Arduino TV clock with vintage clock source (or another stupid Arduino clock, or "The Ping-Pong Clock")

A while back, the boss presented me with a gift ...

"Found this, I thought you'd like it"

It's a pair of 460KHz Crystals in a standard B7G glass valve envelope. Pretty.












It's sat on my desk for months ... I keep looking at it, and wondering what to do with it...

About the same time, my mate Alan gave me a small CRT video monitor.













I think it's come from a reversing camera from a lorry...










After working out the power pins on the rear, it springs into life, and a stupid plan is hatched....

Take the crystal, and build a CD 4060 oscillator/ripple counter, divide the output down to produce ~28Hz (460000/16384). Feed the 28Hz into an Arduino interrupt pin, and get it to run a clock. Run the TV out library as well, and produce an image on the screen. Brilliant. Stupid. All in one go.

Now, with a crystal oscillator, we need to know the crystal's "load" capacitance. This is formed by the two capacitors off each crystal leg to ground. Every crystal needs these. Get it wrong, and it's unlikely to reliably oscillate (if it does at all). For example, the usual Arduino crystal of 16MHz needs about 22pF on each leg to reliably start up. We'll also need to bias the 4060 so it always starts. Regular readers of this blog will know I used to include a 1 megohm resistor (unnecessarily) across the arduino crystal in my stand-alone designs. We'll need this in our oscillator. (We didn't in the ATMega328 because the bias is supplied by the IC). We'll also need a load resistor. The load resistor is there to make sure there's a voltage on the crystal with which to start the oscillation.

The issue we have is we have no idea of the load capacitance required, or it's required load resistor. There's a rule of thumb about load resistors. 1mm thickness of crystal = 1K ... our crystal is a little over 1mm thick, let's opt for 2.2K. Capacitance? No such rule of thumb. I was going to put 2 adjustable 100pF capacitors in each leg, and twiddle until it would reliably start. Sadly, I could only find one 100pF capacitor... so that went in one leg, and a 22pF fixed capacitor on the other.

Result? Nothing. No amount of twiddling of the 100pF capacitor helped. OK, add another 22pF capacitor to the circuit to give 44pF ... and it's oscillating, but it frequency isn't stable. Remove the two 22pF capacitors and replace with 100pF ....


... and bingo! ... 28.061 Hz pops out of the Q14 pin. It's reliable and steady as a rock. It's a shade off frequency, but crystals do tend to drfit with age, and I doubt my load capacitance quite meets the original spec. It should be 28.076171875Hz.. (460000/16384)








It's all built "fugly" style on a bit of perf board.



























Here's the circuit

The pulses are fed into an Arduino Uno which allows a bit of development to go on, and a discovery is made (should have read the read.me!) .... you can't use an interrupt when using the TVout library, as the interrupts are being used to generate the timing need for video ... no problem, we'll use another arduino to generate the video and pass the data to it over a serial interface..

Now Arduino no1 is just doing the final bit of dividing down and outputting seconds over it's hardware serial interface, it's given it's own 16MHz crystal and mounted on the perfboard along with a 7805 to provide the 5V. The Arduino Uno is now used to develop the video software.







Before long, we have a rather nice clock display running.... (The photo doesn't do it justice, this is the tiny colour monitor I use in the workshop, and the photograph has artifacts..) . There's still plenty of memory left though ....








How about an animated "Pong" (Copyright Atari, the dawn of time) clock? Oh this is getting silly... Yeah, OK...

There appears a number of Arduino "pong" clones on the web, that will (allegedly) sit happily with TVout. After trying, I can't find one that works properly. Shoddy ball/bat collision (it's at best hit and miss 🤣), the ball flying right through the bat half the time.. we don't want that, so I re-engineered the code to suit.

Next thing... We don't want the monitor on all the time, consuming power and wearing the CRT cathode out. So some means of switching on the monitor when someone maybe looking at it. I did contemplate using a passive infrared sensor (PIR), but that's overkill. A simple sound detector will do. We can implement this on Arduino 1. 

