For understanding the design demands for the charge indicator and the technical characteristics of the resulting design, one needs at least a basic knowledge of electricity and electronics and preferably also some experience with it. But for people who are not trained and experienced, it should be possible to build, test and calibrate charge indicators as long as a clear, step by step manual is available. That is what this chapter will try to provide.

People who are interested in why the charge indicator was designed this way and how it works, are encouraged to read annex D: More about Charge indicators. If technical problems arise that can not be solved which the help of par. 5.4.5, one will have to refer to annex D. Then it should be possible to work out what part of the indicator does not work as it should, and find a remedy for this.

There are clear `economies of scale' in building charge indicators. Learning how to build them, buying the components, getting the equipment that is needed and making the Printed Circuit Boards will take about as much time irrespective of whether one builds 5 or 100 pieces. Also the actual building, testing and calibrating will go much faster as one gets more experienced and builds some simple, handy devices. So from that point of view, it makes sense to buy indicators from abroad as long as only a limited number of indicators are needed (see par. 7.4), and not start building oneself. This saves a lot of time and also reduces the risc that the project will run into problems because one essential device as the indicator cannot be built locally yet. Then one could start producing indicators in a later stage, when a steady supply of indicators is needed since more chargers are built and installed. There is no clear number of indicators above which it becomes attractive to start producing them locally, this number depends on:

• Technical skills and spare time of the one who is expected to do the job.
• Anticipated difficulties in getting the necessary equipment and buying components.
• The costs: Generally indicators that are built locally, will be cheaper when only component costs and labour costs at the wage level of a local engineer are considered. But when a well-paid development worker should do the job, things are different.
• The importance that is given to being independed from outside support. When the development worker who started the project and who could easily maintain contacts abroad, is about to leave, maybe time presses to have local production established. Of course this also depends on how reliable and how fast this outside support is expected to be (see par. 7.4).

Then there is the possibility of having indicators built in a workshop in-country:

• In bigger cities there are probably workshops specialised in making electronic devices according to a given electronic circuit or even developing them starting from the design demands. For the project this means that there is no need to learn how to build indicators and get the equipment. Those workshops experience the same economies of scale, so this option is only attractive if a high number of indicators can be ordered.
• In smaller cities, there will be electronics repair workshops interested in building indicators. However, it is unlikely that they could build indicators without technical support and the project might need to provide the equipment and help with technical problems. Advantages are that these workshops might know very well where to get parts cheaply and that in the future, indicators can be built cheaply and close to where they are needed.

The batteries are the single most expensive item in the investment costs of a firefly micro hydro scheme. In the operating costs, replacement costs of worn-out batteries will form the major part of the money that is spent outside the community itself (the charging fee is also a major expense, but this money remains in the community). Poor care for batteries and the resulting poor life span can lead to disappointing performance and operating costs being too high for poor potential users.

A main cause for premature battery failure is deep discharge (see par. 5.4.2). In a battery that is often discharged too deep or that has been left in discharged condition for a long time, the plates gradually get covered with a hard layer of lead sulphate which makes them ineffective. High temperatures and big changes in temperature promote this. This process is called sulphatizing and it makes a battery lose its capacity to store electricity (VAN DER MEER, 1990, see also annex C: Sulphatizing).

To reduce this effect to an acceptable level, normal car batteries should be discharged to only 50 % of its capacity (so: 50 % of its capacity should be left unused). Solar batteries are designed for `deep cycle' and can safely be discharged until 80 % (so: 20 % of capacity left unused).

