessary for calculating an ordinary average.
threshold   The threshold increases according to a cubic function so that a wide range of individual differences in GSRs will be detectable by the meter.

 
Alternative Without a PIC
Although somewhat less flexible, it is also possible to create a basic Truth Meter circuit without the use of a microcontroller as shown below.
 
Bill of Materials (BOM)
Quantity Material Vendor Part #
2

0.75x3.5” loop Velcro

ebay  
2

0.75x0.75” hook Velcro

ebay  
2

1x3” strips brass or copper foil

www.maximum-hobby.com 0.002 Brass Foil (42 gauge)
2

10” wire

mouser  
2

2-pin header

mouser 517-6111TG
1

prototyping breadboard with wire kit

ebay / sunpec  
2

AA batteries

mouser 573-15A
1

2xAA battery holder

mouser 12BH324A-GR
1

dual op amp (MCP6002)

mouser 579-MCP6002-I/P
2

3M3 res

mouser 660-CF1/4C335J
1

1M res

mouser 660-CF1/4CT52R105G
1

100K res

mouser 71-CCF55-100K
1

10K res

mouser 660-CFP1/4CT52R103J
1

1K res

mouser CFP1/4CT52R102J
1

220 ohm res

mouser 660-CF1/4CT52R221G
2

0.1uF cap

mouser K104K15X7RF53L2
1

10nF cap

mouser 594-K103K15X7RF5TL2
3 diodes mouser 78-1N4148
1

red LED

mouser  
 
In this circuit, a voltage divider is used to convert Rskin to voltage. The signal is then band-pass filtered from 0.48-4.8Hz to "auto-calibrate" to individual baselines and remove high frequency noise. The signal is then amplified 100x to reach the a voltage high enough to light up an LED. Using diodes to set the (+) inputs to the op amp about 1.6V above V-, the output voltage of the circuit stays just below the threshold of the LED until a GSR occurs. Depending on the current/voltage properties of the diodes used, usually 2 or 3 diodes will be needed to set the "virtual ground" of the circuit 1.6V above V-.

Below is a picture of the circuit on a breadboard.
 
 
 
This circuit uses the dual op amp MCP6002. Other op amps may be used, but it is important that the op amp used can operate within 1V of the negative rail.

R1 may be substituted swapped out to change the amplification. 200K will provide 200x amplification and will make the circuit more sensitive.

This circuit will run on a range of voltages (tested 2.5-6V) depending principally on the operating voltage range of your op amp.

 

This resistor code calculator may also be useful.
 

How to Make a Truth Meter
Make Magazine Vol 26
http://www.make-digital.com/make/vol26?pg=104#pg104
http://makeprojects.com/Project/The-Truth-Meter/703/1
 

 
Also see my other biofeedback wearables / wearable computing accessories
 
 

  All text and images by Sean M. Montgomery are licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
Based on a work at www.produceconsumerobot.com.
Creative Commons License

 
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