Introduction: MintyBeam: Bug a Room and Transmit Voice Over a Laser Beam (also a Super Spy Ear/Stethoscope)

Picture of MintyBeam: Bug a Room and Transmit Voice Over a Laser Beam (also a Super Spy Ear/Stethoscope)
INTRO: This instructable will show you how to make a laser voice transmitter and reciver housed in the infamous altoid tin (or any other clandestine container). If you choose, the receiver can be modified to also work as a Super Spy Ear AND an audio amp for your iPod etc. You can choose to make a RED, INFRARED (invisible), and GREEN version! The red is the easiest, smallest and cheapest to make and transmits almost 4 blocks (2/10ths of a mile), the infrared has obvious invisible top secret spy applications and lastly the green transmits CRAZY far and looks cool. You can power these circuits with a standard 9volt battery for several hours or choose to use a 23A which fits nicely into the altoid SMALLS tin but only powers it for about 25 min on full power...oh and the transmitters are adjustable so you can have a lot or a little laser light come out depending on how far/bright you want it.

THEORY: A LM386 audio amp chip takes voice from an electret condensing mic and modulates the intensity of a laserbeam, then the receiver uses another LM386 chip connected to a phototransistor to demodulate the laser and drives a speaker, headphones, or any other audio out (into a computer would be cool to filter sound!)


Step 1: Parts List

Picture of Parts List

LM386N (2)
8 pin IC socket (2) (optional)

10K (2)
10k Variable micro trimmer (2)
100k Variable micro trimmer (1)....if making the modified receiver

.1uf disk (2)
10uf elctrolytic (2)
220uf electrolytic (1)

Generic Phototransitor (1) 

650nm (RED) 5mw 3-3.2 volt or 980nm(INFRARED) 5mw 3-3.2 volt or 435nm (GREEN) 5mw 3-3.2 volt (all from amazon)
Hack just about any laser pointer/cat toy (I'll show you how)

Electret mic (1)

Perfboard (2)

N-type battery holder (or other hack...ill show you)
9v Battery clip

Altoid Tin and Altoid SMALLS Tin

Micro 2 lead switch (any switch with 2 leads is fine) (1)

Headphones or 8 ohm speaker
1/8" audio out jack (1) .......or 3 if making modified receiver

Hot glue gun
double sides foam tape
Solder/Wire etc.
I like Heat shrink tubing to make things look nice but thats just me.

Step 2: Schematics

Picture of Schematics

I drew these schematics to closely show how to arrange and solder the components on the perf board so it might not look like a typical schematic.

Step 3: Hack a Laser Pointer (optional If You Don't Just Buy a Laser Module From Amazon)

Picture of Hack a Laser Pointer (optional If You Don't Just Buy a Laser Module From Amazon)

Find a way to remove the laser module from the pen, it will have a switch, and may or may not have ground and power wires connected to it. A laser cat toy has nice wires already attached, but my green laser pointer did not.

For either case you must solder wires across the switch terminals so that it stays "ON" when power is applied; in essence you are just bypassing the switch.

If you dont have wires attached, solder a wire to the ground (the spring) and the brass housing.  I roughed up the brass housing with some sandpaper to make the solder stick better.

Add hot glue and heat shrink tubing to protect your new laser module!!!

Step 4: Solder the Circuit

Picture of Solder the Circuit

I always put in the IC first and just solder as I go, it doesnt really matter the order or layout as long as it sticks to the schematic but I think my layout is nice....please note that pin 2 and 4 of the LM386 are connected together. Pay attention to polarity of the electrolytic capacitors and make sure everything that needs grounded is grounded.

You'll notice that both circuits are almost exactly the same except the Laser does not have a 220uf capacitor connect to pin 5.  ALSO, the transmitter has a mic and the reciver replaces the mic with a phototransistor.

Step 5: Solder Your Switches/plugs

Picture of Solder Your Switches/plugs

The 10k pots are always tricky, the "lone pin" of the three (the middle) is the input, and one of the other pins goes to power and the other to ground.

Solder a switch to a 9volt battery clip for the receiver .

The transmitter, if housed in a SMALLS tin will need a N-type battery holder, to save space no switch is included (I just put the battery in to turn it on) If you can't find a N-type battery holder you can simply solder the leads to the  battery terminals then to a switch, just be careful not to get the battery too hot with a soldering iron or see if hot glue will keep good battery contact.

