Golborne Vintage Radio

Several years ago I built an ESR meter from a design in the now defunct ‘Television’ magazine, which has given a good account of itself, but I’ve long since lost the copy of the article and PCB artwork.

I decided it was time to make another ESR meter, as I work in a spare bedroom and also an outside workshop, so it’s handy to have two of most items. I looked around for designs on internet, and three ESR Meter circuits caught my eye. The two designs which I considered but rejected were:

This one:

http://ludens.cl/Electron/esr/esr.html

(A transformer to wind, and no PCB layout – built ‘ugly style’)

And this one:

http://www.qsl.net/iz7ath/web/02_bre..._esr/index.htm

(Doesn’t need a transformer, but doesn’t have a PCB – just a circuit).

The one I opted for is at this link. I favoured it because it is built on a PCB, uses easily obtained components, and there’s no transformer to wind:

http://www.members.shaw.ca/swstuff/esrmeter.html


Schematic: http://www.members.shaw.ca/swstuff/esrschematic.png

Original Parts Layout: http://www.members.shaw.ca/swstuff/esrbuildit.png

Original x-ray of parts placement: http://www.members.shaw.ca/swstuff/esrxray.png

Those should be viewed alongside my updated layout and parts placement, showing the changes to the layout. Most parts placements are the same.

I claim no credit for the design, which was the work of VE7IT, but in the sprit of homebrew and experimentation I’ve adapted the original design to my own requirements. I posted my own version on another forum in May 2010, not expecting the interest that it generated – 45 replies and more than 7,000 viewings. Over time, based of feedback I’ve received, I’ve made further modifications and refinements to make the project easier for others to build. For example, the original instructions were vague as to how to wire the zero adjustment pot and the meter. The only reason to have the zero adjustment pot on the front panel was to adjust the meter for full scale deflection as the battery voltage gradually ran down. Because I incorporated a fixed 5V+ regulator, this pot can be a preset one mounted on the PCB, so I modified the PCB layout accordingly, making wiring-up more straightforward. The meter can be built in a plastic or diecast project box – I housed mine in a little oak box I made, which is 4 inches x 5 inches, and 2 inches deep.

Some constructors weren’t sure how to wire an LED to warn that the meter was switched on. I therefore added a series limiting LED resistor and two pins on the PCB to supply an LED.

There are only 8 off-board connections:

meter +/-;
cap test leads;
9V +/- battery leads (via a switch);
LED.

The only off-board items needed for the latest PCB layout are an on/off switch, an LED (which can be omitted if desired), a battery, the meter movement, and the test leads. I’ve attached the latest PCB artwork, and an x-ray of the PCB showing the updated layout with the changes made. All other component placements that I’ve not mentioned on that updated PCB layout, (capacitors, resistors, diodes) are as per the original layout, and to make any sense, the original and updated layouts and the original circuit must be studied alongside each other. I’ve also attached some pics of the completed PCB, and how it’s mounted inside the case.

The layout isn't critical, and can be built on plain matrix boad rather than a PCB if desired, by following the lines of the PCB, wiring components beneath the matrix board.

In this posting I’ve just covered the key points, but I’ll put out a more detailed post, so that anyone who reads this and is sufficiently interested to want to make the project has got more information on how it works, the component list, troubleshooting tips, calibrating the meter, scope traces, etc.

Hope that’s of interest.

David

Hi,

Many thanks for adding this informative and interesting thread David.

David very generously gave me one of his circuit boards together with full build and operating instructions; the design is compact; well thought out and nicely put together. Once I can get into my workshop I'll make a wooden box to copy David's excellent design. I've seen a completed instrument and it is an asset to any workshop.

Although I'm a woodworker and understand David's box making technique using comb joints perhaps David you could also give details of your comb joint making jig which in itself is an interesting and useful little project.

I bought my meter from China and including postage it was very cheap it arrived within a few days of paying for it using PayPal.

Nice one David; very impressive and such a lovely little project.

Kind regards, Col.

More info on my homebrew ESR meter and ESR meters in general.

