A student in one of my scouting groups recently asked if the human body conducts electricity, to which I replied, "yes, it does." She then asked if I could show her how it does this. Well, not wanting to stick my hand in the nearby 115VAC outlet and show her firsthand how electricity conducts itself through the human body, i simply promised her that the next week the scouting group would meet, I would have something to demonstrate this. So I thought about this for a little bit and came up with a very simple circuit to demonstrate this property of human conduction. The circuit I simply named, "The Body Light."
The circuit's aim is to demonstrate that the human body is indeed a conductor of electricity and that by completing an electrical circuit by using the body itself as a "wire", can illuminate several bright light sources. The student will see firsthand the property of electrical conduction through the human body and also see that the resistance of a human body varies depending on how tightly the electrodes are handled as well as which parts of the body are touching the electrodes.
Next week when demonstrated, the kit proved to be quite successful. The students just loved the Body Light, as it was now called, and couldn't get enough of it. They connected themselves into large chains of individuals and could activate the circuit with 5 and more bodies connected hand in hand. Even the scouting instructors loved the kit and i was so surprised at how entertaining it was despite the fact it was electrically so simple.
This instructable outlines the design and construction of the Body Light. It is very simple and can be built in a very short time with minimal parts.
Video of the Body Light 1.0 in action:
Step 1: Features
- Operating voltage range: 12-15V
- Open Collector Output for LED / Light - 8A, 100V max. voltage
- High quality screw terminals for hook-up
- Mounting holes for board mounting
- Onboard LED to indicate power
- Ultrabright white LED onboard to show when conduction exists
Step 2: Technical Information
One of the stationary electrodes is connected to the 12VDC, and the other stationary electrode is connected to the input of the LM386 amplifier which acts as a comparator. Resistive divider R3 / R4 creates a referene voltage of approx. 1V which exists on the inverting input of the LM386. When a human puts their hands across the two stationary electrodes, the 12V is conducted through that person which charges up the 1uF capacitor, C4. When C4 is charged up greater than 1V, it will trigger the LM358 to a "high" output which will drive and turn ON transistors Q1 (2N2222) and Q2 (MJD122T4). Transistor Q1 (2N2222) illuminates an onboard white LED while transistor Q2 (MJD122T4) is a high power darlington transistor which can drive high power lighting sources such as halogen and incandescent bulbs, as well as LEDs. In this particular case, a BA15S automotive LED bulb is utilized.
The 100k resistor, R2 on the input to the LM358 provides a path for capacitor, C4, to bleed down when the circuit is disconnected at the electrodes, while 1uF capacitor, C4, is there to provide noise immunity for the comparator. If C4 was not present, you would get false trips. C4 also provides a brief delay between the time the electrode circuit is closed and the onboard and external lights illuminate.
There is also an onboard 12V voltage regulator (LM7812), U1. This converts the input voltage to 12V. Please note, that if a voltage source less than 14V is utilized, the output voltage of the LM7812 will merely equal the voltage at the input. This just ensures that the output voltage to the electrodes never exceeds 12V. If you are using a 12V power source, the LM7812 can be omitted altogether.
- (1) Resistor, 2k, 1/4W (R1)
- (1) Resistor 100k, 1/4W (R2)
- (1) Resistor 10k, 1/4W (R3)
- (4) Resistors, 1k, 1/4W (R4,R5,R6,R7)
- (3) Capacitors, 0.1uF, 50V (C1,C2,C5)
- (1) Capacitor, 1uF, 50V (C4)
- (1) Capacitor, 2200uF, 35V (C3)
- (1) Diode, 1N4002 (CR1)
- (1) Diode, 1N4148 (CR2)
(1) LED, Blue, T1 (D1)
(1) LED, White, T1-3/4 (D2)
- (1) LED, Bulb 1156 BA15S Type Automotive LED (D3)
- (1) 1156 BA15S Base
- (1) LM358 Op-Amp (U2)
- (1) LM7812 Voltage Regulator, 12V (U1)
- (1) 2N2222A Transistor (Q1)
- (1) MJD122T4 Transistor, High Current Darlington, 8A, 100V (Q2)
- (3) Terminal Blocks, 2-Position (TB1,TB2,TB3)
- (2) Drawer Knobs (from Hardware Store)
- (1) 12V, 2A Plug-in Power Supply
- Wood Base (Walnut used in this project from local hardware store)
Step 3: Assembly
Assembly is very simple and straightforward. Simply insert parts into PCB board and solder away. Be sure the LEDs are installed with the proper polarity as well as C3, the 2200uF electrolytic capacitor. The complete manual and assembly guide is shown here as a PDF document. It includes full instructions.
Step 4: Performance
Step 5: PCB and Kit
This circuit can be easily built on a breadboard or if your handy, you can also etch your own.
Complete Kit and details here: http://www.easternvoltageresearch.com/bodylight10.html
Enjoy and good luck with your project!