OR you can easily house the transmitter with a 9 volt battery in a normal size Altoids tin. The advantage of using a 9 volt will be apparent when you start burning through A23 batteries in 25 min haha.

P.S. the transmitter circuit also works with a "wal wart" power adapter if you wanna config your spy setup to plug in, you could invent a cool lamp or radio or something that has a laser transmitter in it that operates when the power is turned on or even use a PIR motion sensor to turn on your transmitter when people are in the room!! MUWHAHAHA


Picture of MOUNTING

Drill holes for in a large altoid tin for the pot, switch, and audio jack.
Place the circuit using double sided foam tape to insulate the parts from the TIN (causing a short). 
Use hot glue to secure and insulate wires.
Use the foam tape to mount the phototransistor in the middle of the underside of the tin lid.


Drill a small hole for the mic, and another for the laser.
Mount circuit with double sided foam tape
Use hot glue to mount the mic to the underside of the lid *VERY IMPORTANT*, if you remove the black fuzzy stuff on the front of the mic and mount it that way ( i think it looks cooler and is easer to see the mic hole and use a smaller hole instead of the whole mic sticking out) you must mount it with hot glue in such a way as the front of the mic does not touch the tin or else you'll get nothing but buzzing and screeching sounds. 

Of course you can always house these circuits in whatever your imagination tells you, a book would be cool! 

Step 7: USE IT!!

Picture of USE IT!!

PLEASE be careful of your eyes! Power up your receiver and transmitter. Align the phototransistor with the beam and with the volume turned LOW slowly increase the sound. Its a good idea when testing to have some tunes playing at the transmitter end. You may hear buzzing if you point your receiver at lightbulbs due to a 60 cycle hum. Experiment with the pot on the transmitter to adjust laser power.  I've noticed that some laser modules either like to be fully on or will turn off when adjusting the pot, others like to be adjusted and can be faint or full blast. 

It's truly amazing how far and clear this transmits. Our eyes (assuming you are Human) are the most sensitive to green light and the green laser transmits farther than I can aim it,  so I'm not really sure how far this baby can really go. I did however max out the red at about 2/10 of a mile. The infrared laser transmits maybe 50-100 feet? I'm not really sure since its hard to find the beam ( its invisible) And keep in mind these are whimpy <5mw laser modules.... can you imagine a 20mw or 50mw laser! Yikes! 


Some idea's I'm working on are 
1. use a solar panel instead of phototransistor
2. use a fresnel lens to focus diffuse laser light back to a dot on the receiver at long range
3. use a parabolic dish at the receiver end to boost signal
4. use a computer program like audacity to filter sound in real time
5. use an arduino and IF THEN statements to turn on transmitter when certain conditions exist
6. more powerful lasers
7. eliminate perf board and see if i can fit it inside a pen or marker 


Step 8: MintyBeam Receiver Modification! Add More Spy Functions!

Picture of MintyBeam Receiver Modification! Add More Spy Functions!

With just another switch and  2 more audio  jacks and changing the 10k Pot, you can ADD these functions to your receiver and have the ULTIMATE SPY AMP with interchangeable inputs and outputs!

1. A super Spy Ear !
2. A BOOST button to go from 20x to 200x amplification!
3. Record what your hearing on a digital or tape voice recorder!
4. Plug your receiver into a computer for realtime audio filtering/recording!
5. When your done spying, sit back take a sip of your martini and use the receiver to BLAST music or movies from your phone, iPod, computer or the secret conversation you just recorded!

Solder a switch to the positive lead of the 10uf capcitor on pin 1 of the LM386, this will control your BOOST.
Where the Phototransistor is replace it with a female audio jack (remember where the emitter went)
Solder your phototransistor to a male audio  jack (remember which wire is the emitter).
Solder an electret mic to a female audio jack (or male depending on your application --i put mine in a Hi-Liter)
and make note of + and - wires 
Make a double sided male audio jack cable (make a note of which wire is which when solderi it)...or just buy one 

NOTE: You must keep track of all the wires coming and going from the audio jacks ..... It can be frustrating if you hear buzzing from an incorrect connection and you can't find where it comes from ..... I used my multimeter to make sure which wires are connected to which terminals on the jacks and made sure it all matches up (ie the emitter, the + lead on the mic etc)
 Also know that you must have everything insulated (hot glue, foam tape) or you'll get buzzing or a short. Sometimes it's best just to scrap the altoid tin and use a plastic project box if you can't isolate the shorts. 