One of the most useful items of test gear for renovation vintage radios and other electronic gear is an ESR Meter for testing electrolytic capacitors. It should be said though, that valve radios generally have few electrolytics – generally, just a smoothing and reservoir capacitor, and a cathode bypass capacitor in the output stage, often 25uF 25 Volts, and often duff. TVs, transistor radios and other solid state equipment on the other hand, often have many physically small electroytics which dry out over time and develop a high ESR. Hence, except for valve radios perhaps, an ESR meter is a worthwhile thing to have, and can be made for less than a tenner.

Over time, electrolytic capacitors can develop a high series resistance, and though they may read OK on a capacitance meter and exhibit low leakage, will act as though there is a resistance in series with the capacitor. (From experience, small, low voltage electrolytics seem prone to developing a high ESR - cathode bypass caps etc, and I’ve cured a dim LED display in a VCR through changing a cap found on test to have a high ESR). As well as helping to find caps with a high ESR, the meter will of course exonerate those that are good, pointing to faults elsewhere on the equipment under investigation.

How does this particular ESR meter work?

All ESR meters work on a similar principle - the application of a low amplitude AC signal usually of a frequency of between 100kHz - 200kHz to the capacitor under test, and measuring its ESR. Most electrolytic capacitor values have an ESR of much less than 10 Ohms which means good caps test at very close to full scale, (zero ESR) and bad caps test at little or no deflection – very high ESR.

To make any sense, the following comments need to be read alongside the circuit diagram at the link mentioned in my earlier post: http://www.members.shaw.ca/swstuff/esrschematic.png

The square wave oscillator is as simple as an oscillator can be – just one, cap (R1) one resistor (R1) and one gate of the 74HC14N IC (a hex Schmitt trigger, widely available for just a few pence). At pin 2 of the IC - the output of the oscillator - there should be square wave of 100kHz or thereabouts - the actual frequency isn't important. (I’ve tried various 74HC14N ICs in this design and the frequency of the square wave oscillator has ranged from 100 – 105 kHz).
Pin 2 of the IC goes to the input of the other gates at pins 3,5,9,11 & 13. The outputs of those five gates at pins 4,6,8,10 & 12 go to the five 680 Ohm resistors R2,3,4,5 & 6, and form a buffer and low pass filter. At the junction of those resistors, which are all coupled together on their outputs, there should be a waveform of approximately 250mv peak to peak at about 100KHz. This low amplitude AC signal is fed to the AC amplifier - a 2N2222 transistor. The next stage is the input protection stage (D5, D6, C5 etc) which protects the meter from damage should it get zapped with an un-discharged capacitor attached to the test leads. From the junction of R8, D5, D6 and C5, there should be a waveform of about 180mV peak to peak, but it’s important to note that this is only when the test leads are shorted, or a good cap with a low ESR is connected to them.

This AC waveform then passes via C2 to the base of the 2N2222A which is an AC amplifier with a gain of approx 10.5. Its role in life is to raise the 180mV p-p input at the base to nearer 2 Volts p-p at the collector. This then passes to the meter rectifier D1,2,3 &4, for the meter movement. Note: Without the test leads shorted, or a known good electrolytic cap with a low ESR attached to the test leads, there will be zero Volts at the input of the 2N2222A, nothing at its output, so no deflection on the meter.

The meter rectifier enables the meter to function as an expanded scale AC voltmeter. Full scale is zero Ohms, midscale is approx 5 Ohms. There is no DC output until approx 75 Ohms of ESR is seen at the test terminals, (ie, a bad cap), at which point the meter will barely deflect.

Component changes from the original design:

R10 - shown on the circuit as 25k - is not a preferred value, so I used a 22k, which seems fine. The same goes for the pot which is used for adjustment to get FSD on the meter, in series with R17. The pot was specified as 25k, but I used 22k, (now a preset on the PCB). R17 is shown as 10k on the circuit, but I found I had to reduce it to 3k3 to enable the meter to adjust to FSD.

Either or both of these values may need to be altered by experimentation depending on which type of 50uA meter is used, if a different one is fitted from the one that I used from ESR Electronics Ltd. The values aren't critical, so long as you can achieve FSD with the test prods shorted.

