0
\$\begingroup\$

I recently visited a museum with my son, and we both enjoyed an exhibit where there was a partially completed circuit that was completed when you touched the two end points or joined hands and each touched one of the end points. The setup had a D battery and a small array of LEDs. The LEDs would light up when you completed the circuit with your body.

My son asked me to make one for him at home, so I took an array of LEDs from a flashlight and soldered together a similar setup. The problem is that it didn't work when you completed the circuit with your body (it did when you used something with less resistance, like a wrench).

What am I missing here? I'm no electrical engineer (I'm a computer science/math geek but I do like to tinker around with electronics and I have a working knowledge of residential electrical setups), but it seems to me the issue is related to the resistance of our bodies. If that's the case, then how could it ever work like it did in the museum with such a small power source? How can I get my experiment working?

\$\endgroup\$
2
\$\begingroup\$

Always be very careful when designing circuits that are ment to put a current trough your body. It would be dangerous to let the current for the LEDs actually flow trough your body. What the people at the museum probably did is use some kind of amplifier.

The resistance of a human body (including the skin) is somewhere between 10 kilo ohm and 100 kilo ohms, so with a voltage of say 9V, you could only get a maximum current of 1mA. For a normal LED a current of 20mA is needed.

A simple type of amplifier is a darlington transistor. You could make a simple circuit as shown below. A small current that goes trough the body (shown as resistor) is amplified by say 1000 times. This would mean that to light the LED, a current of 20 micro amperes is needed, this can be considered safe.

Another option is to use a MOSFET. MOSFETs switch on when a certain voltage is applied, so the approach is a little different. A tiny current goes trough the body and trough R4, creating a voltage on the Gate of the MOSFET, making it conduct. In this case the current trough the body is even smaller, about 1 micro ampere.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ I'm afraid that the MOSFET in the circuit on the right will be quickly fried by ESD. \$\endgroup\$ – fgrieu Feb 22 '15 at 1:41
  • 1
    \$\begingroup\$ @fgrieu is totally right. I've added a small ESD protection circuit at the gate of the mosfet to make it more reliable. \$\endgroup\$ – Douwe66 Feb 22 '15 at 7:56
  • \$\begingroup\$ Nice answer. Usually an IC has ESD protection inside, check the datasheet. However for the ESD circuit shown it is rather ineffective unless a fixed resistor (for instance 1 kOhm) is placed between "body" and the gate input. \$\endgroup\$ – HKOB Feb 22 '15 at 13:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.