The Lamp – Parabolic Microphone

The FIX project officially finished in June but I’ve still got some FIX requests hanging around my studio (luckily this was a free service and no-one seems to mind). For 6 months thoughts of these requests have been sitting in the back of my head nagging away; “isn’t it about time you did something about…” – enough already. I decided to use the Christmas break to tackle the most annoying one.

planet lamp

Here it is. A “Planet Lamp” that was listed as an “Ugly Lamp” on the FIX request form.

lamp-microphone

I decided to turn it into a parabolic microphone.

I think the biggest difficulty I had with this FIX request was that I personally don’t find the lamp ugly, it’s actually one of my favourite designs – I have the floor standing version in my lounge room. The “Planet Lamp” (this one is actually a “Studio K” task lamp) was designed by W.P. (Bill) Iggulden in 1962 for the Australian firm Planet Lighting. The lamp became a design icon and was in production for over 30 years (which is why they are so prevalent in second hand stores around Australia).

This particular lamp was in a fairly typical condition, some small dints, scuff marks, a little loose at the joints, but otherwise working. I considered a number of possible modifications ranging from a simple re-spray to re-casting the entire lamp in flexible silicone rubber. None of my ideas seemed right. After quite a few months I realised I was focusing entirely on the word “Ugly”, I was trying to modify the “Ugly” into the “beautiful” or “interesting” but I couldn’t escape the feeling that in this case ugly = beautiful = interesting. So I decided to shift my attention from the lamps form to its function. If I couldn’t turn the “Ugly Lamp” into a “Beautiful Lamp” I’d make the “Ugly Lamp” an “Ugly …something else”. That “something else” became a parabolic microphone. (why a parabolic microphone? – not really sure, probably something to do with me being a geek and a fan of the 1974 Coppola film The Conversation)

parabolic mic dish

I wanted the modification to have minimal impact on the lamps existing form with all electronic components concealed within the lamp structure. The parabolic reflector is housed in the lamp shade

volume control

and a headphone socket and volume control was mounted on the lamps base (this image is a close-up of the volume knob).

An extensive internet search uncovered a large number of possible circuit designs for the parabolic microphone. Most of these were based around the operational amplifier chip (Opamp). One of the most popular circuits is a home made headphone amplifier called CMoy. It’s named after the designer Chu Moy who first presented the design in this online article. Versions of the CMoy are littered throughout the internet and they can even be purchased on eBay, assembled inside a mints tin. The CMoy is a very simple, low power amplifier. It’s cheap to build and is designed to run on a 9V battery; making it perfect for my parabolic microphone.

CMoy amplifier

The basic CMoy circuit looks like this. The ratio of resistor 2 (R2) to resistor 3 (R3) sets the amplifier gain; this can be calculated with the formula Gain = (R2 + R3) / R2. IC1 is the Opamp. There are a great number of opamps available; they are used for all sorts of signal amplification and buffering. Douglas Self (a writer for Electronics World magazine and author of the book Self on Audio, Newnes, 2000) has a personal web page devoted to opamps and their audio application. Self’s site is extensive and extremely helpful (and very amusing, while your there check out his Museum of Retro Technology). I decided to use a TL072 opamp. As Self points out, it’s not a particularly good choice for audio amps but it has two distinct advantages; it’s easy to get hold of and it’s cheap. I got mine from Jaycar for AU$2.35. The TL072 has two opamps onboard and although the signal from the mic will be mono I decided to use both the opamps and amp the left and right headphone signals separately. This has the advantage of allowing me to use a stereo headphone jack which will function with both stereo headphones and a mono earphone.