Where the normal jack was for the speaker, just add another one connected to the same spots on the perf board.
Switch out the 10k Pot for a 100K pot.

Now  plug in your phototransitor to the MintyBeam input and receive your laser signal, and plug in your headphones or speaker to listen AND plug in your recorder to record!.....its a little too quite cause your 2 blocks away, hit your BOOST button...whoah thats way better!  Wait..... there is a conversation in the other room that is pertinent to your mission....unplug your phototransitor and plug in your mic....slip the mic under the crack in the door and record every whisper. Mission accomplished, now unplug your mic and recorder and plug in your iPod, kick down the BOOST and blast some tunes and sip a've earned it!

Step 9: Make a Spy Stethoscope to Listen Through Walls

Picture of Make a Spy Stethoscope to Listen Through Walls

I just figured this out! Works AWESOME! I'm in Nursing School so I had an old stethoscope lying around....just added my electret mic and heat shrink tubing and presto! I can hear my heart really loud too. If you pull off the head of the stethoscope you can cram it under doors too hear on the other side hehe.


ecc0927 (author)2017-12-08

This is AMAZING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

drdan152 (author)2017-01-02

Hello! Thanks for your concern but if u look at the design ..... this is an optical device..... it does not even have an antenna! They are just audio amplifier circuits driving lasers and speakers not radio transmitters.

dash.riprock.37266 (author)2014-12-30

Use 18650 Li-ion batteries instead, buy a battery holder from ebay and you'll have lots of power.

exagerated (author)2013-01-20

hey my circuit uses a lot of energy and the 9 volt battery lasts upto 30 mins plez help me to make it last little longer

odoylemark (author)exagerated2014-05-08

You can wire additional 9 volt batteries together in parallel!

I know way late response but here is a simple link also.

good job!

John C Sunil (author)2013-04-08

Can i use the phototransistor which appears dark bluish black colour PLZ REPLY BACK!!!!!! I want to make this for my science fair!!

John C Sunil (author)2013-04-08

I like your project very much and want to build it badly!!!!!! I have some doubts
1 Can i use a 50mW laser with this circuit?
2 The laser is a 3v powered laser so if I use it with 9v in the circuit wouldn't it get burnt?
3 Is there any precautions to take to prevent burning the laser? By using something like a 4.5 v or 3v
4 Can i use a ir receiver with the laser?
5 Can i use a led and a Cds LDR instead of laser and phototransistor?

sgases (author)2013-03-21

i made the receiver circuit and im getting noise sound from the speaker. im only using a photoresistor and traditional 10K potentiometer :| help me please

zaphodd42 (author)2013-03-03

i know its been a while since this post, but i just finished my own version, totally based on your ideas! i had a blast and learned a ton. i used etched PCBs (because i just learned how), and modified some gorilla pods to magnetically mount the tins to them. in the photos, youll see i made a "collector dish" from polymorph plastic. thanx for a great 'ibble. on to the next weekend project!

exagerated (author)2013-01-20

hey also 1 more problem that my laser isnt that bright which it should i have tried changing the battery but that doesnt worked for me

allsparkunleashed (author)2013-01-05

hey.. can i use an ldr instead of phototransistor????

or a TSOP 1738 ?? will it work instead of phototransistor??

allsparkunleashed (author)2012-07-06

can i use anyother light source instead of laser diode??

Good question !!!! My ORIGINAL idea was to use an IR led and it worked great but only transmitted like 6 inches then burned up the led after about a min ..... So maybe not so great ..... I even got a nice show of sparks and burned plastic kinda cool ..... I think the circuit just has too much amplification for an led but I wonder if a regular old light bulb or maybe a heavy duty led that take more voltage would work ..... Lemme know know If u try it :-)

fahadshihab (author)drdan1522012-08-04

better use an IR led.better for spying because it's not visible for human eye

but very low range rit??

akshay+jadhav (author)2012-08-25

10k Variable micro Trimmer is what I am not getting,

please tell me which one to buy is it this....

OR this

Please Reply

Please Reply or send the Link for the Part which I should Buy  ! !