However, users will be temped to use their battery too long:

• Everytime a user has his battery recharged, it costs time (to bring and collect the battery) and money (the charging fee). These costs (both in time and in money) are the same whether the battery was heavily discharged or only slightly used. So to get the most out of their time and money, users will discharge their batteries up to the limit. And if users don't have the time or money, they will be tempted to use it for a few days longer.
• Sometimes, a battery can not be recharged, for instance because the charger is broke or it is not available because it is charging other batteries, the water supply has dried up or the canal is damaged. Then again users might still want to have light and use their battery a little bit longer.
• Probably, users are familiar with dry cell batteries. These gradually produce less current and at a lower voltage, which can be noticed easily (e.g. in a flashlight, the light goes dim). For them it might be difficult to accept that a lead-acid battery needs recharging before it is empty and even before it becomes noticeable that lights go dim.
• The damaging effects of deep discharge come slowly and invisible and by the time the effects become noticeable, the damage is done. Even though users might know about the long-term costs of deep discharge, they might act based on short term considerations like the ones above.

In order to avoid deep discharge, users should be able to measure how far their battery is discharged. This is where the charge indicator comes in: It measures the state of charge of a battery (there are also other types of devices to measure state of charge or prevent deep discharge, see annex D.1: Strategy to prevent deep discharge).

Then the strategy for preventing deep discharge of batteries could be like this:

• A cheap charge indicator that is affordable to all users.
• Using the indicator should be as easy as possible:
  • The reading of the indicator should be easy to interprete. Therefor the indicator does not give a reading in Volts or in % charged, but in 3 colors of LED's that can light up. Green means that the battery is full and useable, yellow means that state of charge is less than 50 % and if the battery is a car battery, it needs recharging, and red means that state of charge is less than 20 % and also a solar batteries need recharging.
  • The indicator should draw the attention of the users, rather than users having to remember to check their battery. This can be done by connecting the indicator to the switch of a lamp that is often used. When this switch is operated, also the indicator is activated. It does not give a stable reading immediately, but it `walks' to its final reading in a few tenths of a second. This means that one will see a `moving' light, and a light that changes color, which makes the indicator even more conspicious.
• The charging fee should be kept as low as possible (see par. 6.4), so that the costs of charging do not form an incentive to postpone it as long as possible.
• The distance between users and the charger site should be low so that bringing and collecting the battery takes as little time as possible. This means that users living far away from the charger, must be discouraged to join in with this charger, but could be encouraged to start with a charger of their own.
• Users should understand that the responsibility for preventing deep discharge lies primarily with them. For users to handle this responsibility well, they should be informed about the effect of deep discharging on the life span of their battery, and on how to prevent this.
• As a backup to the users own responsibility for having their batteries recharged in time, the operator of the charger should measure battery voltage with a tester before charging a battery. When the battery of a certain user was discharged too deep several times, the operator should give a serious warning to the owner (and maybe check his/her charge indicator in case the owner insists that the battery was not used too long).
• As a second backup to the users own sense of responsibility, the operator should make notes of the charging process. When a battery is fully charged after less hours than it needed when it was still new, it means that it is losing capacity and that the owner should be warned.
• As a third backup, batteries that are worn out prematurely should be investigated and the results made known to other users. This could serve as a last warning. Unfortunately it will come too late for the most careless users whose batteries might break down first.

Buying component is not as easy as it would be in western countries, especially when one tries to buy cheap. At least that was the experience in Manila. There are two problems:

• Availability of components in third world countries. This means that buying all parts that are needed, can cost quite some time.
• Sub-standard quality components might be sold. So either one has to find suppliers that are likely to sell good quality parts, or one has to check the parts before buying them. However, for checking parts properly, one needs to be quite experienced. See also annex D.5: checking components.
  The only thing that is easy to check is whether the leads on the components are clean or oxidized. If the leads are clean, at least it saves trouble during soldering them. If they are oxidized, the component could very well be O.K. but it indicates that it has been in stock for a long time or has not been stored properly. This all makes it look like it comes from a second quality lot.

