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by Stan Swan Cartoon by Barney Walker Thinking inside the box! Many modern electronic components sell very cheaply and some circuits – even using microcontrollers – need just a single supply around 3.3V. However, cases, switches, mounting hardware and battery holders can be disproportionately costly – often more than the high-tech electronics within! Here’s a neat, low-cost solution. G iven the cost of project cases, if you’re on any form of budget it’s often tempting to leave the project “air insulated”, ie, with no case. But damage to exposed connections and flying wires may be a risk. And it certainly doesn’t look too professional, either. Because many projects have only a 74  Silicon Chip few components – a microcontroller and little else in some cases, quite small cases can be used. So here’s a different approach: use a switched battery box to not only hold the battery... but hold the project as well! It’s a cheap solution, with battery boxes selling for just a couple of dollars or so. It’s educationally beneficial too, as it will prompt builders to the sort of compact layouts and neat wiring that are required of today’s designs. Switched battery boxes come in a variety of sizes, most often to suit AA and AAA cells. Two, three and four cell sizes are relatively common but others are available. The common ones can be obtained from most electronics siliconchip.com.au However, this Paxolin-based board is prone to cracking unless guillotined or first deeply scored on both sides. A box-cutter knife carefully guided along a ruler is satisfactory for this. The boards offer convenient supply/ ground rails under the DIP position, which can be broken or bridged as required. Coarse sandpaper can be used to smooth board edges. Jaycar’s spray-on circuit board lacquer (NA1002) protects, makes soldering easier and enhances board appearance. Protoboards By removing the inter-cell ribs a circuit board or even a small protoboard can be fitted in the space made available. Run a        3.7V lithium or (better still) a 3.2V LiFePO4 battery and you have a self-contained project, with battery and on//off switch. The PCB here is cut from a Jaycar HP9558 experimenter’s board. retailers and also on-line. They’ve traditionally been black, but transparent and coloured ones are also available. A summary is shown below. Circuit board The idea is to replace one or more cells in the battery box with a small PCB or other board containing the circuitry. This may or may not require the removal of inter-cell ribs to fit larger boards. Obviously, you need to determine the size of circuit that can fit into the switched battery box. This also depends on the size and number of cells it was originally intended for. Rechargeable batteries are more often than not sold by size, not by letters. Battery box 2 x AAA 2 x AA 2 x AA 3 x AA 3 x AA 3 x AA 4 x AA 4 x AA 4 x AA 4 x AA You need to know that the battery referred to as an “AA” type is also known as a 14500 (being 14mm diameter by 50mm long – the extra “0” on it tells you which is the longer length) and a “AAA” cell is 10mm diameter x 45mm long, or 10450. The table shows the various sizes of boards which can be accommodated in the various switched battery boxes. If you have the knowledge and facilities, a small PCB could be designed and etched to suit the project. Such is the ideal solution but many constructors may not be able to do this. An alternative is to cut a generalpurpose experimenter’s PCB to size – such as Jaycar’s “Ultra Mini Experimenter’s Board” (HP9556) and “IC Experimenters Board” (HP9558) Jaycar Altronics Number Circuitry Comment code code of cells size (mm) PH-9288 S5055 1 x LFP (3.2V) 10 x 45 Suits very small layouts PH-9280 S5042 1 x LFP (3.2V) 14 x 50 Suits centre rail “fingers” PH-9280 S5042 1 x ALK (1.5V) 14 x 50 Niche use voltage boosting n/a S5041 1 x LFP (3.2V) 30 x 50 Remove 1 box cell rib. n/a S5041 2 x LFP (6.4V) 14 x 50 Suits 5V/USB circuits n/a S5041 2 x ALK (~3V) 14 x 50 Suits centre rail “fingers” PH-9282 S5043 1 x LFP (3.2V) 45 x 50 Remove 2 box cell ribs PH-9282 S5043 2 x LFP (6.4V) 30 x 50 Remove box cell rib PH-9282 S5043 2 x ALK (~3V) 30 x 50 Matches 3xAA LFP PH-9282 S5043 3 x LFP (9.6V) 15 x 50 Suits even 9V circuits These are the most common switched battery boxes – the most expensive sells for less than $3.00. LFP means lithium ion phosphate (LiFePO4) while ALK are your common “primary” (ie, non-rechargeable) alkaline cells. siliconchip.com.au As an alternative, some of the larger battery boxes may even fit a portion of a solderless prototyping board. The technique used to trim a breadboard to 46mm x 30mm can be followed from the pictures. This essentially uses side cutters and a box knife to initially remove the unwanted mount wings, followed by a thin backing slice made to reveal a line of spring contacts. Once these contacts are removed a fine hack saw cut can be made through the empty plastic channel. With care two such trimmed boards can be made from a single 23 x 12 hole mini breadboard. Sand the rough