A small electret microphone is amplified using an op-amp, and the output fed to Arduino 1's A0 pin.
This input is measured a few times, averaged, and compared to a value. There's an interesting bit of code here, whereby the audio is "rectified" in software, this means any negative audio is "flipped" over,  and a negative peak has the same value as a positive one (I've got a cunning plan for this bit of code, watch this space...). Anyway if this level increases above a certain point, there's some noise about, and we can switch our monitor on.

The output pin feeds a BC547 transistor, which in turn switches on a P-channel FET as a high-side driver, which supplies 12V to the monitor. A minimum on time is specified, as the monitor takes 8 seconds for the CRT cathode to warm. Even with the monitor off, Arduino 2 is still doing it's thing and creating the video waveforms and dealing with time.  

A temperature sensor is added to display the temperature as well... 

This is connected to A0 of Arduino 2. It's just a cheap 10K NTC thermistor. 

Meanwhile, we need a method for setting the clock. 3 push buttons are added, one for hours, one for mins and one for set, and connected to Arduino 2. When Arduino 2 starts up, it automatically enters this screen, and the hours and minutes can be set with the buttons. Once the set button is pressed, a reset pulse is sent to Arduino 1, and it's restarted, setting the seconds back to zero.

I also added a bit of code to enable easy calibration of the clock. Grounding pin D6 (pin 12 on the actual microcontroller) puts Arduino 1 into a calibration mode, whereby it outputs minutes elapsed since start-up and seconds over the serial interface. Disconnect Arduino 2, and connect Arduino 1 to an FTDI converter and monitor the serial output. Open the arduino monitor , and enable time stamp. Write down the time the sketch started, and the difference between that and the current elapsed time. Leave it for hours, then calculate how far it's drifted and use this value to alter the calibration value in Arduino 1 (old cal factor * ( number of seconds expected / number of seconds counted) . Excellent, it now keeps good time.

Excellent ... what about a chime?

I don't want just a beep or something from the micro... I'd like a proper chime.

(It's at this point, it begins to dawn on me I may have taken leave of my senses...)

A small solenoid is purchased from eBay... 

...and a bicycle bell...

.. all conjured up into a chiming assembly! 

... and a driver circuit created to be driven from Arduino 2.

So the final thing looks like this ...

The vintage crystal is top left, with the 14 bit ripple counter to it's right, and the 7805 voltage regaultor to the right of that.  The horizontal ATMEGA328P is our seconds generator, and controls the power to the monitor, the microphone and amplifier can be seen just below it. The high side FET is there too, and runs very cool, as the monitor draws about ~520mA when running. The vertical ATMEGA is creating the video, and also drives the chime, via the low side FET under the stripey ribbon cable, which leads off to the three push buttons, hours, minutes and set. Video is connected to the monitor via the small coax just above the ATMEGA.

The final schematic.

The clock display itself has three modes.


Clock ...












... pong ...












 ... and 3D cube ...

The software can be found, as usual, on my github page https://github.com/andydoswell/stupid-video-clock , and I've included the TV out library as well, as I've modified it so it compiles without issue, has a "degrees" symbol and tweaked the timing a shade. 

Now, I'd better start thinking about a case for it... some sort of perspex thing?

My colleague , Alan, has given it a good name... "Well, it goes "ping", and plays "pong" ... it's the ping-pong clock" 😁

Mercedes 500SL Speedo Repair.

Julian rang..

"The speedo's packed up on Curtis' merc, apparently there's some burnt bits, can you have a look?"

Yeah ... why not?

It's from a 1986 Mercedes 500SL

It's an electro-mechanical speedo. The speedometer itself is basically a big voltmeter. The odometer (mileage) is driven by a small solenoid.

The electronics is driven by an IC, an ITT UAF2115, and the datasheet for it is available on the web.
Pulses come in from the speed sensor (presumably on the gearbox), and this IC interprets them and creates the appropriate voltage for the speedo, and pulses to increment the odometer. There's some timing components to set the range.