When the meter is built and working, to calibrate the meter dial requires only four low value resistors - eg, two 1R in parallel = 0.5R, one on its own = 1R, the two in series = 2R. Then 2 x 10R in parallel = 5 R, then added to the 1R resistors, = 6R, 7R, and a 10R on its own = 10R. (8 & 9 R can be inferred between 7 -10). I calibrated the scale on my meter up to 15R, which took up about two thirds of the scale but most good caps will have an ESR of just a few Ohms. (The scale isn't linear).
in my experience capacitors either have a high ESR and hence are useless, or a very low ESR and are good, so the precise value isn’t all that important. The easiest way to check a capacitor about which you are doubtful is to see how it compares with a known good one. The meter scale works like an Ohmmeter such as an AVO – with zero resistance when meter is full scale deflection. Thus, when the test prods shorted, the needle move right across to the RH side of the scale, and is then adjusted for zero. The scale then reads backwards to the left. 75% full scale deflection is about 2 Ohms, 50% FSD is about 5 Ohms, and 25% FSD is about 15 Ohms.

A little more about the modifications and rationale behind them:

The designer (VE7IT) used four rechargeable batteries to provide the 5 - 6 Volts needed, but rechargeables soon self-discharge if not used. Fine for radios is use regularly, but unsuitable for use in low current devices which are used infrequently, and then only for a few minutes at a time.

I preferred to feed the 74HC14N IC, (which is rated at 2 - 6V), with a regulated 5V from a 9V PP3 battery, which will last for ages. 5V+ regulator so I could power the unit from a 9V PP3. I therefore modded the PCB artwork to incorporate a cheap and simple 5V+ regulator, the TS78L05, which will provide a fixed 5 Volts until the PP3 voltage falls to just below 7 Volts. This regulator is no larger than a transistor, needs no heat sink, and fits easily on the PCB. As the meter only draws no more than 25mA, a PP3 will last a long time.

I felt that some of the component pads on the original PCB were a little small, so I beefed them up. I made a couple of other minor alterations to the tracks, and I also provided an extra hole at the RH end of C5 on the PCB to allow for different sizes of .47 400V caps used for C5. Some 0.47 caps have radial leads, others, axial. Whichever hole is best for the cap that you find should be used. I re-routed one of the tracks (Pin 6 of the IC to R3) as there was little clearance from adjacent tracks. I also moved the position of C1. All of these mods are highlighted on my diagram of the component overlay, which should be compared with the original.

The meter can be housed in a small diecast or ABS project box, but I’ve taken to making my own enclosures from oak offcuts, with comb-jointed corners, as they cost me nothing to make and are evocative of crystal set days - at least I think so! You can either fit 4mm test sockets, into which test leads can be plugged, (as with a multimeter etc), but I preferred directly wired short test leads fitted with croc clips.

When observed on a scope, at the base of that transistor there should be 0.0V p-p with the test terminals OPEN circuit, and about 180mV p-p 100 kHz (or thereabouts) when the test terminals are SHORT circuit. (You can’t measure this on a normal multimeter without an RF probe, as multimeters on the AC range are for 50 Hz). This amplified AC signal passes from the output of the transistor (the collector) via C3, to the meter rectifier, which rectifies the AC voltage to DC and presents it to the 50uA meter, via the sub-min preset 22k pot, which is adjusted to zero the meter at FSD. There is no DC output at all until approx 75 Ohm (equivalent to a very high ESR from a duff cap), or below, is present.

The test leads aren’t polarity sensitive – they work either way around on the cap under test.

Parts List:

The list of parts is self evident from the circuit, but I’ve received several queries about 'missing' caps and resistors, so this list and notes about the components might help anyone contemplating building this device:

Parts list:

Resistors:

R1 – 1k0
R2, R3, R4, R5, R6 - 680R
R7, R8 - 10R
R9 - 100k
R10 - 22k (25 k specified)
R11 - 2k2
R12 - 100R
R17 - 10k
R18 - 1M0 0.5W
LED Series resistor, if LED to be fitted: 330R
All 0.5 Watt metal film or carbon film resistors (as sold in the UK by ESR and others).
Zero pot to set FSD: 22k vertical sub-min preset. (25 K specified – see note below)

Note: There are no resistors marked R13 – 16 in the circuit, which uses a total of fifteen resistors if an LED is to be fitted, as listed above. I can only assume that the prototype had more resistors, and the author didn’t re-number the resistors in the final version.