[What am I talking about? Well, generally if you insert a mono plug into a stereo headphone socket then one of the channels, the right channel I think, gets shorted out. This is not generally noticeable because the mono plug will still pick up the left channel. If however the stereo socket is being supplied by a mono signal with left and right channels simply tied together then a mono plug will short out both channels resulting in silence from the earphone. By providing separate amplification for left and right channels a mono plug can be used in the socket and still deliver a signal.]

electret

I used this electret mic insert to provide the signal for the CMoy amp. This is the type of mic insert you’ll find in almost all consumer microphones; they are cheap (about AU$2.50), small (10mm dia.) and sensitive. Tomi Engdahl has a detailed examination of the electret microphone here and a circuit diagram showing you how to hook one up. (He also has a mass of other useful electronic information and links at his website)

microphone circuit

The mic circuit looks like this. As you can see it is very simple; the positive terminal on the electret insert is connected to a +ive supply (generally anything from 1 to 9 volts) through the resister R1, the negative terminal is connected to ground and capacitor C1 is used to block the bias voltage from the audio output. For my mic the +ive supply is 4.5V, R1 is 10K and C1 is 3.3uF.

full circuit

The complete amp. circuit (with both left and right channels) looks like this. The 50K POT controls the output volume and the 6.8nF and 10uF capacitors provide low and high pass audio filters. Signal gain can be calculated as; G = (1.5 + 22) / 1.5 = 15.6 times mic input.

Op-amps generally require a split rail power supply (with balanced positive and negative voltages) and although the TL072 can operate from a single supply rail I had difficulty with this configuration so I opted for the standard split rail supply. A supply voltage of +/- 4.5V was obtained from a 9V battery by using a voltage divider. The power circuit looks like this:

power circuit

In this circuit the 9V supply is split in half by a pair of 4.7K resistors and a virtual ground is created between the batteries +ive and -ive terminals. SW1 (an ON/OFF toggle switch) and D1 (a red LED) are both mounted on the side of the lamp shade.

lamp-microphone head

As mentioned, the volume control (50K POT) and headphone socket are mounted in the lamps base. The connecting wires follow the same path as the lamps original wiring; exiting the rear of the lamp shade and continuing through the arm of the lamp to the base.

headphone socket

With the lamps original power cord removed, the headphone socket fitted neatly into the vacated hole. The volume control replaces the original ON/OFF switch.

The rest of the electronics, including the 9V battery is fitted inside the lamp shade.

circuit installed in lamp

The battery is held in place by a Velcro strap; for easy replacement when needed.

The parabolic dish was a bit of a problem; I searched a number of op-shops for a suitable shaped bowl. Eventually I decided on this plastic mixing bowl.

mixing bowl

It’s not a perfect parabola, but it was easy to cut and I love the colour.

marking the bowl

I sat the lamp shade on the back of the bowl and traced around the shades front edge. Then I cut the bowl to size with a pair of scissors.

sanding the bowl

I also sanded the inside of the bowl to remove the internal markings. This gave the dish a soft matt finish. Fixing the dish to the lamp was surprisingly easy. Using the discarded bowl pieces I cut two strips of plastic, one was glued inside the front edge of the lamp shade, the other to the back of the dish. When the dish is pushed into place these plastic strips interlock and stop the dish from moving.

fitting the bowl

In front of the dish you can see the electret mic. It’s positioned at the dishes focal point at the end of a brass tube. The whole mic assembly is covered in black shrink wrap.

using the lamp-microphone

So, does it work? …well, yes and no. It does amplify the sound and it’s slightly directional but it suffers from the small size of the parabolic dish; it’s not really in the same league as the gadgets from The Conversation. A larger parabolic collector would definitely increase it’s effectiveness; but despite these short comings I think it turned out pretty well.

Thanks to Anna for submitting the lamp to FIX, I’ll be returning it to you shortly.

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17 Comments

  1. Posted February 14, 2005 at 12:18 pm | Permalink

    Beautiful work! I especially like how you were able to keep everything inside the lamp, without any extra boxes hanging around. I agree with you that this would have turned it into a lamp-hack project, instead of an, er… brainwashing (lampwashing?) identity-swapping kind of project.

    I suppose you could use the lamp’s signal as the input of a noise-canceling circuit, too, in which case it would almost become a portable cone of silence. “Is the lamp pointing at you? Then I’m not listening to you!”

    Anyhow, what a cool project!

  2. Russ
    Posted March 1, 2005 at 12:22 pm | Permalink

    Good work!

  3. Me
    Posted March 1, 2005 at 12:23 pm | Permalink

    Wifi Directional antenna?