First about the kind of shops where electronic components are sold:

• Shops geared to electronics hobbyists. They usually have many commonly used components in stock, but maybe only a few pieces of each type. The people serving in the shop might provide useful advice, as they might be trained to repair electronic devices. Probably they sell interesting books and testing equipment. Sales volume is not very high and consequently prices cannot be cheap. This is the kind of shop that is common in western countries. It is unlikely that many of such shops exist in third world countries.
• Wholesalers. They sell large numbers of each type to workshops building electronic devices. Quality must be O.K. and prices low or they will lose customers. If there is electronics industry in a country, such wholesalers must be there also. In western countries, such wholesalers are not interested in selling small numbers of parts but in third world countries they might be since labour is cheap and the costs of handling a small order are not so high.
• Suppliers to electronic repair workshops. In western world, such workshops will keep their own stock of components and order directly from wholesalers. In third world countries, electronic equipment will be repaired much longer. So there is a lot of work for small electronics repair workshops and there must be suppliers of components to these workshops, at least in the bigger cities. To keep costs low, such suppliers will only sell parts, keep their stock as small as possible and try to buy parts as cheap as possible. Consequently prices are low, but you will not get much useful advice there, it is unlikely that they have all components you need in stock and there is a risc that the parts they sell are faulty. In Manila, this type of supplier could range from a small shop to a large company with different, large outlets.

Clearly suppliers to electronics workshops are not a safe choice in case one is not experienced enough to test the parts before buying. If the electronics hobbyist shops are not around and wholesalers are unwilling to accept small orders, the best answer is to look for assistence in buying the parts:

• A small electronics repair workshop. They will know which suppliers are cheap and reliable, and maybe they can test parts or get their money back in case parts turn out to be faulty.
• A workshop specialized in building electronic devices and capable to order directly from wholesalers.
• Maybe the `expatriate circuit' provides contact with employees from a western multinational that has its own electronics workshop and is willing to help.

Both a small repair workshop and a larger production workshop might become useful contacts in the future when the development worker who started it all has gone and indicators should be produced locally. A list of all parts that are needed for the indicator is given in the next paragraph.

The electric circuit of the indicator is given in fig. 5.3.

It would go too far to explain in detail how the indicator circuit works, this is done in annex D: More about charge indicators. Also it is not necessary to understand this exactly as long as one is able to build, test, calibrate and install indicators correctly. All parts needed for the indicator are included in the parts list, see table 5.2.

Fig. 5.3: Electrical circuit of home system

There are several ways to make PCB's (Printed Circuit Boards). By far the most common way goes like this:

• Copy the desired copper pattern onto a transparant plastic sheet.
• Prepare a piece of blank PCB with a light-sensitive laquer (ready-to-use blank PCB with a light-sensitive laquer can also be bought).
• Fix the sheet on the PCB with light-sensitive laquer and expose it to UV light.
• Develop the light-sensitive laquer. On the places where no UV light has come (because the sheet at that place was black), a layer of laquer remains while at the exposed parts, blank copper of the PCB comes to the surface.
• Etching away the blank copper.
• Removing the laquer layer.

Finally the holes for the components are drilled and often the copper is covered with a protective layer against oxidation. Then the PCB is ready for fitting the components.

For unexperienced people, it seems a bit overcourageous to try producing PCB's in this way oneself right away. If the resulting PCB's would have a doubtfull quality, this could result in many more indicators that do not work because of poor soldering, tiny cracks in the copper strips etc. Finding these errors takes a lot of time and could be quite frustrating for unexperienced people. Also, for building PCB's this way, blank PCB material, several chemicals, a source of UV light and experience are needed. Workshops who sell these materials and can provide advice, might also produce PCB's according to design quite cheaply. Or a supply of ready-made PCB's can be bought from abroad (see par. 7.4).

In annex D.6: Making PCB's, a manual on how to produce PCB's in this way is given, together with simpler alternative ways which could be attractive in case no ready made PCB's can be bought and only small numbers are needed.

So now the problem of getting PCB's is reduced to finding a workshop who can make them, and providing them with a crystal-clear design.