Capacitors:

C1,C2,C3 - .01uF
C4 – 0.047uF
C5 - 0.47uF 400V DC (280V AC) Isolation Cap. See note below.
C6 - 0.1uF
C10 - 10uF Tantalum 16V

C5 is there to protect the meter from the stupidity of a user who might connect the prods to an un-discharged high voltage electrolytic cap, such as a smoothing or reservoir cap in a valve radio, on a cap in a TV, and zap the meter. It goes without saying that for personal safety, anyone working on high voltage equipment - even when disconnected from the mains - should always discharge electrolytics before delving into the set. Should this not be done, at least the ESR meter will be protected, if not prying fingers.

The other caps can be miniature resin-dipped ceramics, which are usually rated at 100V, but anything over 10V should be fine. (As with the resistors, there are some gaps in the list – there are no caps C7, 8 & 9).

IC1 – 74HC14N (NOT 74HCT14N!)
Q1 – 2N2222 NPN transistor.
D1,D2,D3,D4 – 1N1418
D5, D6 – 1N4004.
78L05 5V+ 100mA regulator (If following the mod included on my updated PCB layout)

Note re the IC: The ‘N’ suffix simply denotes a 14 pin DIL package. Any prefix denotes the ID of the maker. In the family of the '74' series of ICs the 'HC' stands for High Speed CMOS with CMOS-compatible inputs.

M1 – 50uA analogue panel meter. New 50uA meters can be found cheaply on internet, though often with a small face. The one that I used is 60mm x 46mm and was supplied by ESR Electronics Ltd, Order Code 124-116, current price (April 2011) is £5.54 + £2.50 + VAT:
http://www.esr.co.uk/electronics/pro..._testequip.htm

(Tel: 0845 2514363 0845 2514363).

Other bits:

PCB (or plain matrix board).
IC socket
Red LED
Croc clips for test leads
Case
On/off switch.
PP3 battery connector.
Project box, wire etc.

All of the above items are available from many UK suppliers, including ESR Electronics, who have no minimum order level or minimum pack size, and charge only £2.50 P&P. Their current catalogue can be viewed and downloaded from here:

http://www.esr.co.uk/

(I have no connections with ESR, other than as a satisfied customer).

The total cost of components - including the meter movement, should be no more than £10.

Testing caps in situ, or not?

There is a debate about this, and one school of thought is that at least one end of a cap under test should be unsoldered and lifted, as some say that the testing of a capacitor while still in circuit can be tricky. Its worth watching the explanation at this link, by John Preher from Preher-Tech.com The two videos were very informative, and may be to some interest to anyone not well versed in electronic components as is the case with me:

http://www.youtube.com/watch?v=GZMQWa0b1xY

In my previous post I attached the updated PCB layoutlayout, and an x-ray of the PCB showing the component overlay with the amendments highlighted. All other component placings are as per the original layout. (Do remember that the layout is as it must appear on the etched PCB - if the press ‘n peel or laser ‘photo print/iron-on’ method is used, the layout must be ‘flipped’ or the PCB will come out back to front!).

A table written by ESR meter guru Bob Parker, of approximate worst-case (highest) ESR values for new electrolytic capacitors at 20 degrees C (68'F) can be found at this link:

http://www.your-book.co.uk/design/esrchart.htm

Or here:

http://members.ozemail.com.au/~bobpar/2003esrchart.txt

As a footnote, the designer referred to the oscillator (gate 1 of IC1) frequency 150kHz, and the circuit shows 156kHz, though neither of these are correct, given the values of C1/R1. As far as I can determine, the formula for calculating the oscillator frequency of the 74HC14N is 1/C1*R1= f MHz, where C1 is uF and R1 is Ohms.

Hence: 1000*.01 = 10. and 1/10 = 0.1MHz = 100kHz.

Having tried several 74HC14 ICs from more than one maker, they all come out at around 100kHz. The actual frequency has no bearing on the accuracy of the meter, and in fact 100kHz is the frequency that many designs seem to operate at. (Using this formula, a frequency of 150 KHz would require R1 to be 665R, and at 156 KHz, 640R)

I mention this only in the event that a curious constructor with a frequency counter might decide to check the actual frequency at pin, and finding that it’s near enough 100kHz rather than 150/156 kHz, might wonder if it’s faulty.

In the next posting I’ll explain the ESR meter construction, testing, setting up and troubleshooting notes.