breadboard edges to finish. The battery Already, we can hear the question: how is the project going to fit in the box if the space is being taken up by batteries? The answer is simple: use higher voltage lithium secondary cells! These deliver significantly higher voltages than traditional NiCd, NiMH or alkalines, at currents as good or better – in some cases, much better. Where three AA or AAA NiCd or NiMH batteries will give you about 3.6V and a pair of alkalines about 3V, a single lithium-ion cell of the same size will give you about 3.7V. There is no “memory effect” that you get with some rechargeable cells. The higher single Li voltages allow fewer cells for the same supply so you can use the liberated space, (as we mentioned earlier, a “footprint” of 14mm x 50mm per AA cell) to house all manner of simple circuitry on a small PCB or strip board. However, lithium-ion cells are not perfect: they have a falling supply voltage with use and are very picky about being run too low. In SILICON CHIP June 2013, we looked February 2014  75 The 2 AA cell (left) and 4 AA cell (right) from Jaycar are typical of the switched battery boxes available. The four-cell unit can have a PCB or protoboard occupying the space of three cells (45 x 50mm) to give quite a reasonable project space. at the merits of safer, longer life and abuse-tolerant lithium ion phosphate (abbreviated LiFePO4 or LFP) types. These are more tolerant than lithiumion and deliver a very steady 3.2V per cell, almost until their charge is exhausted. This means a single LFP can do the work of two 1.5V alkalines. And it will probably last longer into the bargain. Naturally, rechargeable cells will eventually need recharging. It’s recommended that this be done externally with a dedicated smart charger. These are available now and they’re very well priced – and user friendly. We built one of these into the PortaPAL-D featured over the last couple of months (it sells for <$15.00). The one pictured ($AU6.00 from Hong Kong!) charges a “AA” LFP cell in around 90 minutes and can be used from a computer USB port or a dedicated 5V USB supply. Sure, it’s a minor inconvenience having to remove the battery and place it in the charger but we think it’s a very small price to pay. Just one word of warning: the re- This single AA or AAA charger can handle LiFePO4 or NiMH cells and plugs into a computer USB port or a dedicated 5V supply (either mains or car cigarette lighter). It cost just $6.00 from Hong Kong via ebay. 76  Silicon Chip chargeable batteries found online are often (usually?) significantly overrated. In fact, we saw some on ebay recently from a Chinese supplier who actually said in their description “you may not get 2800mAh from 2800mAh batteries. It is industry thing”! At least he is honest, sort of! Some posters in local forums who have done accurate measurements report that many of the cheaper rechargeables online are actually half, or even less, of their stated capacity. Even some “brand name” cells are not immune from this as many are in fact counterfeits. couple of dollars. An initial decision has to be made on the circuitry size, supply voltage and case ergonomics. Most switched battery boxes have a lid which closes over the batteries. This may or may not be a problem depending on the height of the circuit board/ components but if it is a problem, simply remove the lid. For basic circuitry a 2 x AA “14500” (or even a 2 x AAA “10450”) box may suffice – remember that the circuit board cannot be larger than the battery it replaces. Larger boxes could house even trimmed protoboards. LED monitoring Once the box type is selected it can be internally altered to suit, if needed. However boxes tend to differ in their switch positions, so a careful examination of the cell tabs and switch contacts should first be made. Cell tabs are easily removed with pliers and unwanted ribs snip out with side cutters. Excess cell rib plastic may have to be removed with a “Dremel” or similar, a nibbler or even a file. Fit/refit cell contacts/ springs and run the positive supply wire from the switch and the negative from the contact/spring. Many “5V” circuits will run on 6V or so but there are some which will get upset. Sometimes the specifications say something like “absolute maximum 5.0V +/- 10%”, so anything over 5.5V is certainly not recommended. If you use two 3.2V LFP batteries in series you’re obviously going to get around 6.4V. The easiest way to get around this is to simply connect a pair of suitably rated silicon power diodes (eg, 1N4004 for typical low-power circuits) in series between the battery positive and circuit positive. With around 0.6-0.7V voltage drop across each diode, you’ll While the LFP cells are not as demanding when it comes to low voltage, it’s best to avoid running any lithium cell too low. Just as importantly, when charging multiple-cell batteries you need to use a “balance charger” which individually balances each cell voltage. This will give maximum life/number of charges. To avoid over-discharging individual cells, we’ve found using a white, blue or