Stereo to Mono converter - how to do it properly

On occasions you might wish to feed a mono PA input from a small stereo device such as a personal MP3 player or cassette player.

Historical note for youngsters: cassettes are those little plastic boxes full of brown string that your parents keep in cardboard boxes on top of the wardrobe. The de-facto standard of 129 metres of brown string wound on little bobbins is almost but not quite enough to store one whole album. This is what passed for copy-protection in their day. Left in a car for five summers, all cassette recordings of any kind evolved into either Wings or Steeleye Span anyway. This phenomenon has never been fully explained but is thought to be an early experiment in what is now known as consumer subscription expiry.

For example, you may wish to provide some recorded music during the interval between sets at a live gig, or you may be a quizmaster or quizmistress delivering the "music round" at your local pub. Either way, the material is on your personal device and you want it to go front-of-house, through the desk or whatever.

I know what you are thinking. You go to your local electronics store, you get a 3.5mm to 3.5mm stereo extension cable and a 3.5mm stereo to 1/4" mono converter and the job is done. Oh ho ho! No. You risk (at best) distorting your sound or (at worst) frying your kit. That's your decision but here at The Secret Laboratory we like to do things properly.

Here's how to do it properly:

Find a 3.5mm stereo jack plug, a 1/4" mono jack plug, a length of shielded two-core audio cable, two 10K resistors (brown-black-orange) and some insulating tape or heat-shrink tubing.

Solder all the bits together according to the following schematic and following instructions:

On the left, your 3.5mm stereo jack. Source (R) will probably be a red wire - connect it to the "ring" on the jack (you may need a meter to check which tag is the ring but it's usually the tag on the right as you look from the rear of the plug - red ring right is how I remember it). Source (L) might be blue, white or some other colour. Connect it to the "tip" on the jack (usually the left tag as you look from the rear). Connect the shielding (Source G) to the "sleeve" of the jack (that's the BIG tag that also serves to secure the cable when you crimp it). In this particular application it doesn't actually matter if you get the L and the R mixed up but it's good to get it right. Don't mess with the G though, else you'll be pissing signal to ground.

On the right, your 1/4" mono jack going to your PA. Connect the red signal wire (Source R) to a 10K resistor and the other signal wire (Source L) to another 10K resistor, making sure the two wires don't short at this end (that's where the insulating tape or heat-shrink tubing comes in handy). Twist the other ends of the resistors together and connect them to the "tip" (PA Sig) of the mono jack. Connect the shielding (Source G) to the "sleeve" of the jack (PA G) and crimp to secure the cable, just like you did at the other end. Get this right and the whole assembly will fit inside the 1/4" jack housing. Oh, er, now is a REALLY bad time to remind you that you should have made sure the plug housings were already on the cable - I'm sorry I'm sorry I'm sorry...

Right! You have just constructed a simple "audio mixer" - tell your friends! They'll think you're some kind of electronics genius! "Buy an audio mixer? No, I just make my own - it's nothing, really..."

Or: "Yeah Brett, I'd love to come to dinner tonight but I'm, like, building an audio mixer?" Think of the potential to keep 'em hot!!!

I showed my version (above) to Steve Glennie-Smith (Electronics Engineer par excellence) last night and he said "That's quite neat - for a programmer". Praise indeed!

The importance of antennas

No, not "antennae" - that's what insects have. Antennas, as in aerials. Sticky-up or sticky-out bits of wire that mysteriously capture signals from the thick soup of radio-frequency data that's whizzing through the ether as we speak.

Perhaps you've got a long-forgotten Yagi in your loft or strapped to your chimney. A sat-dish on your wall or whatever. How soon we forget the importance of these humble items of hardware, yet we are becoming ever more reliant upon wireless communications technology.

Let me really drive this home with a concrete example:

• No antennae, no automated irrigation systems.
• No automated irrigation systems, no strawberries.
• No strawberries, no Wimbledon.

You see? Tennis itself relies upon technicians willing to spend hour after hour experimenting with bits of bent wire.

Yesterday I visited the home of the perfect strawberry with a view to helping to create the ultimate wireless irrigation management system. Dr. Alcock's technical expertise combined with The Secret Laboratory's 40 years of soldering experience is certain to secure the future of UK sporting excellence for years to come.

Watch this space.

Audio attenuator

I know, I know - everyone is wondering when I'm going to pop a cap in the aerosol of Silly String. I'm still waiting for the Great Hairy Leader to lend me his vidcam. I want the best possible footage for your delectation. More news soon...

The project for March did actually take place. The Hairy One needed an audio attenuator to balance line-out from a sound desk to mic-in on his video camera, so he could record some live gigs without unwanted background noise. A bit of Googling turned up schematics for a suitable mono attenuator. The Secret Laboratory adapted the design to produce a stereo attenuator for the purpose, constructed entirely from salvage. This is recycling at its best.

