Introduction: The 555 Light Theremin

Picture of The 555 Light Theremin

Quick! Make a Tight, Light-controlled tone generator! It's a fun to build little devise that can be made out of easy to find electronic components! Even better is that it has a cool name! The "Light Theremin" (Th-air-ah-min) that I will be showing you how to make today is a tone generator who's pitch varies directly with its light input.
I would suggest putting this project together on a breadboard first and then soldering it up on perfboard if you enjoy it!

Step 1: Do You Have the Parts?

Picture of Do You Have the Parts?

The necessary parts:

* Small wires for making connections
* Female RCA (TV cable) adapter. I use these for power input on all of my soldered-up projects.
* Female headphone jack (1/8 in. / 3.5mm.)
* Phototransistor. THIS IS NOT AN LED
* 10,000pf ceramic capacitor. #103. This is the same as 0.01uf or 10nf
* 4 different valued resistors. 4.7 Kilohm, 10 Kilohm, 22 Kilohm, and 10 Megohm
* NPN transistor
* 555 timer IC (integrated circuit)
* DIP-8 (dual in-line package - 8) IC socket

****If you don't know where to find these things, head on over to and search for them! make sure to look at the datasheet of each item before ordering!****

****Note that both the DIP-8 IC socket and the female RCA adapter plug are unnecessary for building this project on a breadboard.

Step 2: These Are NOT LED's!

Picture of These Are NOT LED's!



Don't mistake these for LEDs. They look essentially identical, but they work in the opposite fashion.
LED's emit light, whereas phototransistors absorb light to control the flow of electricity.

The phototransistor is what makes our Tight Light Theremin work! Tightly!

Step 3: Put It Together! Schematic

Picture of Put It Together! Schematic

As with all of my electrical inst'ables, I include a link to the HD-ish picture of the schematic!

*** How does this work? The 555 timer's tone that it creates varies it's pitch based on resistor and capacitor values. The phototransistor controls how much current can pass from the collector of the NPN transistor to the emitter of the NPN transistor, sort of like a resistor.

*** The output of the 555 is connected to the headphone jack and in turn is heard as an audible tone when listening with earbuds or headphones. Depending on how much impedance your earbuds/headphones have, you may have to fiddle around with the value of the 22 Kilohm resistor. 

Step 4: Final Step!!

Picture of Final Step!!

Now that we have our Light Theremin all put together on a breadboard and well played with, what can we do with it?

I will admit that there is no real practical application for a light theremin. The only thing I can think of is just a little bit of fun once in a while. But all in all, that is pretty TIGHT!!!
Now solder it up! Show it to your friends! And for the love of god, 

Click here to see my other instructables! Including AM radios, Arduino hacks, RTL Logic gates and MORE!!!


S_Hutchinson made it! (author)2015-04-05

I made it! ... Kind of.

I didn't have a 555 timer, so I used a 556, and made a kind of "Atari Punk Console" with phototransistor control.

This is neat, but what are the pros and cons of using phototransitors (coupled with transistors here) vs.using photocells? They're both creating resistances for control based on light. Is there a significant difference in behavior?

jensenr30 (author)S_Hutchinson2015-04-06


I really like how your improved and expanded on my design! =D

The phototransistor works like a transistor, but its base current is derived from light. So the light ultimately controls the current going through the phototransistor.

It seems like the photoresistors works very similarly, but controls the resistance of the device. The photoresistor, therefore, might lend itself better to applications where current can flow in both directions. The phototransistor might not be as well suited to this particular application.

S_Hutchinson (author)jensenr302015-04-06

Excellent. Thanks for the information! Looking forward to experimenting further!

Anton Bäcklund (author)2013-02-05

is this an RCA adapter

where did that comment come from? xD

Ghost Wolf (author)2012-09-10

Will a 3904 npn transistor work for this project?

jensenr30 (author)Ghost Wolf2012-09-11

I assume you are referring to the 2N3904 npn transistor?
something like this

If it is something like that, it should work.
It looks like you have a pretty common transistor so that is a good sign as well.

For the most part, any transistor will work.