pink LED acts as a useful battery status gauge. Usually, these LEDs are bright at 3.2V but begin to dim below 3V and go out at 2.5V – the recommended level to recharge LFP cells. Remember: “if it’s dim then recharge!” While the LED could be left permanently across the battery, this would obviously act as a continual drain and you’d find the LED goes out far too early and far too often! Simply use a small tactile switch in series with the LED to act as a supply tester when pushed. Battery boxes Switched battery boxes come in 2, 3 or 4 cell versions and sell for only a Box alteration siliconchip.com.au ANTRIM TRANSFORMERS manufactured in Australia by Harbuch Electronics Pty Ltd harbuch<at>optusnet.com.au Toroidal – Conventional Transformers Power – Audio – Valve – ‘Specials’ Medical – Isolated – Stepup/down Encased Power Supplies ANTRIM TRANSFORMERS manufactured in Australia by Harbuch Electronics Pty Ltd harbuch<at>optusnet.com.au Toroidal – Conventional Transformers Power – Audio – Valve – ‘Specials’ Medical – Isolated – Stepup/down Encased Power Supplies Toroidal General Construction OUTER INSULATION OUTER WINDING WINDING INSULATION You can even (very carefully!) cut down a proto-board for a solder-less project, as long as you don’t use any high-profile components. This is a three-way AA battery box but could equally be a two-way or four-way. Encased Power Supply easy to lay out circuits that were first end up with very close to 5V without www.harbuch.com.au the losses and unpredictability of a developed on a normal protoboard. resistor. Pty Ltd Harbuch Electronics 3 x AA box: 9/40 Leighton Pl, HORNSBY 2077 2 x AA/AAA box: This may be configured for 2 x Ph 5854 cells Fax (02) 9476 3231 alkaline (3V) or 1 x LFP (3.2V). A single 3.2V LFP cell (available (02) in 9476 both AA & AAA form) will directly run all manner of devices, including a PICAXE micro. LEDs could be mounted in the (enlarged) hole previously used by the flying leads of the battery box. Jaycar’s 3.5mm chassis stereo socket (PS-0132) is around 10mm deep and also neatly fits – it could be used for PICAXE programming or even in-situ LFP cell charging. Jaycar’s HP9556 experimenter’s board readily makes four AA sized “fingers”. The resulting 50mm x 14mm boards offer 20 x 6 solder pads but care will be needed in cutting & trimming to avoid undue weakening of the outer solder pads. Very tight designs may gain a few millimetres by removing the box mid wall and securing the cell instead with Velcro tape. Although of only modest capacity, a narrower 10mm x 45mm AAA cell could even be used on lowdrain circuits. Jaycar’s HP9558 experimenter’s board offers more layout flexibility. It features both normal side supply and spare centre rails, along with “3 a side” contacts. A 20 hole x 10 hole HP9558 “finger” board showed itself as versatile and siliconchip.com.au Although versatile, the alkaline approach is hindered again by the small board housed in the single cell space. A single LFP cell however alllows a trimmed breadboard (11 x 12 holes). 4 x AA box: This largest switched battery box offers very useful versatility and may even suit as a mini test bench when used with jumper leads. A two-alkaline supply will allow the same internal rail circuit board approach as used on the single LFP powered 3 x AA box. LFP cells are more expensive than AA so their use may be unjustified for some simple projects. However, when gutted to just a single LFP, it’ll even house a trimmed (20 x 12 holes) breadboard, complete with full supply rails and even top and side hole ID lettering. Head room on a breadboard is a rather tight 5mm, but this doubles to a tolerable 10mm when housing a soldered board. Enhanced board real estate now allows all manner of circuits to be designed and built and finally neatly housed in a battery box. Sufficient internal cavities remain to allow diverse indicator LEDs, I/O connections and INNER WINDING CORE CORE INSULATION Comprehensive data available: www.harbuch.com.au Harbuch Electronics Pty Ltd 9/40 Leighton Pl, HORNSBY 2077 Ph (02) 9476 5854 Fax (02) 9476 3231 sensors to be mounted as well. Finish: Neat external labels can affixed and covered with clear tape for protection. It’s particularly important to also indicate the cell type the circuit is intended for, else sub-standard performance or over driving may result. Dummy cells can of course be used – a single LFP in series with a dummy cell gives 3.2V, which almost matches the voltage of two alkaline cells. The prospect of connecting several LFPs in series may tempt, but cell matching (relating to differing cell charge, discharge and aging characteristics) may become an issue for long term projects. Simple setups can readily however be organized, and a couple in series (2 x 3.2V = 6.4V) will be good for 5V USB style delivery (after regulation), three for 9V (3x 3.2V = 9.6V) with four in series (4 x 3.2V = 12.8V) even capable of a healthy and lightweight 12V supply. Just don’t try and jump start your SC car with them! Resources: Conveniently located at www.picaxe.orcon.net.nz/bbox.htm February 2014  77