Thank You !  ! :) :)

The first one is out of stock.....just had been to that shop

Can i use the second one instead ?

drdan152 (author)akshay+jadhav2012-08-27

Yeah sure it's just not as small :-)

akshay+jadhav (author)drdan1522012-09-07

I am doing this project,
So sir I do need ur help

I got a laser module which can barely handle 3V,so whenever i connect it just damages any idea how I could lower the voltage or any modification as per ur suggestion ?


drdan152 (author)akshay+jadhav2012-08-25

The first one is perfect !!!! Get that one or equivalent.

B.F.L.M (author)2012-08-17

So, Theoretically speaking, would this work if you removed/disabled the mic (maybe by adding a switch) and bounced the laser beam from the transmitter off of a window and into the receiver? Would you be able to hear whats on the other side of the window?

I would love to build this but i want to know if the above works so I know if i want to include an on/off switch for the mic on the transmitter.

Many Thanks,
{[Happy Building]}

drdan152 (author)B.F.L.M2012-08-19

That would be soo cool but in all my reasearch on "laser listeners" as they are called they really don't work all that great or at all and the aming is insanely hard and impractical and frustrating! Thats why i came up with this design is so i can still use lasers as a room bug. I guess theoretically YES you could experiment with it and is should work but im not sure how close you would have to be to the window and the window would have to be a certain kind so that it would vibrate with sound. If your goin to try it out i would build the reciver FIRST and then use a normal laser pointer in a constant on position and try bouncing the dot off the window into the reciver....and see if its worth adding an on off swithc to the transmitters mic!

Have fun experimenting!!!!

B.F.L.M (author)drdan1522012-08-19

Excellent! Thank you very much ^-^

Many Thanks,
{[Happy Building]}

jdaley99702 (author)2012-04-25

Could you use a cone type shape or possibly old satelite dish to collect the beam and then put the photo resistor where the signal collects? I guess that would defeat the small size of it but for longer distances that may give a larger target and easier boost

fahadshihab (author)jdaley997022012-08-04

why can't you use a concave mirror?????????

drdan152 (author)jdaley997022012-04-25

Theoretically yes ..... I have even seen like a white PVC pipe that collects the light into the phototransistor at the other end ..... A fresnel lense also seems to work really good at collecting light ..... U can get an 8 1/2 by 11 inch at a bookstore !!!!

drdan152 (author)drdan1522012-04-25

What's crazy is this thing seems to work BETTER when the laser light has had a chance to diffuse at long distances

jdaley99702 (author)drdan1522012-04-25

Hmm I wonder if the diffusion leads to the extra static being lost because it's not the main part of the sound being transmitted?

allsparkunleashed (author)2012-07-14

hii.. i got the laser and still i am getting a humming sound of different frequency.. i am not geting the corect sound.. wen i touch the mike the sound changes a bit... thats all.. what should i do??

How far away from the transmitter is the receiver ? Sometimes if your too close it won't come through clear ..... Farther away is better ..... So try aiming it from farther away and experiment with the alignment ..... If that does not work can you send me some pictures of the pcb you made and maybe I can look at the soldering and tracing ..... And see if I can spot the problem ..... I've made like 10 of these things and they always worked great ..... So maybe you have a wire crossed or your too close ? My email is if u can send me pix

allsparkunleashed (author)2012-07-09

hii.. i completed the receiving part.. and i almost cmpleted the transmitting part. oly thing i need is a laser.. am on my way to buy one..
mean while i thot of using an infra red led instead of laser.. and whenever i bring the ir led clos to the receiving part, its making a humming a bee humming.. i am not getting a perfect sound from the mic even if i adjust the 10 k pot.. will this problem b solved wen i replace the ir led with the laser??

Hey !!!! It sounds like you built the receiver well and it works ..... It likes to hum like a bee when it senses light it beeps too ..... When I used an IR LED
it screeched and I occasionally got clear audio then it burned up and i got humming then nothing. I'm sure If u use the laser all will be fine :-) which laser are u gonna use ? Red ? IR ? GREEN ? If u live in the USA most grocery stores sell a laser cat toy in the shape of a mouse that has a really nice laser diode with wires attached already ..... You just have to hack it to bypass the off button.

can u help me in tuning the right frequency??

is this enuf??

