Fullscreen HTML5Med Res (??MB)HTML4

Aaagh! Does your Digital Multimeter lack an auto power-down facility? Many don’t – and if you forget to turn it off, next time you go to use it you might be tempted to say naughty words. This little circuit, which will cost just a couple of dollars, will stop a DMM chewing through batteries when you forget to turn it off. While this auto simple power-off circuit is intended for Jaycar’s tiny QM1502 DMM, this could be applied to many other battery-operated devices. By Stan Swan S hoppers in Jaycar stores may have spotted their cute new orange QM-1502 mini DMM (digital multimeter) and wondered if such throwaway-priced “toys” can be serious! At its bargain price – five bucks – it’s hard to know if one should laugh or cry at such trivial test gear prices. I recall (as an early teen) paying ~$500 in today’s money for a far inferior analog meter. For schools and hobbyist needs they look extremely appealing – you can never have too many multimeters of course! But, even at only $5, are they worth a punt? Although superior meters abound in the $40 range (and should certainly be considered for those serious about electronics), this little DMM merits a closer look for newcomers, schools and hobbyists. With dimensions of just 94(H) x 46(W) x 26(D)mm it’s almost laughably tiny (the footprint being smaller than a credit card) but range settings are very clear and the display is just as bright and clear. Build is surprisingly good, with a sturdy switch action and current drain an astoundingly low 250µA from the small cylindrical A23 12V battery. A full set of normal electrical readings, even including transistor gain, are featured. The (unfused) 10A DC current range remains usefully connected even with the meter turned off – handy perhaps 78  Silicon Chip for occasional monitoring of a photovoltaic solar array or battery charger. The 12V supply voltage is not critical and the DMM runs well from a fresh 9V battery, with the “low batt” symbol only showing at around 7V. Demand current, which “surged” to 400µA on resistance with shorted leads, otherwise remains near 250µA. Aside from newcomers, schools and toolbox spare use, the DMM is also suitable as an inbuilt panel meter. That’s right – just switch it to the setting you want, internally An A23 battery might be rated at 12V but the opened-up view at right shows why they don’t have much capacity – they’re merely eight button cells packaged together. siliconchip.com.au I forgot to turn the DMM off . . . again! connect leads and supply and build it into the project! Note however that it’s not possible to power the DMM from the same circuit that you are monitoring. This arises because many basic DMMs use a classic ICL7106 COB (chip-on-board) for measuring and display. The “COM” socket on such meters is at 3V lower potential than the +ve terminal of the battery inside, as such an arrangement also allows negative voltages to be measured. What to do? So what’s the downside of such a cheap meter? External 9V battery supply: Alkaline types of ~500mAh capacity (of which perhaps half will be available before the supply falls below 7V), are cheaper than A23 (but not much!) and are very widely available. They won’t fit inside the QM-1502 case but could be readily mounted externally, perhaps along with a supply switch. For many users this approach may be appealing. Several weak points emerge. For example, the meter sensitivity (or insertion resistance, if you like) is only around 1MΩ (most DMMs now are far better) and the supplied probes are low quality – do not trust them for mains work! We also found some difficulty in inserting the leads into the multimeter sockets – they do go in, but. . . The leads can readily be upgraded (you’ll probably pay as much for better leads than the whole DMM, perhaps more!), Or you might like to replace the test probe ends with some more sturdy crocodile clips. Even with decent test leads we sill have a major reservation about safety. While the DMM has ranges for 500V AC and DC, we’d be very hesitant about using it for this type of measurement. In fact, we’d go so far as to say it should only be used on low voltages – 50V AC or DC – and we’ve even prepared a warning label to stick on the meter. Several supply workarounds were considered, with an eye to cost effectiveness – it’s unjustified spending much more than the ~$5 meter cost on enhancements! Space inside the meter case is tight but there’s enough for some additional compact circuitry, although simpler approaches may appeal (if only for constructional ease). In approximate order of complexity, these include: Case mounted switch: A dedicated supply switch can also prolong the life of the DMM’s rotary switch, as a pre-selected range can remain ready for immediate use at power up.    (Many a DMM fuse is blown by “knob twiddlers” when No auto off However, the meter’s most annoying attribute (or nonattribute!) is the lack of auto-power-off. Despite the multimeter’s low current drain, the low capacity of 12V A23 batteries (which are merely eight button cells packaged together) means that you’re likely be plagued by flat batteries, especially if you forget to turn it off. And A23 batteries can be elusive and costly (often around $5) – which may exceed the cost of the meter! As alkaline A23 batteries typically have a 55mAh capacity, only a few dozen hours or so of operation will result before the supply drops too low. Although tolerable for a conscientious user, this equates to approximately a weekend, so failure to turn off the meter on Friday will likely see it flat by Monday. Educators who’ve found their class meter batteries dead mere minutes before 30 surly youths arrive for a Monday school lab session will keenly appreciate this particular “electro-angst”… Even with meagre supply needs, it makes no sense to leave devices wastefully on when battery replacement costs are high. siliconchip.com.au This is what the DMM looks like when it’s opened up. Shown here very close to life size, you can see that there’s room on the inside back of the case to add a simple auto power-off circuit. May 2013  79 + + meter current ranges are selected with the meter paralleled to the supply!) In conjunction with a low drain LED (often still visible at a mere few hundred microamps), a still-powered meter would then readily be noted in a dark storage cupboard. Solar Power: Although using a few tiny photo-voltaic cells from cheap calculators or solar garden lights (most provide ~3mA at several volts) is tempting, an array to supply >7V would be difficult to neatly mount on the front of this small DMM. Meters are often used in poorly lit places indoors as well. TO DMM's POWER SUPPLY TO DMM's PADSSUPPLY POWER PADS – S1 S1 DMM's NORMAL DMM's 12V (A23) NORMAL BATTERY 12V (A23) BATTERY – 1k 1k G G 3.9M 3.9M SC SC 2013 2013 D D S Q1 2N7000 Q1 2N7000 2N7000 2N7000 S 100F 100F D S G D S G DMM DMM AUTO AUTO POWER POWER OFF OFF Orientation switch: Mercury position switches (Jaycar SM1044) are relatively costly and may annoy users when the meter is in unexpected working positions. Fig.1: the circuit is simply a switch (Q1) which is turned on by pressing S1 and it turns the DMM off once the 100µF capacitor discharges, to conserve the battery. Auto power off – microcontroller or IC: The popular PICAXE-08M microcontroller can shut down totally after some minutes but a sleeping PICAXE will draw tens of microamps, which over time will still drain batteries.    Such an approach is rather an overkill anyway, as of course a micro can do far more! Additionally, the cost of the IC and extra components will likely exceed that of the DMM. needed, which are cheaper and fit better in the DMM’s case than larger types. A breadboard trial with the 2N7000 verified that a momentary push on the switch, with a 100µF electrolytic, paralleled with a 3.9MΩ across the gate, held the meter on for about 10 minutes before the low battery symbol appeared. This period should be enough for most users but could be readily altered with different value discharge resistors, An A23 battery, with its 350µA drain could stretch to perhaps hundreds of such test sessions. Simple components are used, and the total bill of materials should only be a few dollars. The benefit of such an enhancement may be educational as well as financial, especially for those fresh to electronics. A “hands on” understanding of RC discharge and FET action should result and organising the few components to fit the DMMs interior may help new comers develop skills with compact circuitry. Auto-power-off – FET capacitor discharge: John Crichton’s recent “Circuit Notebook” time-out switch (SILICON CHIP January 2013) showed most promise and has been the approach adopted. Small signal FETs High gain Darlington bipolar-based auto-power off circuits exist but the popular (and cheap) 2N7000 (N-Channel enhancement mode FET with an insulated gate is superior as, being a FET it has negligible gate current. A major practical benefit of such gate supply switching is that only low value (10-100µF range) electrolytics are 80  Silicon Chip Construction For such a simple circuit, a PCB is hardly warranted (and would likely cost as much as the DMM). Therefore a small (5x5 hole) Veroboard offcut was used and the