When Gavin & Craig from Bus Stop VW came via Bardwells to see if we could make a unit that acted as a ‘phone ringer extender’, we were initially pretty excited. I mean how often do you get to work on something as iconic as a full-sized Tardis?
But let’s back the story up a little here…
Briefly, the workshop at Bus Stop is a very loud environment. Air tools make a heck of a din, and when the boys are working on four or five different vans simultaneously, most of them using either air tools or loud mains-driven models, there was absolutely no chance of hearing the phone should it ring.
So, the boys came up with the idea of having the phone inside its own ‘booth’ at one side of the workshop.
As they are all huge Doctor Who fans, why not make it as a Tardis -Doctor Who’s famous time and distance traveling ship?
So, after scouring the Interwebs for photos, drawings and articles about arguably the BBC’s most famous sci-fi creation, they built a full-sized exact replica.
There was, however, one problem.
When the guys were using their loud power tools, they couldn’t hear the phone in the box when it rang!
This is when they got in touch with Chris at Bardwells, and he kindly suggested us.
To cut a long story much shorter, yes, we delivered the box towards the end of last December, and yes, it worked. Apart from one little detail.
If you watch Doctor Who regularly, you’ll see that the light on top of the Tardis actually dims and brightens with the swooshing sound as it takes off or lands. This was the detail we were missing. When the phone rang on our unit, the light merely came on, then went off again as the phone was answered.
Sooooo, after much head-scratching, we totally re-designed the circuit to include a PIC (Programmable Interface Controller) that once triggered will brighten and dim the mains lamps completely in time with the swooshing sound as the Tardis lands or takes off.
Out went the old-fashioned relay, in came a much more modern PWM (Pulse Width Modulation) circuit controlled by a PIC12F675, a little 8-legged beastie that can be programmed in ‘C’ via Microchip’s PICKit3 programmer and software.
Another important factor came in to play here, too. The isolation factor.
Isolation with a relay is really simple -you just make sure nothing you are switching comes into contact with the thing you are switching it with.
Also, the Big Boys at BT get a little, er, ‘upset‘ should you try to connect anything whatsoever to their phone lines, and with good reason. They don’t want any Tom, Dick, or Harry adding circuits willy-nilly to their shiny, big, and above all expensive telephone circuits and potentially their exchanges -which are very, very expensive.
So, the unit had to be totally isolated from the phone system. No problem -we’ve already done that via a photo-isolator.
Then, our circuit needed to be isolated from the mains. An 0 – 6, 0 – 6 mains transformer and another opto-isolater came to the rescue here. One side of the 0 – 6 would power our circuit -including the PIC, comparator and analogue circuitry at 5 Volts, with a 3.3 Volt regulator hanging off this for the sound sampling chip. The other 0 – 6 winding powering the ‘beefy’ FET that controls the brightness of the lamp.
THEN, we needed to isolate this all from the audio output. As we’ve previously noted, this box is wired to a very good sound system, and we didn’t want any unexpected flashes or bangs as we plugged it all in. This is where a simple 600 Ohm isolating audio transformer came in handy.
This is a quick photo during construction, before any of the ‘major’ wiring took place.
The device mounted on the aluminium heatsink is the main chopping FET. This is the bad boy that actually controls the brightness of the mains lamp. Strictly speaking, and looking at the maths involved, this FET shouldn’t actually require a heatsink, but we thought it for the best, and anyway, it gives the box a nice ‘heft’ as you handle it. The unit will quite happily handle a power of about 500 Watts before anything gets even slightly warm.
The square red thing to the left is the 0 – 6, 0 – 6 mains transformer, while the long chip down to the bottom right of the board is the 3.3 Volt sound chip that plays the sounds of the takeoff then the theme tune.
The two little 8-pin devices sat on the board beneath the heatsink are the analogue comparator circuit to ensure the circuit switches off as the phone is answered, while the other one by the green electrolytic capacitor is the little PIC we used to control the brightness via PWM.
Here’s another view, this time with the clear top on.
A note to say that the bright light that seems to be coming from the top of the heatsink is actually the flash of the camera. The slightly dimmer LED in almost the dead centre of the board is a tell-tale to show that the PWM is working -it glows and dims in time with the bigger light mounted on top of the Tardis itself.
Because we were ‘proud’ of this circuit, and the way it all fits together and works, we decided to mount it in a box with a totally clear lid. This meant not only could you see the various LED tell-tales to show the various states of the circuit -the power supplies, the PWM, and even an indicator to show that the sound chip has trigger- it also means you can see (for once!) just how neat and tidy our wiring is. A quick inspection of the other two controllers will reveal a rat’s nest of different coloured wires that really isn’t pretty.
If you glance to the left of the circuit board, to the left of the larger sampling chip is the audio output transformer that is wired directly to the output jack socket. The phone line comes in on the bottom left via the two white wires.
To the right of the box is the mains input -top right, which is just a standard IEC ‘kettle’ socket as used by computer power supplies the world over.
The other, white wire to the right is the main output to the large lamp sat on top of the Tardis. It is this that dims and brightens as the PWM is working.
To build this circuit, although it doesn’t actually look much, was very, very challenging.
Having to ‘juggle’ the different voltages -and even power supplies, yet make sure everything was properly grounded took quite some doing.
In fact, whilst we were building this circuit, late one night, we made an error that resulted in a wonderful flash of bright, white light, and the fuses blowing for our entire workshop. We had completely unintentionally touched an earthed piece of equipment to the heatsink of the FETs. (This still shouldn’t have happened -this heatsink, like virtually everything else is meant to be electrically isolated. It certainly is now!)
SO, that resulted in the purchase of a new keyboard, a new mouse, a new 10-way USB hub, and a new motherboard for the computer. We’d blown the USB system of the entire computer into orbit. Oooops.
Luckily, nothing else was ‘zapped’, so on receiving the new motherboard, we just slotted it in, and away it went as usual.
It is at this point we can safely cue in the inevitable reader’s comments-
“Couldn’t you just take it back a couple of minutes and stop yourselves from touching the live heatsink? It is The Tardis, after all!”
Very funny, and yes, we’d thought of that, but only the Doctor had the correct co-ordinates to ensure we went back safely.
All being well, we’ll finally install this at some point on Wednesday 11/3/15, and we’ll try to get some shots and maybe a brief video of the full Tardis in action.
…Would you like a jelly bean?