Ghost Wolf (author)jensenr302012-09-18

Ok thanks :)

jensenr30 (author)Ghost Wolf2012-09-20

no problem

Jimmy Proton (author)2011-04-30

How can you tell the difference? I have a lot of LEDs and I dont know which ones are IR, phototransistors, or LEDs.
I once found an LED looking thing but instead of clear plastic it was solid back, I don't know what it is...

jensenr30 (author)Jimmy Proton2012-03-28

also you may want to try recording the possible IRLED with a digital video recorder. they can pick up light wavelengths that are greater than red (i.e. IR)
i have found this is a quick way to check, but it probably not the most reliable.

jensenr30 (author)Jimmy Proton2011-04-30

**It is kind of funny how we must tell the difference. If you have an led looking thing that doesn't output light when a moderate current and voltage are applied to it, you are probably looking at one of the following:
1. Phototransistor
2. Infrared phototransistor
3. Infrared LED

How to discover which of the three possibilities your LED-looking thing is.

First, shine IR light at it with an IRLED.
Second, shine light from a clear, white LED at it.
You shouldn't get a reading from both.
If it doesn't respond to either, it may be an IRLED itself. in which case you should apply a moderate voltage and current to it and direct it towards an IR-phototransistor. You will now be able to tell what devise you have.

**I have some of those black-lens LED-looking-things. there is a very good chance that they are infrared phototransistors. I believe that the blackness on the lens is to filter out visible light and leave just IR light.

Jimmy Proton (author)jensenr302011-04-30

Well it doesn't seem I have any but I can just buy a bunch on ebay and that would make sense because infrared light is the lowest frequency near visible light and black (or dark purple (UV)) is the highest and the dark case would filter out those higher frequency to allow the low frequency light through.

jensenr30 (author)Jimmy Proton2011-05-01

thats good idea.

Krayzi99 (author)2012-03-23

To tell truth, I have used an un-modded LED as a phototransistor, though with Arduino.

elephantinc (author)2011-07-18

Do you think this will work with only 4.5V (I want to use batteries)?
Will I have to alter any of the resistor values?

jensenr30 (author)elephantinc2011-08-19

I bet you will be able to do that. Post a reply to this when you try it!
: )

elephantinc (author)2011-07-19

Also, why not just use an LDR?

elephantinc (author)2011-06-25

Do you have a video of it in action?
What's the output quality like? Does it produce recognisable notes?

jensenr30 (author)elephantinc2011-06-25

I should have a video, but I haven't made one.

It would be incredibly hard to play recognizable notes. it's pitch varies directly with light intensity.

MidnightSon (author)2011-05-02

Cool one !
Keep up man !

jensenr30 (author)MidnightSon2011-05-02

thank you! I really appreciate the support!

The nerdling (author)jensenr302011-06-08

and following

The nerdling (author)jensenr302011-06-08

me=faving this

pfred2 (author)2011-05-01

I've made similar in the past with photocells. They vary their resistance which is what 555 timers look for. Makes for a lot lower parts count circuit too. What is the advantage of using a photo transistor in your circuit over simply using a photocell?

jensenr30 (author)pfred22011-05-01

To be honest, I can't find cheap photoresistors on the internet. doesn't sell them, only phototransistors so I use them.

pfred2 (author)jensenr302011-05-01

They do seem awfully pricey to me. I just looked and I have 16 in one of my junk drawers.  A dozen were salvaged out of an old deli scale. The rest I picked up here and there.

Jameco seems to have them for $1.49

I even have made a similar circuit with just 2 transistors and a cap. it sort of squealed when it was hit by light.

I think the circuit was billed as a fridge alarm or something silly.

jensenr30 (author)pfred22011-05-02


TSC (author)2011-04-30


jensenr30 (author)TSC2011-04-30

thanks alot!

TSC (author)jensenr302011-05-01

Your welcome!

Jimmy Proton (author)2011-04-30

Thats pretty sweet, so it's a device that can allow you to hear light?

jensenr30 (author)Jimmy Proton2011-04-30

Yeah! that is a very interesting way of putting it, but you are in fact right. It turns light intensity into a tight, audible tone.

Jimmy Proton (author)jensenr302011-04-30

Kinda like that Tesla Spooky Radio thing but that thing is way cooler!

About This Instructable




Bio: I'm an 18 year old Electronics enthusiast. I have completed five semesters of schooling at Minnesota State University in Mankato. I'm pursuing a ... More »
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