David

ESR meter construction, testing, setting up and troubleshooting notes.

In my first post, the picture of the meter that I attached had no meter movement in it is they’re out of stock at ESR Electronics just now. Hence, to give an idea what the finished meter looks like, I’ve attached a pic of my original one, which had the zero adjustment pot on the front panel, but which is now a preset pot on the PCB.

These additional notes, based on help that I've given to several constructors who have had teething troubles building the meter, might help less experienced constructors to build and set up the project, and if need be, to troubleshoot and get it working. As with all projects it’s important to check that components are the correct value, particularly those which have obscure markings. Even resistor markings aren’t so straightforward these days - often using the 5-band marking code, as at Maplin for example. Also, ensure that diodes are connected the right way round.

The 2N222A transistor comes in two case styles, the more common of which is the metal ‘TO18’ style case. It also comes in the plastic TO92 case. On the ‘x-ray’ picture of the updated PCB, I’ve shown the pin-outs of each case style.

A word of warning though – there are less common TO92 cased 2N222A transistors in which the emitter and collector are reversed. It’s therefore wise to use a metal cased one, or to check the connections on a transistor tester to be sure.

Setting up and troubleshooting:

Initial tests and setting up:

On completion of the project, set the meter adjust pre-set pot on the PCB to midway position, and before inserting the IC, switch the meter on, and check that there is 5V present at pin 14 of the IC socket. (Black test prod to pin 7 of the socket – red to pin 14). If all’s well, insert the IC, clip the test prod leads to each other, and the meter needle should deflect to the right. If so, adjust the preset pot on the PCB until you get full scale deflection on the meter. It should not need adjusting again.

Calibrating the dial:

Having zeroed the meter with the pre-set pot so that with the test prods shorted it shows full-scale defection - zero Ohms, you can begin calibrating the dial using a few low value resistors as follows:

Two 1 Ohm resistors in parallel = 0.5 Ohm, one on its own = 1 Ohm, two in series two Ohms, then more 1 Ohm resistors up to 10 Ohms, and so on. On the 50uA meter that I used 1 Ohm was about 80% of full scale, 5 Ohms mid-scale, 15 Ohms about 30%. Really, any cap approaching 10 Ohms is highly suspect, so it’s the low Ohms end we’re interested in, at the RH end of the scale.

What if it doesn’t work?

Carefully check all the components, especially the correct orientation of the diodes, Q1, the voltage regulator, the IC and C7 (the only polarity sensitive capacitor). Using a magnifying glass, check for any solder bridges - for example between the pins of the IC socket. Check that you have 5V+ at pin 14 of IC1. If not, is there 9V at the input of the voltage regulator? If so, suspect the regulator. Check that you've used the right IC - 74HC14N. (Not 74HCT14N). It helps if a scope is to hand to check the waveform at various points, and to gain an understanding of the various elements of the circuit.

Getting the meter to read Full Scale Deflection:

The original circuit specified a 10k resistor for the 50 uA meter shunt, (R17) though the designer refers to having to reduce that to 4k7. I found that I needed to reduce it still further to 3k3. Make sure you’ve used a 3k3 resistor for R17 - not a 10k resistor for R17 as was in the original design.

Though the elements of the circuit aren't complex, it does help in troubleshooting if a scope is to hand, but it's worth stressing that when testing and setting up the device, unless the test leads are shorted out, there will be no deflection on the meter and 0.0mvpp at C2 into the AC amplifier (Q1 etc). I’ve attached some pics to show the waveform at various parts of the circuit in a working model (with the test leads shorted), and have shown the scope switch settings.

The pics a are:

1) At pin2 of IC1 - the output of the oscillator/waveform generator, (which simply consists of one resistor, one cap, and one gate of the hex Schmidt trigger IC).
2) At the base of Q1
3) At the collector of Q1

If the meter doesn't work, it's unlikely to be the transistor or IC that are at fault as they're not especially delicate. But as the ICs are just a few pence each, it's easy enough to swap one to see if the original is indeed dud.

From my post-bag over the least few months, it's clear that lots of these little gizmos have been built and are in use, and though it's not a complex project to build or set up, quite a few constructors have had difficulties. Invariably it's been due to shorted tracks, dry joints, polarity of diodes wrong, wrong value components (by a factor of 10 or 100), IC not inserted correctly, or the meter movement connected with reversed polarity. It’s wise to test all components such as resistors and caps before insertion to check that their values are correct.