The circuit board is removed by removing the 4 rubber mounts, and desoldering the solenoid and speedo

Tracing the circuit through, 12V comes into the circuit, and is filtered by a C-R-C circuit. The R bit of this is the bit that has burnt up. It is (was) quite a substantial component. Poking around with a meter, and sure enough, something after the resistor is dead short (in the picture above, you can see I've already swapped it out)

How's that second capacitor looking? Dead short! Bingo!

It's a Frako brand 22uF 16V fellow. I've had these particular caps fails dead short before. This one looks like it's popped it's seal too...

It's replaced, along with the resistor (33 Ohms) and we're in business. Feeding some pulses in from my signal generator, and the speedo is reading.

I change the other capacitor as well, as it's probably not far behind the first ....

Curtis reports that the speedo is reading perfectly. Great.

Another saved from landfill...

Dangerous Prototypes.

There's been a bit of an upsurge in traffic to the website of late. After a poke round the google stats thing, it's from the very excellent dangerousprototypes.com, home of the Bus Pirate and other great, open source, projects.

Firstly, thanks to Dangerous Prototypes for linking here

and

Secondly, thanks to everyone who clicked through and I hope you enjoy your stay!

(Not a) Triple Tube Geiger Counter

A not inconsiderable number of years ago, I bought a box of parts at a car boot sale for not a lot of money.

In amongst the wire-wound resistors, bits of tag strip, valves and other delights, was this small assembly....

I took it into college (it was that long ago) to see if it could be identified...

Most of the lecturers were a little puzzled at first ... "Pencil vacuum tubes?" , "Some sort of neon?" ... then one lecturer (whose real name escapes me at present, but we used to call him "Toke") identified them as geiger-muller tubes "Russian, by the looks". Top man... I wonder where he is today?

There are three tubes in total, designed to detect different energies of radiation.

This side is fitted with a DOB-50 tube. There's scant information on the internet about this tube, except it seems to have been made in poland, and used extensively by the polish army.

It needs between 390-490 volts to operate, and the left electrode shown here electrode is positive.

The top tube here is a DOB-80, again a polish tube, and there's more info about this on the web...

Operating voltage 490-590, and sensitive to Gamma, Beta and X-rays

The bottom metal tube is a CTC-5 (STS-5) and has CCCP on it, it's a hard beta tube, and has the date code for 1969 on it. It wants between  380V-480V.

After a bit more googling about, it seems I have the internals from a DP-66M dosimeter probe , as used by the Polish Army. Apparently, these have only become available on the market relatively recently, so I don't know what mine was doing in a box of junque in the 80's ... anyway, let's make it do some work!

So, two little switching power supplies, one for the DOB-50 and CTC-5 as they have similar operating voltages, and one higher voltage one for the DOB-80, a simple op-amp comparator to output the pulses to an Arduino to process and display the results. 

The power supplies are controlled using two MC3406AD's, driving an IRF840. I'll just refer to the component numbers on the top supply, the bottom one is almost identical. The back EMF from the inductor L2 is rectified by a UF4007. There's a feed back loop, R14,15 & 16 and the pot is used to adjust the HT.. The supply is smoothed by a C-R-C filter C13, R19 and C14 (which is absent from the lower supply, I was concerned about the top supply driving two tubes)  R22 limits the current into our tube's anode. R25 provides a load for the tube, and the output is fed to a comparator opamp, which drives Q5 via R29, and triggers the arduino input.

I've included a meter driver on the output of the arduino, for that authentic "3.6 roentgen, not great, not terrible" moment, a speaker output for added "China Syndrome" atmosphere, and an interface for an I2C display, for some actual empirical data.

I've also added a pin to monitor the battery level.

A PCB is duly designed, and ordered from PCBWay.









A pleasant morning spent stuffing the board.... note some of the through hole resistors are mounted off the board, in an attempt to prevent them tracking to the ground plane.

Note I've not fitted the Arduino yet, let's get the power supplies sorted first.






First, measure between pin 5 of each power supply IC and ground and set each HT control to give the maximum resistance to ground, this should give us the lowest output voltage to start with.

Now we're going to be dealing with high voltages here, not a fat lot of current behind it, but it will certainly bite if not kill, so be wary.

Connect up a 5 volt bench supply, and current limit it to about 500 mA.