  4. John M`
    Posted March 2, 2005 at 12:24 pm | Permalink

    THIS IS AWSOME KEEP UP THE WORK !!! ” : – )

  5. Posted March 2, 2005 at 12:25 pm | Permalink

    Wow, I really like this design, i like the circitry for the amp as well, I might just have to use that ;) good job, keep it up! -Leif902

  6. Posted March 2, 2005 at 12:26 pm | Permalink

    Your warranty just expired!

  7. Willem Boogers
    Posted March 4, 2005 at 12:30 pm | Permalink

    Nice project. However on electronics, some savings can be achieved: to feed both L and R channels you don’t need two amplifier circuits. Just:
    - take the leftmost terminal of the 30 Ohms resistor near the R channel output
    - connect that to the same leftmost terminal in the left channel (left side of the 30 ohms), and
    - remove the entire amp cct in the lower half of the drawing.

  8. Justin
    Posted March 6, 2005 at 12:32 pm | Permalink

    Just an idea… Why not make the lamp functional as well?(using LEDs for heat consideration of course) So you can have a super spy lamp!!!!! lol Anyway I love the project :-D…

  9. Sathish Kumar
    Posted June 16, 2005 at 9:36 pm | Permalink

    Wow, very fine kind of its work!!!!!! I am copying ur idea little bit :-)
    I will use just plastic bowl and a mic to construct a parabolic mic.
    thank you

  10. Bill Zellers
    Posted February 5, 2006 at 9:47 am | Permalink

    I love the lamp Parabolic Microphone. Can you please tell me what is it’s maximum range if placed outside?

    Bill

  11. may lam
    Posted February 23, 2006 at 4:21 am | Permalink

    Hello there. I have a request which you may find both interesting and annoying. But first let me say how amazed i am to have been directed to an RMIT-related site, where our friend Michael Douglas (do you know him?) teaches somewhere in the industrial design area.
    We’ve been living in London for a year, and one of the things we brought with us when we moved from Melbourne is a Planet lamp. I love Planet lamps, have done ever since the 70s when I was a teenager and admired their utility and good sense. I don’t know if they’ve gone out of production, but anyway, the one we brought from Australia has lost the spring in its switch. I have taken the whole thing to pieces, carefully keeping them all, and was trawling through Google looking for ‘planet’, ‘lamp’ and ‘repairs’. I suspect the Planet lamp is an Australian phenomenon, but you may be able to tell me what I need to know. Is this the sort of thing i could get fixed without sending it to a Planet lamp specialist? which, judging from Google, may be a bit hard to locate in London. I would send it home to Melbourne if I needed to; I could detach the head and just send back the bit with the switch. it’s perhaps absurd and sentimental, but I love Planet lamps, they’re part of my history. I hope you can help and await your reply. May.

  12. Posted February 28, 2006 at 8:58 pm | Permalink

    Hi May,

    Yes I know Mick, he lives around the corner from me and were good friends. I’ve actually repaired a planet lamp for him in the past. Sound’s like your lamp has the same problem his did. If your lamp is similar to the ones I’ve seen then the replacement switch you need looks like this. This is a pretty standard item and you should be able to find it in a local electronic store (I’ve no idea who Maplin are).

    If you do decided to do the repair yourself keep in mind that mains voltage in lethal (and this sort of DIY stuff is probably illegal). Be very careful and make sure you re-connect the earth wire to the lamps frame (it may save your life).

    Alternatively, take the lamp to a qualified electrician and get them to replace the switch, they shouldn’t have a problem with it.

    Good luck,
    Scott.

  13. Gooky
    Posted March 7, 2008 at 6:49 am | Permalink

    Just a bit late to the party… :P
    I’m trying to have a go at making this circuit (though admittedly I probably know less than I should about it >_>) and I was wondering what kind of voltage was needed on the capacitors. I think most of the ones I have are 10v or 16v, except the 6.8nF which is 63v. Should any of them be larger? Dunno if this’ll ever get seen or answered, but figured I may as well try…

  14. smitchell
    Posted March 8, 2008 at 10:10 pm | Permalink

    Hi Gooky,

    The capacitors I used were as follows.