If anyone does build this project, and after checking all of these points, still has difficulty getting it to work, I’ll be happy to help and advise.

Hope that’s of interest.

David

(13-04-2011, 10:27 AM)Retired Wrote: [ -> ]Hi,

Many thanks for adding this informative and interesting thread David.

Although I'm a woodworker and understand David's box making technique using comb joints perhaps David you could also give details of your comb joint making jig which in itself is an interesting and useful little project.

I bought my meter from China and including postage it was very cheap it arrived within a few days of paying for it using PayPal.

Nice one David; very impressive and such a lovely little project.

Kind regards, Col.

Thanks for reading my post Colin, and for your kind comments.

Meters and China are a sore point with me at the moment. I ordered a digital frequency counter module to make a lathe RPM meter on 8 April. Still not arrived - they said allow up to 35 working days, and that's about up, so I'll have to e-mail them I guess. They do a no quibble re-send or money back, but I'm wondering if the components are sourced from Japan, and possibly affected by the tsunami.

First time I've had any hassle - normally stuff is here within two weeks. I've found Asia Engineer especailly reliable, but they didn't have this particular module, so I used another firm.

The router jig that I made for making comb jointed boxes was inspired by having seen so many similar boxes in 1920s/30s equipment such as crystal sets and test gear. I thought it would be rather nice to try to replicate those boxes rather than use ABS plastic or aluminium boxes for certain projects. They're a bit time-consuming to make, but then it's a hobby, and hobbies are about the enjoyable use of scarce leisure time, and developing one's skills and resourcefulness. I keep a keen eye out for scraps of oak, so mostly, the boxes cost nothing to make. I have in mind to make a reproduction of a 1930s Australian homebrew battery 2-valve receiver called the 'Hikers' Two' and to house it in such a box, but with a lid, rather like a Crystal set. That radio featured in a boys' magazine called The Lamphouse'.

I got the design for the router jig from a video entitled 'Making Router Jigs and Gadgets' by the late Roy Sutton, but the plans which should have come with the video were missing, and by the time I found that out, the shop where I bought the video had closed down! I just followed the video and more or less guessed the dimensions of the jig.

When I get a few spare minutes, I'll take some pics of the jig, and if anyone is interested, I'll try to draw some plans of the dimensions. Meanwhile, I've attached a pic of a test piece I made in two contrasting bits of scrap timber, which gives an idea of the end result. I rounded the corners on that test piece, but generally I leave the corners square.

You can of course buy comb-joint jigs, but they're not cheap. I always find it odd in practical hobbies that so many hobbyists spend large sums of money on things they can make for little or nothing. I know that you share my view that perhaps the most surreal aspect of woodturing is when woodturners buy chisels and gouges with handles, which they can make in minutes for a few pence from an ash blank from a firewood log. Most chisels and gouges for woodturing are available unhandled, and it's useful practice in spindle turning.

Its not for me to pour scorn on how other people spend their hard-earned money, and I guess that my own outlook on this is influenced by having grown up in the post-war years of austerity, governed by mottos such as 'make do and mend', waste not - want not' etc.

David

David,
Indeed. Make the tools to make the tools, or in my case usually make the gadget to take something to bits or put it back together. On my shelves in the garage I have all sorts of odd-shaped 'things' which I've made to do specific jobs, like the pullers to get the bearings out of my Vertical Mill. The bad news is that I can never throw them away, just in case I have to do the job again.
I think I'll leave the woodwork to you and Col, but some info on the jig would be good 'cause I could always make one. You never know, it might galvanise me.
Digital RPM Meter - shock horror. You need one of these - see attached. Of course on my Chipmaster you just dial it up on the Variator. I must get Jamie to generate a couple of emoticons for 'sticking tongue out' and 'putting thumbs in ears and waggling fingers'
Alan

Thanks for reading my post Alan, and for your interesting comments, and showing off your posh RPM meter!

