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!
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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!
is this an RCA adapter
jensenr30 (author)  Anton Bäcklund2 years ago
where did that comment come from? xD
Ghost Wolf2 years ago
Will a 3904 npn transistor work for this project?
jensenr30 (author)  Ghost Wolf2 years ago
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.
Ok thanks :)
jensenr30 (author)  Ghost Wolf2 years ago
no problem
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 Proton3 years ago
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 Proton3 years ago
**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.

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 Proton3 years ago
thats good idea.
Krayzi993 years ago
To tell truth, I have used an un-modded LED as a phototransistor, though with Arduino.
elephantinc3 years ago
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)  elephantinc3 years ago
I bet you will be able to do that. Post a reply to this when you try it!
: )
elephantinc3 years ago
Also, why not just use an LDR?
elephantinc3 years ago
Do you have a video of it in action?
What's the output quality like? Does it produce recognisable notes?
jensenr30 (author)  elephantinc3 years ago
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.
MidnightSon3 years ago
Cool one !
Keep up man !
jensenr30 (author)  MidnightSon3 years ago
thank you! I really appreciate the support!
and following
me=faving this
pfred23 years ago
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)  pfred23 years ago
To be honest, I can't find cheap photoresistors on the internet. doesn't sell them, only phototransistors so I use them.

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)  pfred23 years ago
TSC3 years ago
jensenr30 (author)  TSC3 years ago
thanks alot!
TSC jensenr303 years ago
Your welcome!
Thats pretty sweet, so it's a device that can allow you to hear light?
jensenr30 (author)  Jimmy Proton3 years ago
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.
Kinda like that Tesla Spooky Radio thing but that thing is way cooler!