A print lay-out on scale 2:1 is given in fig. 5.4. Mind that the component lay-out is printed as seen from component side, while the copper pattern is printed as seen from copper side. That is why on the component lay-out drawing, the copper pattern appears in mirror image (see the blue, dotted areas and the thin lettering).

Fig. 5.4: Component layout and copper pattern of the Printed Circuit Board for the indicator.

This lay-out is quite compact, it occupies a minium of space, is quite sturdy and consequently the PCB needs only 2 screws to fix it to the front cover. At the same time, this compactness leads to disadvantages: There is too little space for components if they happen to be larger than the standard ones and then some bending might be needed. Also the copper pattern is quite detailed and might be difficult to print properly in less well-equipped workshops.

The lay-out of fig. 5.4 shows the placing of the components that were included in the electrical circuit of fig. 5.3, and the copper pattern that is needed to connect them in the right way. There are `islands' onto which later the leads of components will be soldered, and there are copper strips that connect the islands.

The coppern pattern of fig. 5.4 could be given to a workshop as a design for the PCB's. However, it is better to provide them with a transparant sheet that can be used directly. In fig. 5.5a, the same design is reduced to the correct size and 12 copies are placed on the smallest standard size PCB board. Here the copper pattern is printed in mirror image because it is best to have the transparant sheet in mirror image. Then the sheet can be laid on the blank PCB with the printed side facing down directly onto the light sensitive laquer and the clearest image will result. The copper pattern of fig. 5.5a can be copied directly onto thin tracing paper (the slightly transparant type of paper that is used for technical drawings) or transparant sheet with an ordinary copying machine. Please note that only special heat-resistant transparant sheet can be used in a copying machine, ordinary plastic will melt and make the copier jam. Check the quality of the copy carefully for:

• Are no strips interrupted by cracks or different strips or islands connected by stains.
• Are the dimensions still correct. The cadre around the 12 copies should measure 160 x 100 mm. If on the copy, the length deviates more than 5 mm from this 160 mm, the transparant is unusable because the pins of the LM 3914 chip would fit badly in the holes of such a PCB.

Fig. 5.5a: Twelve copies of the PCB copper pattern (in mirror image) fitted on standard size PCB board. Note: For correct dimensions, print this figure at 300 DPI (Dots Per Inch).

Fig. 5.5b: Alternative: Eight copies of PCB design with current shunt. Note: For correct dimensions, print this figure at 300 DPI (Dots Per Inch).

If the tracing paper copy or transparant is O.K., it can be used directly by the workshop that will make the PCB's. There are many qualities of PCB material and if the workshop asks which one you prefer, choose for `epoxy' (colored white, slightly transparant) of ca. 1.0 mm thick. This is stronger and more resistant to humidity than the `pertinax' (light brown) that was used in older days. Especially the copper layer is fixed stronger on the base material and this reduces the risc that the iron wire that forms the switch will break loose. A disadvantage of epoxy is that it contains glass fibres that makes drill bits get blunt quite rapidly.

If the workshop will drill the holes, specify which holes have to be bored with what diameter, see below.

It makes sense to check the PCB's before accepting them, possible errors are:

• A clear, sharp image of the copper pattern. The thickness of the copper strips should be close to the thickness of the strips on the transparant, and there should be no grains of copper scattered on places that should have remained empty.
• Short-circuits between different copper strips and islands, or interruptions within one strip.
• If the holes were bored in by the workshop, the holes should be right in the middle of the islands and with the right diameter, see below.
• The print has been made in mirror image. A correct PCB will have the text `charge indicator FIREFLY' readable at the copper side. If it is not, the PCB's are practically useless. Therefor always specify `component side' or `copper side' on a PCB lay-out.

After the pattern of fig. 5.5a is printed on PCB board, the 12 individual PCB's can be cut loose with a hacksaw and the holes must be drilled. Drilling is not difficult but when the workshop is willing to do this for a reasonable price, it saves a lot of work.