Something's very wrong with mine.... I can't adjust the output voltage above 290V, and my FETs are getting hot, and it's making a nasty buzzing noise. ... Checking though my work, and I've fitted 33nF and 22nF capacitors in place of C7, C3 and C1 resulting in the switching frequency being far too low..... thankfully nothing is damaged.

Firstly set up the "top" power supply, measuring
the high voltage from R19 to ground.

Adjust using R15, and set for 390V












Now, following the same procedure for the bottom power supply, this time adjusting for 550V.

It's worth noting here, that removing the 5V supply to the board, the HT collapses very quickly to safe levels. There's not a fat lot of capacitance here to stay charged up, and the feedback loop discharges this quickly.




The more observant of you will notice that I've socketed the arduino. For some unexplained reason, I've got and used pin 0 and 1 for other things than serial coms, making it impossible to programme in situ. Silly.

After a little bit of tweaking, I've got readings from the CTC5 tube, but nothing from either the DOB-50 or DOB-80.  On closer inspection of the original board, on which the tubes were mounted showed the wiring to the tubes is very corroded. Using some sockets recovered from a DIL socket, I cleaned up the tube pins and connected straight to the tube.






Still nothing. I switched the CTC5 tube into the DOB-50 circuit, and it counts ... Sure enough further testing shows I've got two duff tubes, and looking at them under the microscope shows cracking around the glass where the pins enter... Damn. I suppose they've been kicking round in a box of junque for years, I shouldn't be suprised...

I've included the code for the triple tube in my github, although it's not anywhere near finished and does contain errors, feel free to use is as a starting block only. I'll concentrate on a single tube variant, as that's all I've now got...

So, using the existing PCB, things are looking good. The top power supply is removed, and the lower adjusted to 420V to better suit the CTC5 tube.

I've added a link wire from arduino digital pin 7 to pin 3, so we can use an interrupt to count the pulses, rather than polling.

Software can be found on my github at https://github.com/andydoswell/geiger_counter

The pulses are counted. The instantanious Counts Per Minute (CPM) is based on the time elapsed between the last two pulses, also an actual CPM is calculated (the actual number of counts in the last minute), a long term average (since start up), and counts per hour.

The peak level is displayed on a meter, and decayed slowly in the software.

When the software is started, the meter displays FSD (allowing FSD to be set with R35) , and then the battery level.

The meter display looks like this when in use... a couple of pulses in quick succession will give a high peak reading ... if it stays that way, begin to panic....

(Excuse the B&O VU meter, I had it to hand!)





LCD display looks like this... Battery Level is displayed to left, followed by the instant CPM (CPM) and Actual CPM (ACPM). On the next line is a trend indicator, showing if the last Actual CPM is greater or less than the LTA (Long term average), and actual counts per hour (CPH) .






I'd like my counter to be portable, so I can wander round the countryside after the virus has passed, and scare people... So we're going to need a power supply, preferably re-chargeable.


I've got some of these modules from eBay, normally found inside small power banks. The USB socket is used to charge a single LiPo cell. This in turn, feeds a 5V boost supply. Almost ideal, except the 5V booster is always running, and although there's a protection circuit in it to prevent the battery volts falling too low and damaging the cell, I'd rather it was switched, so a modification is needed. You can see in the photo, the USB charging circuit is very much on the left of the board and the booster on the right...


All we need to do is cut the track that connects battery + and the booster, and fit a switch. We also want to monitor the battery voltage, so we can take this from there to JP7 (which feeds A0) on our PCB.

First things first, connect a battery (I used a 18650 cell) and carefully set the output voltage to 5.0V using the small pot on the board, just below the output terminals.. Now remove the battery, and cut the track where marked on this helpfully annotated photo...









And solder on some wires as shown... The green wires go to the on/off switch, and the thin grey wire goes to JP7 on the PCB, to monitor the battery voltage.












A suitable (if a bit too big!) enclosure is ordered from Electromart2000 on eBay. I've had a few enclosures from them, and they seem to be of good quality, not too expensive, and made here.









The front panel cut outs for the meter and display are drawn out in LibreCAD ...









