Difficulty Operating Solenoids with an Arduino Uno

If you're going to get into electronics, treat yourself to a Digital MultiMeter. Using a low impedance AVO will give rise to all sorts of confusion, because putting the meter in circuit changes the impedance of whatever you've put it across.

My choice of transistor would be BC441, but only because I've got some. It claims to be good for 1A.

Don't get a digital multimeter! They are cheap and readily available for a reason - easy to manufacture whereas an Avo is a precision instrument and therefore rather more expensive.

Stick a digital on a varying waveform, and you will be unable to read it because the display will be flickering up and down. An AVO, or indeed any reasonable analogue meter will rapidly steady on a reading, ok the needle may oscillate rapidly, but you will have a much better idea of the voltages.

In respect of the impedance, an Avo, if my memory serves correct, will be using its 10V range with the battery being 9V. Thus the internal resistance of the Avo will be of the order of 200Kohms (10v at 20Kohms per volt), and I would suggest that 200K is effectively negligible in this context. Even measuring 1.5V will require the 2.5V range, and again at 20Kohms per volt, the Avo internal resistance will be 50Kohms, sufficiently large enough to be negligible in this circuit.

Finally, consider the so-called accuracy of a digital - say 3 & half digits plus or minus 1 digit or something similar. But lets not get bogged down in this - just think of the tolerance of the resistors being used in the circuit plus or minus 1% if you are lucky. More likely 2% or even 5%. Which means that the so-called accuracy of a digital is blown straight out of the water due to the tolerance of the components.To explain a bit further, suppose you have two 220 ohm resistors in series across the 9V battery. In theory, the voltage at the junction should be 4.5v - provided both resistors are exactly equal. If one is slightly high, and the other slightly low, then the voltage at the junction will not be 4,5V., indeed with 1% tolerance resistors, the centre voltage could correctly be anything between 4.2v to 4.8v. So, why bother measuring a voltage to say 2 or more decimal places?

Andrew talks about good practice to have a resistor between base and emitter. He is quite correct and suggests a value of 10K. Fair enough, but in a lot of instances, a much higher value would be just as satisfactory. Many years ago, my then manager "lumbered" me with repairing some digital display units of which he had arranged for a local firm to build 130 units. The agreement my manager struck was that I would repair those that didn't work, but he didn't bother telling me, so when this chap turned up with 30 duff units, I was as you might expect, suitably less than impressed. Anyway, most of the faults were ic's incorrectly inserted, but I did find two design faults. One was a particular manufacture's ic which under certain circumstances did something different to all the others. The cure was to not use that manufacturer in that position. The other problem, which I admit took me a long time to sort out, was what is termed Icbo leakage from collector to base on the input transistors. This was cured, as Andrew says, by connecting a 33K resistor across base and emitter. We also arranged for all those units which had gone out into the field to be suitably modified.

Regards,

Peter G. Shaw

Fluke digital multimeters quote 0.5% on DC voltage 2% on AC. Less than £200 new. I don't think they will be using randomly selected 1% resistors. Used AVO 8 £500 with the joy of special batteries to add to the cost. I think I'll be OK with my digital multimeter for the foreseeable future.

Martin C

Good grief. I had no idea that they are that expensive. I was given mine and have been offered another.

James.

Having earned my crust in electronics for 45 years, and it's still paying my pension, I can't imagine going back to an analogue meter. AVOs were in their day the standard instrument but are now outmoded. Given a good-enough DMM is about £20 and you can buy a simple kit to make an oscilloscope that's good enough for simple debugging for the same if you're prepared to build it, I can't see any benefit in using an AVO unless you already have one. I used to have an AVO Minor, having not used it for 20 years or more I tested it a few weeks ago and the movement stuck at several places on the scale so it has been recycled. I also have 3 DMMs which all get used, as well as an old analogue 'scope and a new digital one - again only the modern scope is used for 99% of the time, in fact since I bought it I can't remember using the old Scopex one.

Though I'm in John Haine's not impressed camp, I do own an AVO8, which was a gift. Like most tools it has virtues and shortcomings! For me the shortcomings outweigh the meter's advantages but we're all different.

In favour:

• Precision instrument (outstanding back in the day)
• Built to last and to take abuse; a practical rather than a laboratory instrument. Exceptionally solid construction - thick Bakelite front-panel mounted on a steel box. Further protected by a heavy leather case.
• Large easy to read scale with consistent calibrations.
• Well thought out ranges
• Does both DC and AC amps up to 10A
• Analogue movement gives quick indication of life in a circuit when actual values don't matter

Disadvantages:

• Very expensive new. Hobbyists own them via Government Surplus, ex-professional, or ex-educational. Often expensive second-hand too because people crave quality rather than functionality. Seriously expensive compared with modern alternatives (where modern means anything later than 1970!)
• Take up lots of bench space
• Heavy to carry (shoulder-strap job)
• Analogue movement slows down taking accurate readings. To be accurate the user has to select the most suitable range, and to read the scale carefully, including reducing parallax with the mirror. Also has to get the polarity right!
• By modern standards Avo 8 accuracy is ordinary, many cheaper meters do just as well.
• The Avo's relatively low impedance makes it dodgy for electronics - it was designed for valves, not low voltage semi-conductors.

