Electronic – Detect electrical signals from plants


My daughter wants to do a science fair experiment that involves reading the electrical signals in plants, but the cost of a multi voltmeter with data logging is very high. Can anyone suggest a setup for less than $100? Maybe she doesn't need as complicated a set up if she tests one plant at a time? But I believe that the sampling rate has to be at least 3000 s/sec.

Here's a setup for a similar experiment:
A PXI-4071 digital multimeter (National Instruments, Austin, TX, USA) connected to .2-mm-thick non-polarizable reversible Ag/AgCl electrodes was used to record the digital data. The PXI-4071 high-resolution digital multimeter delivers fast voltage measurements from 10 nV to 1000 V, current measurements from 1 pA to 3 A and resistance measurements from 10 µΩ to 5 GΩ. A

Update: One of the researchers in this field answered my daughter's email and said that she could use a KEITHLEY DMM. Would that still require an amplifier? The only affordable ones on ebay (models 169, 177, 179) only have a digital read-out, no PC interface. I'm guessing she needs some sort of output recording device, like an oscilloscope? What about a Radio Shack MM with PC interface and op-amp (on a breadboard?)?

UPDATE: She ended up using a RS digital multimeter w/PC interface. The software was easy to use and the data could be saved to text files on the computer. For each test, the DMM took a reading every second for 100 seconds (there are other options too). The MM was set to record mA. I chlorided 30 gauge silver wires with bleach for the electrodes. Those worked OK, but maybe could have been thicker and not bent as much. The DMM did measure a strong reaction from the plant when it was stressed with high heat (brass wire attached to brass woodburning pen) applied for 5 sec. The baseline voltage seemed to vary alot from day to day (over 20 days), but I'm thinking that's because the electrode was touching a different leaf or part of a leaf on each day. Next time, it should be in the same place for each plant over all the days. Thanks for everyone's advice!

Best Answer

If 3000 samples per second are needed, I would suggest the data logger to be build on some ATMega with built-in DAC and several analog channels. Through the UART this setting will be connected to a simple PC.

The hard part is to build the mV precise amplifier for the ion probes. This amplifier has to be very high impedance, with high gain and probably with differential input. But there are cheap and precise opamps these days, so it is not a big problem IMHO. At least $100 should be more than enough.

I can't point you to exact reference for this analog part of the project, but I have some memory that I saw something similar in the great book "The Art of electronics". Search there for some schematics.