The R1s are 10K and provide the bulk of attenuation. The R2s are 1K and serve to sink optional power from the camera input to ground, fooling it into thinking it really is dealing with a powered mic (you lose a bit of signal here as well but, hey, attenuator - there's a clue in the name). The C1s are 104 ceramics. You could use almost any similarly rated ceramic caps here - all they are doing is making sure that no DC power gets from the camera to the desk. This model provides approximately 21dB attenuation.

I really wanted to etch a custom board for this project but, when I sketched in on paper, it looked like three strips of Veroboard, so I built it on three strips of Veroboard.

I put phono plugs on the desk end (line-out) and a single 3.5mm stereo jack on the camera end (mic-in). Worked a treat.

I wanted to post a photo here but the device was needed on the night in question. I'll post a photo as soon as I get the device back.

Results for February's project

Thanks to everyone who responded. There were six genuine voters and two virtual voters. Analysing the responses was not as straightforward as I had expected.

Kate voted for option 3. Kate's cat also voted for option 3 but I suspect this was just Kate's way of trying to wangle some extra representation and so I'm accepting only the one vote (sorry Kate's cat).

Cosmic Tink Tank voted for option 3 and included some interesting suggestions to enliven the proceedings. I won't be using the suggestions but, hey, what a creative mind.

Mille Vache voted for option 2, which I would have been delighted to run with but unfortunately Mille rendered option 2 redundant by publishing the recipe in advance of project selection and execution. Never mind - it was a good service rendered by Mille and I trust everyone will try the recipe (I certainly will).

Angie voted for option 2 but see Mille Vache above. Sorry Angie. At least you've got the recipe now.

Andrew voted for option 2. Yep, that's right. Sorry Andrew. Blame Mille Vache. Enjoy the honeycomb!

Su voted for option 1. I am counting this vote as valid even though I know for a fact that Su is voting that way only to try to put me on the spot. You don't believe I can do this number theory stuff, do you Su? I have only one thing to say: Tom Westerdale, Birkbeck College. He taught me everything I know about number theory, set theory, logic and computability. Although I'm a wee bit rusty, I've still got my notes...

Anonymous voted for option 1 but then disqualified himself/herself by voting twice for option 1. A bit harsh maybe but Anonymous could be anyone - perhaps even Su trying to inflate her representation. Anonymous did almost redeem himself/herself by suggesting that option 3 would be more fun but, in all fairness, if I'm invalidating this person's vote for option 1 then I must do the same for option 3.

So there we have it:

Option 1: one valid vote (Su).
Option 2: three valid votes (Mille Vache, Angie, Andrew) but option rendered redundant by early disclosure.
Option 3: two valid votes (Kate, Cosmic Tink Tank).

A clear win for option 3. I kind of thought it would go that way.

And here you are - the very can that's going to get it. The contents conform to European directives 76/769/EEC and CE 3093/94. I'm not saying that's good and I'm not saying that's bad but one way or another it will do no more harm to the environment than if it were deployed more conventionally, at a party for instance. The only difference is that my special party will be over in seconds. The only way to reduce impact is to ban this stuff altogether and that would probably be a good thing.

When is it going to happen? Well, I had planned on doing the deed later today but a friend of mine offered the use of his professional quality video camera in place of my humble mobile phone video capture facility. I must admit I couldn't resist the opportunity to capture some high-speed vid and render it in slow-mo for your delectation.

Do not try this at home. See you in March!

Your chance to vote on February's project

February has been a slow month for the Secret Laboratory. I have several serious projects on the go but there will not be any results until late March at the earliest. Therefore, as infill, I have decided to test the intellectual orientation of my readership. You may vote upon the following three last minute projects for February, the most popular of which will be published here.

1. Decidable Predicates. How to decide whether numbers possess a given property.
2. How to make that yummy honeycomb stuff they put in Crunchie bars.
3. What actually happens if you shoot an aerosol can of "Silly Party String" with an air pistol.

You may vote 1, 2 or 3 by responding to this post.

A mystery...

An unknown agent appears to be testing The Secret Laboratory. This morning a package arrived from an anonymous sender. The address was written in block capitals, so no handwriting clues. No return address on the package, no note inside and the package was franked with a "Royal Mail postage paid" label, so no postmark. Who could it be from?

Inside: two mugs of Welsh origin, each bearing a conspicuous dragon emblem; a small jug of Scottish origin; a brown paisley scarf of English origin; a newspaper clipping from The Wirral News reporting upon the activities of a certain rock band with whom I once had some association.