For drilling the holes, there are 3 options:

• With a special, small electrical drill for electronics and miniature work, and the drilling frame that goes with it. Such a drill has the right high speed for drilling small holes, drill bits fit in easily and the drilling frame makes it easy to use without danger of breaking drill bits. Especially for drilling a large number of holes in epoxy material, you could buy `hard metal' or `diamant' drill bits instead of the usual `HSS' quality drill bits for metal.
• The next best solution is to make a small electrical drill yourself. This is described in annex E.3: A small electrical drill.
• Using a normal electrical drill. If there is a smoothly operating drilling frame with it, you might use this in the normal way (with the PBC on the working surface below, and moving the drill up and down. If there is no drilling frame or it does not move smoothly, it is easier to fix the drill (vertically in the frame, or horizontally on the table) and move the PCB towards it by hand. Speed is low, there is a risk of breaking drill bits and it is a lot of noise, but it works.

There are 4 kinds of `islands' on the copper pattern:

• The normal ones for ordinary components. These should be drilled with 0.8 mm.
• The ones with a larger outside diameter for the trimmer R₄. These should be drilled with 1.2 mm. One mm will also do but then one has to make the leads of trimmer R₄ thinner by squeezing with pliers.
• The square ones, for the copper wire switch, the diode and for the connections to the outside. These can best be drilled with 1 mm, so that the 0.75 mm² wire fits in tightly and strong. Drilling with 1.2 mm will also do but then these wires will be fitted a little less strong.
• The ones with two concentric circles besides the chip. These are for the M3 bolts with which the PCB will be fitted to the front cover and should be drilled with 3 mm. This can only be done with a normal electrical drill.

Clear copper will oxidize readily after it has been touched (the sweat from your fingers contains acids). A greenish-blue copper oxide layer will form and soldering later on will be more difficult. Of course the PCB's can be cleaned before soldering, but it is best to prevent oxidation by applying a protective layer on the PCB's before storing them. Electronic shops sell special sprays for this, but fixative (for charcoal drawings) will also do. Even grease or vaseline will do as long as it does not conduct electricity (check this with a tester on the highest resistance range).

Also after the indicators have been built, tested and calibrated, it makes sense to apply a protective layer on the PCB's. At least it will make them look better and make soldering easier when you would have to repair them.

First a safety issue: A hot soldering iron has about the worst temperature with respect to creating burns. It is so hot that your skin will be severely burned before you feel the heat, while it is not hot and large enough to radiate so much heat that you will notice it before touching it. Therefor:

• Find a means to put it away safely when you don't hold it in your hand. Some types have a hook with which it can hang somewhere. Or just stick it into a heavy pan, piece of steel pipe or anything that is heat resistant and will not topple.
• If it falls: Don't try to catch it when it falls, jump back.
• Keep away children.

Fitting the components is not difficult. Basically, the work goes like this:

• Keep the component lay-out (fig. 5.4) and the parts list (table 5.2) at hand. These provide enough information for fitting the components correctly. The picture of fig. 5.6 could be helpful in case of doubt and it shows how the components can be fitted neatly.
  There is no need to check whether the way components are interconnected by the strips on the PCB, fits with the electric circuit of fig. 5.3 and it could be quite confusing to try to check this.
• The sequence in which parts are fitted doesn't matter much. For practical reasons, it is best to start with the parts that are smallest (resistors, diode) and/or most difficult to fit (the chip or chip connector, since it has so many leads that should all fit in the corresponding holes).
• On fig. 5.4, choose a component to fit. Use the parts list for finding the right resistor, capacitor etc. that should be fitted at that place and check how the polarity should be. Possible errors are:
  • Fitting the wrong resistors. So check with the color code in the parts list or measure resistance.
  • Fitting a capacitor, the diode, a LED or the chip with the wrong polarity. So check with the polarity information in the parts list.
• Stick the leads of the component through the right holes in the PCB and bend the leads so that it will not fall out.
• Cut off excess lengths of the leads with a side cutter. The bend should not be cut away so far that the component will fall out again. On the other hand, the end of the lead should not come close to another island or copper strip.
• Solder the leads of the component onto the island on the PCB.