Imported into Laserweb ...










and the front panel is laser cut with a 3.5W Laser etcher... Fume extraction on!





... it took a while!







This is speeded up 4000 times!



After an hour and a lot later , the front panel is finished.











The parts are still just held on, and are knocked out...













Looks good :) As I was now using a 50uA FSD meter, I added 91K in series with it, to allow adjustment to FSD using the pot on the board.

















The on off switch is added, the LED and speaker, then ....












... I knocked my one remaining good tube off the shelf and promptly stood on it. Suffice to say it no longer works. I'm not happy.










Still, gives me a great excuse to try some more :)

Some different tubes are ordered from Russia. They take a while to arrive, due to COVID-19, but are superbly packed...












Here's what I've bought, in comparison to the CTC5 tube I ruined...













Here's the spec for the SBM20 ...

Minimum Anode Resistor (meg ohm) 1.0
Recommended Anode Resistor (meg ohm)    circuit diagram 5.1
Recommended Operating Voltage (volts) 400
Operating Voltage Range (volts) 350 - 475
Initial voltage (volts) 260 - 320
Plateau length (volts) at least 100
Maximum Plateau Slope (%/100 volts) 10
Minimum Dead Time (at U=400V, micro sec) 190
Working range (mkR/s) 0.004 - 40
Working range (mR/h) 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr) 29
Gamma Sensitivity Co60 (cps/mR/hr) 22
Inherent counter background (cps) 1
Tube Capacitance (pf) 4.2
Life (pulses) at least 2*1010

The huge SMB19 tube

Operating Voltage Range (volts): 350 – 475V
Initial voltage (volts): 260 – 320V
Recommended Operating Voltage (volts) : 400V
Minimum Dead Time (at U=400V, micro sec): 250us
Plateau Inclination: 0.1%/V
Working temperature: -60 to + 70 C
Counting speed: max. 2000 imp/s
Inherent counter background (cps) 1.83 Pulses/s
Interelectrode Capacitance 10pF
Load Resistance 5 – 10 MOhms
Sensitivity to gamma radiation:
MED – 3.0 mR ∙ s -1;
247.5 mR -1 ± 26mkR -1
Length: 195mm
Diameter: 18mm

The SBM 19 tube is apparently very sensitive...

Now we know how delicate these tubes are, I'm going to make some enclosures for them from some PVC pipe. I'll cut a window in the pipe, in an attempt to prevent any low energy beta from being absorbed by the pipe...

34mm OD plastic PVC drain pipe, and end caps...












Window marked out, and ends drilled....












... and the window cut out with a rotary tool.













Some dense packing foam (actually the foam the replacement tubes came in) is marked out, and some foam washers made to support the tubes.











The washers are (very carefully!) pushed over the ends of the tube...











... and pushed down inside the tube, along with a piece of wire to connect to the electrode at the other end...










I bought some PCB 6.3mm fuse holders which are just the job for fitting to the tube's electrode connector. Here's a tip, don't get them from eBay, they're very expensive. I got a pack from RS for much less money.








Nice ...












Repeat at the other end...












The end caps are the sort that usually push into the ends of tubular furniture. Ideal for the job.











One is drilled to take the BNC socket, and the wires are soldered on, observing polarity...










The process is repeated for the substantially larger SBM 19 tube. The electronics is checked, and the voltage adjusted to 400V for these tubes.

The SBM20 tube produces about 5 CPM here, whereas the SBM19 tube is most definitely the more sensitive of the tubes, and produces about 75 CPM. It will even detect the americium source in a smoke detector, giving a noticable increase over background.

SBM20 ...













SMB19 ...



I've really enjoyed this project. What it has brought home though is the need for a 3D printer in the workshop. I could have 3D printed all the cases for the instrument, and probes, and saved hours of time. If there's any company wishing to "sponsor" me with a machine for review, please feel free to drop me a line below!

Incidentally, want to know what happens if you accidentally short the high voltage to an arduino analogue pin?
An accident occurred during development (these things happen) , unbelievably the Micro still works, except every analogue pin now reads about 40ohms to ground and gives no results!... ah well...