Sifting analytically through my portfolio of friends, I could see tenuous links to many among their number but no single friend satisfied all of the criteria. Those with a Wirral connection might know of my Welsh connection but not necessarily of the band in question. Those with the band connection might know of my Welsh connection but nothing of the Wirral. Those with the Welsh connection would know nothing of my Wirral or band connections. Those who might have judged that the scarf would work well with my tweed jacket and moleskin trousers might have known of the band and my Welsh connection but what were they doing in the Wirral? As for the Scottish jug, well, was this just a Red Herring? Or a Kilmarnock even - that's a plaice in Scotland.

I confess to being utterly stumped.

Band, Scarf, Scotland, Wales, Wirral; that's five factors. I should try drawing a Venn diagram to narrow the range of suspects but, on paper, Venns only work with three factors. Looks like I'm going to have to fire-up the multidimensionaliser tomorrow. It's been in the shed all winter - I hope it still works. Now, where did I put the 2-stroke oil?

The WHUMMMP of clean underpantlessness

It is the sound that every solitary male dreads. What begins as a subliminal thumping grows over the weeks to become a truly disconcerting racket, like unto a teenager learning to play drums in the garage. At last comes an Earth-shaking WHUMMMMP! and the lights go out all through the house. Silence. The washing machine is dead.

This would be distressing for either gender, of course, but the special (and most ironic) tragedy for the male is that calling for an experienced service engineer is like phoning the AA to change a wheel; an admission of extreme girliness. Even if the task has never presented itself before, one feels a masculine obligation to give it one's best shot and fail miserably rather than admit defeat at the outset.

The alternatives are even worse. It's one thing to be sitting in the laundrette in crisp white boxers with "Heard it on the Grapevine" going on in the background whilst your Levis whirl around the tub; quite another thing to be sitting in the laundrette in threadbare nylon paisley briefs with "Steve Wright in the Afternoon" for accompaniment. It doesn't thrill the ladies - trust me, I know about these things.



I would have been particularly upset to lose my Hotpoint Aquarius 1000M. After all, this was the rare edition that included the "Single Bloke" setting on the wash programme selector. No need to understand the intricacies of the other settings - just chuck all your stuff in, select "Single Bloke" and press "On". Everything comes out just like your Mum had done it.

Taking care to isolate the machine from power, water and drainage, I drew breath and heaved it from its lair beneath the fitted cabinets and the sink unit. An adrenalin rush, blood coursing through my wiry biceps, I felt alive.

Seizing a screwdriver, I considered which of the many screws were most likely to grant me access and which were likely to send some delicate internal component tumbling down into an unreachable abyss. Selecting three likely-looking candidates, the top was easily removed and the innards of the beast were laid bare.



As I gazed upon the wonders within, I noticed a spring where no spring should be, then a place where a spring should be but where there was no spring. It took a moment but then a rush of realisation, a "eureka" moment as all suddenly became clear: a spring had come loose. Not any old spring though - a very important spring - the one that holds the thingummybob (yes, thanks Gracie fans, we know...)

But why had the spring come loose? Closer inspection revealed that the hooky bit at the end of the spring had broken off, and this, in turn, was because a certain retaining plate had either never been fitted or had been removed by some rogue service engineer in the past. There was no trace of the missing plate within the machine but the evidence of slots and threaded holes (plus the presence of a retaining plate on an identical partner spring) gave the game away.

No hook, no retaining plate and me running low on clean undies. What to do?

The Secret Laboratory boasts a collection of pliers that would shame a light engineering workshop - 14 to be precise, each with a different purpose. Two of these were selected to fashion a new hook on what remained of the broken spring. The spring now in place, a new retaining plate was fashioned from an old PCI slot blanking plate and secured with a screw of exactly the right type found in my Dad's old collection of "everything you could possibly need". He died in 1998 but to this day he's never let me down. Whatever you need, it's somewhere in his collection - you've just got to work out where he put it (and sometimes this involves bizarre "visualisation" techniques, crystals, incense, chanting and all kinds of hippy shit).



Job done! I reckon that saved me a couple of hundred quid. Another work of genius from The Secret Laboratory!

Note: If you want to try fixing your own washing machine, please observe the usual sensible precautions. Turn off the water supply before disconnecting hoses. Have mops and buckets ready before disconnecting the outlet (you wouldn't believe how much gunk is sitting in the sump). Stick a cork or something in the thing the outlet drained to. UNPLUG UNPLUG UNPLUG! Don't trust the on/off switch or even the wall switch - they don't always do what you think. Remember, some electrical equipment can hold a potentially lethal charge even after being unplugged. If you see something that looks like a little tin can, just walk away and call an expert (one of those babies once threw me across a room and I'm lucky to be here). Finally, if you are of the Compensation Culture, do not under any circumstances attempt to fix your washing machine. In fact, don't try fixing anything. Don't even try staying in bed because I don't want to be sued when you get bedsores. Just close your eyes and try not to exist.