To work faster, one could stick a number of components onto the PCB, bend and cut all leads in one go, and then solder them. If several indicators are to be made, it is best to work on all of them at the same time, so fit the same component on all indicators and then fit the next component. Have the different resistors sorted into little boxes before starting to fit them. This also reduces the risc of misplacing components.

For resistor R₁, R₂, R₅ and R₆, the holes are drilled so close together that they do not fit when placed horizontally. So bend one lead 180° and fit these resistors vertically, see fig. 5.6. Make sure that the blank, upright leads do not make short-circuit with leads from other components.

Fig. 5.6: An indicator with copper wire switch. This one has an older version of the PCB: A lying trimmer is used and the placing of some components is different. The wound-up wire underneath is the current shunt.

For fitting the 10 LED's neatly on a row, a simple jig is a great help. This could be:

• A strip of cartboard or metal that fits between the leads of LED's. This will serve to keep the LED's in strip and at the correct height during soldering.
• A piece of PCB board with 3 mm holes where the LEDS fit in. This will keep the LED's in line, at the proper spacing from one another and at the correct height with respect to one another.
  To make such a jig precisely and easily, glue a piece of PCB board (e.g. one of the left-overs after the 12 copies have been cut out) onto the back of a PCB and drill in one of the row of holes for the LED's with 0.8 mm. Then remove the PCB and drill the holes to size with 3 mm.

On the PCB, the LED's are placed 3.4 mm apart while the lower portion of some types of LED's have a diameter of up to 3.7 mm (the 3 mm size refers to the upper portion). With these LED's, file off a little from the lower portion until they fit properly besides one another.

For the soldering itself, there are also some point to pay attention to:

• Use a small soldering iron (ca. 15 W) with a pointed tip. File the tip to shape if it has become irregular.
• Use solder for electronics. It is a thin wire with resin inside.
• Try to solder fast. Some components will be destroyed if they become too hot during soldering (see below). If the liquid solder does not form neat connections readily, probably the lead or the island you want to solder, is oxidized so that problem should be solved first. After solder has been hot for a bit longer, the liquid solder becomes a bit sticky and difficult to work with because the resin has evaporated. The results are sharp points on the soldered connections and with bad luck, these could form short circuits. So shake off droplets of liquid solder from the tip of the soldering iron and try again with a bit of fresh solder.
• Proper light is very important. The islands are close together while the leads and solder glitter. This makes it difficult to see if connections are soldered properly and no tiny Parts with semi-conducting material inside, are heat-sensitive: Above 150 or even 125 °C, they are gone while the solder will melt only above 190 °C. Heat-sensitive parts in the indicator are the LED's, the diode and the chip. Of these, the LED's are the most sensitive and the chip the least (soldering on one of the 18 leads will not heat up the chip itself that much). To prevent that LED's or the diode are destroyed during soldering, the soldering of one lead should take seconds only. After soldering one lead, allow the part to cool off before soldering the other lead. Another way of preventing damage is by keeping the leads a bit long (5 mm or longer). This makes that the part itself is further away from the heat from soldering and will not become that hot.

If an NTC resistor will be fitted to improve temperature stability, the PCB must be adapted. As printed, the two islands on the PCB onto which the NTC will be fitted, are connected by a narrow copper strip (the one where the `NTC' arrow points at in fig. 5.4). Only fitting an NTC will have no effect because it would be short-circuited by this narrow strip. So if an NTC is fitted, this narrow strip has to be cut, e.g. with a sharp knife. Check with a tester whether the two islands are really not connected any more.

If an NTC is fitted after calibration, the indicator must be calibrated again.