Frankly, I don't need an RPM meter - my woodturing lathe has seven speeds by changing the belt on the pulleys, which only takes a few seconds, and I know instinctively what speed it should be at for a given diameter of wood. Too fast, and it gets scary! You'll know of course that the smaller the diameter, the faster the rotational speed can be, because what matters is the speed at which the material passes the cutting tool. For each rev, a 1" spindle being turned on a lathe will pass 3" of material per rev past the tool, but a 10" diameter workpiece will pass about 30 inches, so though the revs are the same, the amount of material passing the tool is ten times greater. For sanding and friction polishing of wood on the lathe, the speed needs to be much slower, or thin walled vessels can soon overheat and crack.

I'm making the rev counter more as an intellectual excersise than anything - it stops them putting me in a home!

I have the basic circuit from a model engineering mag, but there were no PCBs, so I've designed my own. There are three modules to it - a 12V stabilised power supply, the interface to the cheap and cheerful frequency counter module, which needs 12 Volts and 5 Volts, and the photo-interrupter which fits on the lathe headstock. Because the frequency counter module counts Hz rather than revs per minute, to get it to count correctly it's necessary to fool it by sending 60 pulses per rev, (hence one per second), so to get a correct readout at say 1000 rpm, the frequency counter module needs 60,000 pulses per minute. To accomplish this, a disc about the size of a CD is slotted with 60 1mm slots, which passed through a photo-interrupter (which costs less than a pound) mounted on the headstock. At 1,000 RPM, the beam is thus interrupted 60,000 times by the revolving slotted disc, and this signal is fed to the frequency counter via the interface, whereupon the frequency counter sees what it thinks is 1kHz, so it displays 1,000.

That's the theory, but of course, the difference between theory and practice in practice, is often greater than the difference between theory and practice in theory! I've designed and made up the boards, and the slotted disc, but I'm waiting for the frequency counter from the PRC, if ever it gets here. If it all works, I'll post it under homebrew. The 12V stablised PSU could be used for other projects, or could be altered to any other voltage with a different regulator; 9 Volts for exmple, as a battery eliminator for transistor radios.

Hope that's of interest.

Night night,

David

Hi David,
Thats a mightly familar bit of kit!
I do recall spending valuable time with you (and thank you again) for helping build mine which works perfectly.

What a great and invaluable instrument it is and I do have a spare undrilled PCB for this if anyone is interested?

REgards

Rob

Hi Rob,

Ah yes, I remember it well!

You and Josh, if I'm not mistaken?

Nice to see you on this forum Rob, and glad to hear that your meter is a worthwhile addition to your test gear.

I've built lots of bits of test gear over the years, often for a pittance. One such item as a crystal calibrator which has square wave outputs at 1MHz, 100kHz, 1 kHz and 100 Hz. This is handy as a marker to calibrate homebrew receivers (or any receiver come to that) and for checking the calibration of a scope, as those frequencies correspond to 1uSec, 10uSecs, 100uSecs, 1mSec and 10mSec respectively. I recently looked up the construction article, which was in Everyday Today International Electronics 'ETI' (now sadly defunct and incorporated into EPE), and was shocked to note that it was back in 1981 when I made it. It uses 3 BC109 transistors, and four CMOS ICs, all still available cheaply (under a pound for the lot of 'em), four caps, eight resistors and a 1mHz crystal. Apart from the PCB and a little project case, the whole lot shouldn't be more than a fiver.

If there's any interest in such htings, I'll be happy to post some pics etc on the forum.

Have fun!

David
(AKA David G4EBT elsewhere).

David,

I don't know about the Tacho being 'Posh', although it seems to give reasonable results. I was given it as it was going to be skipped. I must try to check it sometime.

I agree with your comments about speed. After all, we're not generally in the business of removing the maximum amount of material in the minimum time as Industry is. I generally just dial up something that looks plausible. On my old belt drive lathes I generally leave it where it is unless it's wildly out.

On the subject of home-made Tachos see attached. Once again I captured a few of these on the way to the skip. If I remember rightly they are part of the Plessey 801/802 Shaft Signals System. As you might imagine it's built like the proverbial Brick Outhouse to resist the rest of the heavy machinery associated with the Winder. That really is a chunk of 6" x 4" steel angle in there. However, there is also a nice slotted disc and a couple of sensors on the little PCB, which might assist with your experiments. I believe I have your address here somewhere. Shall I put them in the Post?
Alan

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