    Electrolytic:
    3.3uF 63 volt
    47uF (x2) 63v
    220uF (x2) 63v

    Greencap Metallised Polyester:
    6.8nF (x2) 100v

    These are probably a over specked, you’re probably not going to need these voltages. – but 10v is quite low.

    The original Chu Moy amp uses a 0.1uF 100v on the input and 220uF 25v caps on the power circuit.

    best of luck,
    Scott.

  15. Posted January 25, 2010 at 3:49 pm | Permalink

    Hi Scott. I saw your reply above to May who was asking for replacement switch. (2006) and you said looks like THIS. but there is no link.

    Do you have a dick smith or jaycar part number? I need a couple of those switches or the springs that go inside them at least. ;)

    Otherwise I’ll be without a lamp for a while since prices on ebay are silly.

    Please email me the correct switch specs, I couldn’t find them anywhere on the net.

    PS. There was rumours that planet lamps are still being made in sydney. I sent the company an email from their barely functional website to ask about them but never got a reply. I was wondering if you had any info.

    The rest of your website/blog looks great…I will have to investigate it more when I have time.

    Cheers.

    Bill

  16. Posted January 25, 2010 at 6:41 pm | Permalink

    Hi Bill,

    Jaycar seem to have changed their supplier since I last looked. This is the ‘equivalent’ switch they now stock:
    http://www.jaycar.com.au/productView.asp?ID=SP0736&CATID=28&form=CAT&SUBCATID=652

    or this one from Dick Smith looks like a better fit:
    http://www.dse.com.au/cgi-bin/dse.storefront/4b5d4cc30307fbdc273fc0a87e010654/Product/View/P7874

    HOWEVER, my memory may be failing me but I thought the switch in my planet lamp was a double throw type (SPDT). Both the switches above are SPST. Theoretically I think you only need to switch the active line, the neutral wire can go straight through to the lamp connection so a SPST switch on the active line will do the job but you will need a terminal block (or another type of shielded connector) to join the neutral wire.

    You might be able to get a SPDT version of this switch at an electrical supply outlet like Middy’s, I haven’t tried.

    I’d be interested to know if your switch is a single or double throw type. The lamps were manufactured for many years, so I guess it’s possible both styles exist. Which leads to your other question – yes, I’d heard that rumour too. I’ve forgotten the source but I was told Planet Lighting do a short run of the lamps each year to satisfy large one-time orders – i.e. building fit-outs.

    Let me know how you get on. Please don’t electrocute yourself – If the above doesn’t make sense, seek some knowledgeable assistance.

    All the best,
    Scott

  17. Posted May 23, 2011 at 8:04 pm | Permalink

    I have designed and constructed a microphone amplifier circuit using LM301 with a line driver SSM2142 and a line receiver SSM2141.The circuit is supposed to pick up any voice/audio by the mic and send it via 100m Twisted Pair Cable to a PC for recording and analysis.I am using an omnidirectional Yodata Laptop Microphone,but i get nothing on the output.The circuit(amplification etc.) on its own is perfectly working but the audio/voice part is not,cracking my head.I have tried a ready-made Phillips microphone on my circuit and i works perfectly but the mic described for the job turns me down..Please help

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  1. [...] The Lamp – Parabolic Microphone Read more | Permalink | Comments | Read more articles in Furniture | Digg this! Source: MAKE Magazine [...]

  2. By GEARFUSE » DIY: Lamp To Parabolic Microphone on July 18, 2008 at 1:35 am

    [...] Link (via) Read More Slinky Desk Lamp: The Coolest Slinky Product EverMedusa Underwater Lamp: Trippy!DIY: Cloud Lamp, For When You Can’t Afford LavaThe Really Big Match LampThe Knock-Off Lamp: a lamp for bowlers TAGS: eavesdrop, flux, lamp, microphone, mitchell, objects, parabolic, scott, spy, vintage SHARE: Stumble Upon, Digg, del.icio.us, Reddit this, SlashdotSend to a FriendPermalink [...]

  3. [...] The Lamp – Parabolic Microphone [...]

  4. By splendid! « rubysunday on January 14, 2009 at 5:05 pm

    [...] also dream of planet lamps, did you know? this guy made it into some microphone. i dig it, and all, but i want the lamp. this is the microphone… [...]