Just looked at your link to eBay ..... That laser is PERFECT for this project.

ok thanku...

i am from india.. i am planning to buy red laser.. 650nm.. i tried to buy from amazon but ther is no shipping to india. i tried to search on ebay but i am not able to find one that suits this.. there is not a thing called laser in electronic shops.. where can i find one??

allsparkunleashed (author)2012-07-07

hii.. my work is almost complete.. i went to a shop to buy some photo diode and photo transistr.. wen i reached my workshop, both these wer mixd together.. both are identical.. both look the same.. i dont kno which is which.. can u help me??
also, can we use an ir led intead of laser?

Oh no !!!! I think I know what you mean ..... Some times when you buy a phototransistor it has a ir photodiode with it ..... But they both look the same sometimes And are easy to mix up ..... You could try to put 3 volts to each with 2 AA batteries and then try to view them with a camera to see which one is emitting IR and which one doesn't ..... The one that doesn't is the phototransistor. If you have an iPhone or a friend with one , the REVERSE camera works GREAT on detecting IR from LEDs.

As for replacing the laser with a diode ..... I tried it and it only worked for about 30 sec then burned up the diode.

sciencetor2 (author)2012-07-06

hey just a thought, since you won a pair of night vision goggles in the grand prize pack, they will make it a lot easier to aim the infrared laser, as they can see the light :)

drdan152 (author)sciencetor22012-07-06

Why YES it would !!!! I didn't even think of that !!!! Sweeeet ..... Thanks for
Your post :-)

sciencetor2 (author)drdan1522012-07-07

You're welcome :)

allsparkunleashed (author)2012-07-06

hey!! i really like this project and i am interested in making one.. i got the pcb and now am working on it.. nice work!!

Thank you my friend ..... It's really fun to build and test !!!!

spystealth1 (author)2012-07-01

Hi, I am in the process of making this. I've seen this design before and it is a great little gadget. I was just wondering if reducing the 10K resistor or trimpot values would increase the laser brightness as my laser doesn't seem to light up very bright unless I yell into the microphone. I believe it's because the laser diode is cheap and needs a tad more power than most to light up all the way. Any help would be appreciated.

spystealth1 (author)spystealth12012-07-01

Also, I just wanted to add that my design uses an old cell phone battery with a charging port at the back so I can recharge it from a wall wart, or attach a solar panel. I also added a microphone port so I can plug in a better mic if need be.
The receiver seemed to buzz or beep very loudly when I prototyped it. Is this normal?

drdan152 (author)spystealth12012-07-01

Also make sure your testing it with FRESH batteries as I noticed even with 12v as soon as the battery got too far drained the laser would act up..The wal wart i used was 12 volts.

As for the receiver I think i forgot to mention that its basically a "light sensor" so ANY light from the Sun or light bulbs (esp. flouressent) even the IR flashes from a tv remote make sounds...depending on the type of light is what type of sound you get...which is kinda cool in itself...for example if you point the reciver directly at flouressent ligh bulbs you can hear the 60 cycle hum in the reciever or if you point your remote control at it you can hear differnt tones like Beeps from each key you press....this is actually how i tested my circuit to make sure it WAS WORKING....i pointed my tv remote and held down the ENTER key and heard loud beeping...the reciver is crasy sensitive so i RARELY had the volume turned up at all...and having the phototransitor inside the lid of the altoid tin seemed to block out unwanted light....this brings up a good point that essentially the circuit works BEST at night.....with no interfering light...BUT i would test and use it in broad daylight and it worked perfect as soon as the laser aligned with the reciever becasuse i think the intensity of the laser blocked out any competing day light or artificial light.

Hope that helps! lemme know if you have any more questions (sorry bout the at work and kinda bored ha)

drdan152 (author)spystealth12012-07-01

Hey! So cool your building it! Ok lets see. I experimented extensively with the 10k resistor and 10k pot combo and I found that it worked great with any laser diode i used...even a cheap one from my cat toy....however it does affect the brightness so go ahead and try differnent combos! What seemed to affect the laser MORE is the voltage and power of the battery!!! For example the green laser diode wouldn't even turn on unless it had 9 volts or more....the cheapy laser module from a cat toy would only turn on for a bit and very dim when powered with 6 volts...but when I used 12v (from n-type battery or 9v from standard 9v) it worked great and the pot could adjust the intensity! SOOOOO I THINK your cell battery doesnt have enough volts as most are 3-6 volts i think

finn12345 (author)2012-06-24

well, well desirved for the grand prize i'll try to make it

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