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

About: Bill Nye the Science guy is my Hero... that and Bear Grylls...would be cool if they combined to become Bear Nye the Wilderscience Guy using science and technology to drink his own pee.
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

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

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)

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

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

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


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!!

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!

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

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.

Spy Challenge

Grand Prize in the
Spy Challenge