If no ready made push-button switch is used, switch S can be made from 0.75 mm² copper wire (ca. 1 mm diameter). The shape of the wire that forms the switch is drawn in fig. 5.7. See fig. 5.6 for how it looks in practice.

Fig. 5.7: Cross section through copper wire switch.

The wire that forms the switch, will be soldered to the PCB and there is a risc that when large forces are exerted on the iron wire, the copper layer will break loose from the base material of the PCB (therefor epoxy PCB material is preferable above pertinax, since with epoxy the copper layer is fixed better onto the base material). Once this has happened, it will be very difficult to repair. To minimise these forces, the iron wire should be bent to the proper shape before soldering it on the PCB, so that after soldering, no more bending is needed. Bending forces during normal use are minimal because the button is placed right above the point where the iron wire is fixed on the PCB.

The two contacts of the switch are just pieces of wire (e.g. excess length of leads of other electronic parts) that are soldered onto the proper islands. The one connected to `B+' is bent flat over the PCB. So when the copper wire is pushed down, it will touch this contact. The one connected to `L+' is bent to form a hook and touches the copper wire switch when it is not pushed downwards, see fig. 5.7.

Check whether the switch can not touch a metal part of trimmer R₄ in any possible way.

A copper wire switch probably won't be too reliable because it is likely to oxidize under humid conditions. To reduce chances of problems, apply some solder on the contact surfaces. Grease would also help, but that will attract dust and then the dust could cause bad contacts. So this only makes sense if finished indicators are packed properly until they are fitted in their housing.

If a ready-made switch is used, the wires coming from the connector block could go directly to the switch, with the `L+' wire going to the connection that is normally `on' and the `B+' wire to the connection that is `on' when the switch is activated. Then the middle connection of the switch must be connected to the island where normally the copper wire switch is fitted. So then on the PCB, the islands for the `L+' and `B+' connections, and the islands where the contacts for the copper wire switch are fitted, remain unused.

Once the current shunt is fitted and wound up to a neat roll, the copper side of the PCB is inaccessible for testing or repair. Therefor it makes sense to first check whether indicators function (see par. 5.4.5) before fitting the current shunt. Without a current shunt, the indicator can't work so then the `B-' and `L-' connection must be short-circuited with a piece of wire.

Addition to internet version: PCB with on-board current shunt

Instead of making a current shunt from copper wire, one could also use the PCB design of fig. 5.5b with an on-board current shunt. With this PCB design, there is a copper strip between `B-' and `L-' that has such a length and width that its resistance is ca. 0.02 Ohm. Of course also thickness of this copper strip plays a role, so it can only be used with PCB material with the usual 0.035 mm copper layer. When in doubt: Check resistance afterwards by having a stable DC current going through it (e.g. from a lamp connected to a battery) and measuring voltage drop over the shunt with a digital tester on mV DC range.

As stated in the parts list, the current shunt consists of 860 mm of 0.75 mm² copper wire. Start with a length of ca. 940 mm, strip off ca. 42 mm of the isolation on both ends and stick the ends through the PCB from the copper side. Bend the long current shunt in a neat way behind the PCB so that it doesn't occupy too much space. Then solder it to the PCB. Now the ends stick through at the component side and a length of isolation can be shoved onto them again. These protruting ends can be used to connect the indicator to the connector block.

To limit the power consumption of the indicator, the LED current is kept to a minimum and consequently the LED's light up only dimly. To make the LED's look brighter, wrap some black electrical tape around the row of LED's. This will prevent ambient light from entering LED's from the side. Then LED's that are off, will look darker and the one that is on, is more conspicious.

The plexiglass cover provides no complete housing for the indicator. It only forms a basis onto which the other parts can be fitted and because it is transparant, the LED's behind it, remain visible. Users themselves could make the rest of the housing using woodcarving: A piece of wood with a space carved out that is large enough for the indicator to fit in. The plexiglass cover with indicator itself mounted on the back, can be fitted onto this piece of wood.

Fig. 5.8: A plexiglass cover.

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