Cheap, Easy Light Probe





Introduction: Cheap, Easy Light Probe

I teach high school physics and I use a lot of expensive probeware to collect data. The only reason I can do this is my school has been collecting the probes over a number of years, building our collection slowly over time. For those who aren't science teachers, probeware refers to a collection of interfaces used to connect a variety of sensors to a computer or graphing calculator. These interfaces can allow for real time data collection and graphing or can serve as data-loggers collecting data over time.

The two largest vendors of educational probeware are Vernier and Pasco:

Not every school has the ability/money to do probeware based labs, however. So, here is a description of how to create a really simple probe that will allow you to see the fluctuations in light caused by alternating current or see the signal from a TV remote control (as well as do other things I'll describe in a future Instructable).

Total cost is less than $5, but you need a computer with an oscilloscope program installed.

Visual Analyser - Free oscilloscope program for Windows.

Audacity - Can be used as a recording oscilloscope.

Step 1: What Do You Need?

-Wire Cutters/Strippers
-Soldering Iron

-Cheap Solar Cell (I bought mine for $3)
-Headphones from the Dollar Store (you could just buy a microphone plug and some wire)
-Alligator Leads (I used red and black, but you can use any color)
-Electrical Tape (heat shrink tubing would be cooler, but I'm lazy and cheap)

Step 2: Destroy the Headphones

Cut one of the earphone off and carefully strip back the wire. You'll notice some strands of wire. One will be a different color (red or blue). Separate the colored strand from the others.

The colored strand carries the signal while the other strands are the common ground. You'll need to strip off the insulation. There are two ways of doing this. You can lightly sand the wire, but you risk breaking it, or you can burn it off. The enamel will burn off readily, but if you burn it too long the wire will melt.

Step 3: Destroy the Aligator Leads

Cut your alligator leads in half and strip the ends. I used two leads, but you could get away with one if you don't mind each side being the same color.

Step 4: Put It Together

Now all you need to do is attach your alligator leads to the headphones. I put the black lead on the common ground (the loose strands or wire) and the red wire on the signal wire (the single colored wire).

Be very careful not to break the signal wire. It's very thin and fairly easy to snap.

Solder it together.

Step 5: Hide Your Mistakes With Electrical Tape

Now wrap the two wires separately with electrical tape and then wrap them together for strength. The added bonus with the headphones I bought is the cassette tape holder. I now have a handy box to store my wires in.

Step 6: Using It.

Attach the alligator clips to the solar cell and plug it into the microphone jack on your computer. Turn on your oscilloscope program or hit record in Audacity. Then you point the solar cell at any light that is changing. If you're using Audacity, you'll have to zoom in to see anything (see the pictures below).

You could solder the wires directly to the solar cell, but I typically don't. This way I can use my probe with any varying voltage source. Just be careful to keep the voltage low so you don't fry your sound card.

I've done some digging (after some comments, thanks) and found a reference to a 2Vpp voltage limit on a SoundBlaster AWE64 Audio Card. You should probably check your solar cell to determine it's maximum output. If you're concerned it would be a simple matter to include a current limiting resistor. The solar cells I've used are rated for a maximum of 0.5 V and I've had no problems so far.


-Any light plugged into alternating current
-Your computer monitor (frequency will equal refresh rate)
-TV remote control. The solar cell should pick up the IR.



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    For anyone reading this I think you need to be aware that it is not current limiting but voltage limiting which is required. I would suggest something like the following image.

    2 replies

    Could you clarify what are the ratings on the diodes and resistors. The resolution on the picture is too low and I can't read it.

    The solar cell produces very little power but could possibly produce more voltage than the PC input can tolerate so the diodes and resistors can be low power rating say quarter watt. The zener diodes are 2v7 and the resistors are 1k0.


    Pasco is the best of all my dad is a professor in physics in the Arab Academy of Science and Technology and maritime transport in Egypt Alexandria most of their equipment is from Pasco and their stuff is really neat as well as Photon.

    I'm having problems. I'm using the solar cell from the top of a solar landscape lamp. I thought I might be able to make two probes, one on each earphone. Will this work do you think? I hooked a multimeter to my solar cell so I know it's working. Do I need to cut only one earphone off?

    2 replies

    I haven't tried to use both earphones, but it seems like it should work. Let me know what you find out. Watch out for the voltage. Some landscaping solar cells can produce too much voltage. The best reference I've found online for the limits of a sound card is 2Vpp.

    Yep .. 2Vpp is right, most try to keep under this as peaking will corrupt the data.

    I beleive the PC audio input is AC coupled which means that you will not be able to read any steady state (DC) voltage. It looks that way from the pictures and I would expect it to be AC coupled as DC has no business in audio.

    5 replies

    This is correct. It will only register a changing voltage. So, any varying light source will result in a readable signal.

    Well the AC coupling will reduce the effective signal the lower it is in frequency. This could effect their measuremets on slow changing sensors. I bet the effect would be noticable at 10hz.

    What should I look for? I've used this with a flashlight on a string to measure the period in pendulum motion. If it was a little off I'd never know. I've also used it with a picket fence (plexiglas with black bands) to calculate the acceleration due to gravity (future instructable) and the answer is within a few percent. In the past I've figured the error was due to the technique of dropping (if it's not exactly straight you can account for the error). Perhaps the error happens due to the way the sound card works?

    Unless you are simply curious (nothing wrong with that) the accelleration from Earth's gravity can be found out easally enough. It is approximately 0 to 60 in 3 seconds. Not including aerodynamic friction of course. Now, if you are simply curious, measuring the speed of sound would be fun. At low altitude (close to sea level) it will be around 750mph. At the top of Mount Everest it'll be around 680mph. Fun hint: the speed of sound is the FAA's speed limit for America's airspace.

    You will find that low frequencies suffer from an attenuation, and (possibly) a phase shift. But for all the applications you mention, neither matters: all you care about is the period. And really, you can design an awful lot of experiments where the precise amplitude and phase don't matter. But do be aware that waveforms will change shape, for example.

    nice instructable. would a mic wired in work as a probe, and if so, would it be useful for anything?

    1 reply

    Yes, you can use a microphone as a probe and it can be quite useful. I have a couple other instructables in the works involving microphones. I'll try to get my measuring the speed of sound one done today.

    Please tell me how to determine the frequency on Audacity.

    1 reply

    To determine frequencies in Audacity just look for "Plot Spectrum" in the view menu. This will give you an FFT analysis of your sound. The tallest peaks will be the frequencies. If there is more than one frequency then you should see more than one tall peak.

    If the frequencies are really low, you may have to measure time from peak to peak in the main window (zoom way in). This will give you period (time for one cycle). Frequency = 1/period.

    If you're doing much with frequency analysis then I'd recommend downloading a copy of Visual Analyser. This has a built in FFT and is much easier to use to determine frequency.

    I would recomend a free program called Winscope. it does most of the functions of your standard osiloscope

    Simple in analog, just connect a solar cell to an audio amplifier (or just headphone). Cheap oscilloscopes: #1.Audio amp+electromagnet+music wire(elec.guitarstring)+strobe light. #2.Audio amp+speaker+laser #3.Small TV with probes on the yoke vertical coil. (Danger for dummies!) A PC is one hell of a Rube Goldberg Mousetrap for most of it's uses, considering it is a billion times more complex than what got a man on the moon, and it's most common use is playing Solitaire. At graduation, those students will have mastered tools less useful than punchcard machines and telegram printers, and probably not even be able to tell the time of day on a ticking watch. I guess I should probably write an instructable on data acquisition.

    In the first step above, please define "probeware". I don't teach high school physics and only know it's used to "collect data" Thanks