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!

  • I think to answer I'd need more than this to go on - maybe you have a link to a site where it shows how this is done. – Andy aka Oct 14 '13 at 15:08
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  • Yes, this is one of the articles my daughter found. – Catherine Gruber Oct 14 '13 at 15:29
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    I would need to read some more papers, but my first reaction to this interesting project is that most low-end commercial/amateur measurement equipment will have an impedance way too low to pick up any signals from the plant. Basically: the act of measuring the voltage will severely impact said voltage. All is not lost, however; it's easy to get an op-amp with an input current 100x lower than my Fluke multimeter, or 1000x lower than a RadioShack multimeter. – Nick T Oct 15 '13 at 0:55
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    You should be able to make Ag/AgCl electrodes by sticking a silver wire in bleach for about 30 minutes, then rinsing it real well. – Scott Seidman Oct 16 '13 at 15:49

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.

  • "The Art of Electronics" probably refers to the book by Horowitz and Hill, which is a great book indeed. – Li-aung Yip Nov 17 '14 at 3:30
  • @Li-aungYip - Yes Horowitz and Hill. I though everyone knows the authors. :) – johnfound Nov 17 '14 at 5:53

I'd go with some low cost DAQ solution like or (haven't used either, but both look OK-- this is simple stuff) or any usb oscilloscope hardware/software package.

You will need a preamplifier for the Ag/AgCl electrodes, which can be built with an instrumentation amplifier like the AD623, whole circuit probably about $20.

As an alternative, have a bunch of little kits with fairly high gain amps for biopotentials, and some of that might be adaptable for your purposes.

3KHz does sound high.

If you could be a bit more exact about what responses she'd like to measure, I might be able to be a little more helpful. Encourage her to build the experiment to address a hypothesis that she can answer.

  • She wants to be able to detect an electrical response in the plant under acute stress, like the application of heat on a leaf. This would be done daily to the plant. Some of the plants will be subjected to blue light at the same time (controls: light only, heat only, etc.). Seeds from these plants will be harvested and grown. Generation 2 will be tested with blue light (no heat) to see if they have the Generation 1 response to heat. So, she doesn't need an quantitative measurement of the signal, but a yes/no, I think. (This epigenetic learning isn't through changes in genes, but expression.) – Catherine Gruber Oct 16 '13 at 13:14
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    Sounds doable. I recommend trying to track down the article: Fromm and Lautner, Electrical signals and their physiological significance in plants Plant, Cell & Environment,Volume 30, Issue 3. You won't need to sample faster than about 100 Hz. – Scott Seidman Oct 16 '13 at 13:23

A DSO-nano or similar oscilloscope might be better for the money, it would allow viewing of waveforms (a cheap old CRT scope for $20 plus a digital camera is another method).

TBH I wasn't aware that plants gave off any electrical signals, you'll likely have a hard time proving it's the plant and not just stray EMC and the plant acting as an antenna.

  • Thank you, we were planning to build a Faraday cage around the plants to block other signals. – Catherine Gruber Oct 14 '13 at 17:15
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    @Catherine Gruber Keep in mind that test equipment will radiate as well. Another thing that's very important is that test equipment will pick up its own radiation! Try to make a setup where you can somehow keep everything intact and just disconnect the plant, so you can get a baseline value for stray radiation pickup. – AndrejaKo Oct 14 '13 at 19:03
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    It may be worth using a "reference" model, EG a dead plant or moist stick in a pot or something, just to confirm that what you're measuring is only present in the living plant. – John U Oct 15 '13 at 9:02

I find it very hard to believe that 3000 samples per second is needed. Human cell processes are on the order of 10's to 100's of Hz and we move a lot faster and have a little more processing/response capability than plants do. I'd be surprised if you need anything faster than a few Hz. As a test case, I believe that a venus fly trap is one of the fastest moving plants and they move slowly (yes there are seed pods that erupt at Km/Hr but that is stored energy).

There is a reason why this eqt is expensive, detecting small signals is very hard to do. There are electrometer grade Instrumentation amplifiers available but even using these devices is very tricky as leakage currents across the surface of material being used can cause notable signals. HAving lower bandwidth/speeds will be key to reducing noise in any circuit you come up with.

Ironically, the least expensive way of assembling this circuit will be the best (i.e. dead bugging) as you will not want to build this on a perf board, or proto board, leakage currents will be too high.

There are probably Instrumentation amplifiers that are electrometer grade as well. I will note that a lot of the devices at that were the "go to" devices are now obsolete. Also note that a lot of manufacturers will sample devices for free, especially for kids and science projects ...

Now this only takes you to a buffered and amplified signal that you then need to stuff into the digital domain, but I note that others have already given you leads.

  • Yes, I'm not certain that we need 3000 s/sec for this experiment. I'm guessing that some indication of signal/no signal would be sufficient. In the research we looked at, they were measuring the signal, not just whether or not there was one. – Catherine Gruber Oct 14 '13 at 18:28
  • In the paper she linked (PDF, 555 kB), there are some signals with a rise-time of <1 ms – Nick T Oct 15 '13 at 1:08
  • Just a point of information, cell processes can be much faster than 100's of Hz. Neural action potentials pretty much start at 1KHz and have information up to 10KHz. Muscle (unfiltered EMG) goes up to about 2KHz – Scott Seidman Oct 16 '13 at 15:42
  • @ScottSeidman given the distance scales are smaller (cell sized) that makes sense. – placeholder Oct 16 '13 at 15:49

You need at least 10 gigaohms impedance - ie higher than the impedance of the plant - texas instruments do a good opamp for around 5 dollars for this purpose - otherwise the current will find it easier to enter your measuring equipment than continue its path through the plant. . 100 samples per second is even enough - if you think an action potential lasts maybe 30 ms. Read the second part of this paper - an artists guide to plant electrophysiology - and let us know how it goes !

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