For what it's worth, I use an ordinary 20kΩ analogue meter and a hand-held Digital Multimeter. The analogue meter is good for quick diagnostic work not requiring accuracy, whereas the DMM shines when actual values are needed. The DMM doesn't care which way round the leads are, and it's digital display is unambiguous. For accurate work digital meters are less error prone. But you have to pay - very cheap analogue and very cheap digital meters are both disappointing!

Since buying a portable digital oscilloscope, I find myself using it in preference to meters. The oscilloscope is high-impedance, and accurate, and it shows what's happening over time. My previous valve and transistor oscilloscopes didn't get used in the same way because they were too heavy and time-consuming for quick dab measurements on a dining table. Same objection to my PC oscilloscope/logic analyser: though it works well it takes too long to set up for simple testing.

Horses for courses!

Dave

Edited By SillyOldDuffer on 30/12/2020 10:23:15

Apologies for going slightly off topic, but I think relevant, what do users of Avo 8s do nowadays about the special 15volt battery that it used? I switched to DMMs probably 25 years ago because I could no longer obtain that battery. Also as others have said its too big for table top use.

Phil

Err, with 1% resistors I think the midpoint voltage will also vary by 1%, ie, 4.455V to 4.545V.

Andrew

I have an old AVO (not sure which model) have not used it for years and am sure it used a total of 3 batteries.

I think BLR154 15v Alkaline Batteries are available and advertised as replacements for the original.

Edited By V8Eng on 30/12/2020 12:22:35

Looking in on this post I was particularly impressed with the circuit diagram produced by AJAX. Can you tell me what program was used to produce it ?

Andrew,

I did think my calculated values were a bit out when I wrote it, but considering the time, which I hadn't realised it was, I just couldn't be bothered to check it. So thanks for the correct values.

The point I was trying to make is that digital is not necessarily the best and that an analogue meter despite its perceived lack of accuracy, is often more than plenty good enough.

I haven't done any serious electronic work for a considerable number of years now, but a bit like Dave/SOD, I do indeed have a pair of analogue meters, and a digital meter. I don't have a portable digital 'scope, but I do have a standard analogue twin channel, single timebase 'scope, and, if I can ever be bothered to mend it, a Tectronic dual channel, dual timebase 'scope. So I have more than enough test equipment available If I need them. Which I don't.

My final comment is that digital is not the be-all and end-all it's often cracked up to be.

Cheers,

Peter G. Shaw

Malc there are a few free electronics packages around. I used to use DesignSpark from RS but switched a while back to KiCad. Both involve a bit of a learning curve but it's well worthwhile.

I'd agree with that. I have an AVO (bought from a friend mostly for old times sake) but it does sometimes get used instead of the DVM. Digital 'scopes are great in many ways, but I've seen a number of professionals get confused due to aliasing giving a misleading picture of the waveform.

Andrew

The best feature of the AVO is that you can stand on it to change a light bulb.

Even when I was using them professionally, my own analogue meter at home was 100k ohms per volt, as compared to the 20 k ohm per volt of the Avo. They were good in their day, said day being at least fifty years back. Their valve tester was quite useful too...

John

Just a quick update. Thank you for all of the advice and help.

I have replaced the BC547 transistors with the 2N2222 type. It is all working as designed now. The next step is to put it all in a box, along with the Arduino Uno, the RTC, digital display and the relays and make sure that it runs properly for a prolonged period.

Assuming that it works as hoped, the next stage is to build a mechanical clock for the relays to operate.

Regards,

James.

This thread means another AVO,(now considered 'valuable' asset?) will sit on a shelf even longer- pre Christmas I had eyed up the old and dusty AVO for the cases potential to house a 180 v DC power supply for a CNC machine - a 2021 project - and recycling at its best!

Downside suitable project box now required at minimal £s and 'currently ' no car boot sales or autojumbles to shop at. full of goodies I can't 'resist'! I really find trips to the 'local dump' quite challenging so many things I could rebuild/modify or just put on a shelf!

Any alternative suggestions/donations/swaps (Watts) out there? That needs a new 'ohm' Near Aylesbury/deliverable in tier 4 times.

Happy New Year and keep safe

From 'Two AVO's ' Pete

James, maybe a silly question, but presumably the relays will supply some sort of solenoids in the clock, in which case why not drive the solenoids directly from the transistors? What did you have in mind for the clock?

To put it in perspective the clock I described here uses a converted quartz movement as a display driven directly from the Arduino pins via a 270 ohm resistor.

What size box do you need? I have a few odd things spare and am in Aylesbury

Hello James (Sorry to others for thread drift)

Thought I was an isolated island of madness! (Near The New Zealand)

Although partly written tongue in cheek I really am looking for a box about the size of an AVO or slightly larger - think desk top PC as maximum although experience of actual PC cases is -never quite suitable..

But for COVID and my shed is COLD I would welcome you for a coffee now - hopefully sooner rather than later!

Pete

Edited By Frances IoM on 01/01/2021 14:11:16