components connected as shown in the wiring diagram. The electrolytic capacitor will not fit into the case if soldered onto the Veroboard in the normal way – it is laid over off the Veroboard and parallel to it, as shown. The supply wiring (ie, from battery to the DMM PCB) was then connected as shown in the diagram. You only need to unsolder two of the wires (ie, those going from battery to the PCB) and connect wires from your Auto Power-Off 3.9M – S + Auto-power-off capacitor discharge: As users are now familiar with such modern devices as cameras, cell phones and PCs going “touch to revive”, a simple switched discharging electrolytic was considered. Quick tests with a 4700µF electrolytic confirmed several minutes hold up until the ~7V “low battery” display came up.    This is readily verified by Q = I x t = V x C, when a 250µA drain at 12V should fall in one time constant (T) to 1/e (37%) of the original voltage (12V x .37 = ~ 4V. Hence the time constant = 12 x 4700 x 10-6/(250 x 10-6) = ~200 seconds.    This approach could suit push switch operation for quick checks (perhaps of circuit charge/discharge currents or supply voltages) but power will only be held on for few minutes. This time will be too short for most users, and can only be extended with larger value capacitors (10,000µF+), or even super-caps,which will be bulky and perhaps costly. V+ TO DMM SUPPLY PADS V– G + D 12V ALKALINE BATTERY “Joule thief” step up: Solar garden lamps use step-up circuitry to drive a 3-4V white LED from a single AA(A) sized battery. Although higher voltages are possible, they’re at very reduced currents and with rough output, requiring smoothing and regulation. – Q1 2N7000 S1 100F 16V 1k STRIP BOARD – LOOKING THROUGH BOARD SIDE OF REAR OF DMM CASE Fig.2: a suggested Veroboard layout and wiring diagram. The Veroboard and 100µF electrolytic capacitor can be secured to the DMM case with double-sided adhesive. siliconchip.com.au The finished modification. There’s not much to it but note that you’ll almost certainly have to lay the electrolytic capacitor over to get it to fit in the case. Otherwise, it’s just a matter of breaking the positive and negative leads from the battery and inserting the auto turnoff PCB. switch in their place. Finally, carefully drill a hole in the side of the case (back section) for the pushbutton switch (its diameter will depend on the exact switch you use). There is a ridge all around the case and unfortunately the hole needs to go through the case where the ridge steps down. But once tightened properly, this should not be a problem. Depending on the size of your switch, you may need to mount the Veroboard underneath the battery compartment – if you do, make sure it’s as close as possible to the battery to avoid interference with the current shunt in the DMM (the thick copper wire near the terminals at the bottom of the case). Similarly, make sure it doesn’t interfere with the on-board fuse. We used a switch probably larger than necessary, originally to maintain Parts List – DMM Auto Power Off (all available from Jaycar/Altronics/etc): 1 NO momentary pushbutton switch 1 2N7000 FET or equivalent 1 100µF 16V electrolytic capacitor 1 scrap of Veroboard (5 x 5 holes) Short lengths of red and black hookup wire Resistors (1/8 or 1/4W, 5%) 1 3.9MΩ (orange white green gold) 1 1kΩ (brown black red gold) siliconchip.com.au the isolation between contacts and outside. However, with our comments about using the DMM on 50V or less, a much smaller switch will suffice. If you do use the switch we used (a Jaycar SP-0702) you might like to remove the large red knob and cut back the actuator a little. One further enhancement also makes the meter more “user friendly”: marking the setting arrow more boldly with a spirit-based pen ensures correct settings. While the arrow and markings are obvious in bright light, they are much less so in dim light! Speaking of light, we’ve found that some of these meters are sensitive to bright sunlight (affecting readings). This can be simply cured with a piece of black electrical tape over the back of the chip (the black blob!). Finally: don’t twiddle the pot! Note – the potentiometer, or variable resistor, in the DMM is used for calibration, so avoid altering its factory setting as re-calibration may then be needed and you won’t have the equipSC ment necessary to do this. Silicon Chip Binders REA VALU L E AT $14.95 PLUS P& P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Australia. Order online: www.siliconchip.com.au/shop or call (02) 9939 3295 and quote your credit card Or use this handy form – Fax to (02) 9939 2648 or post to Silicon Chip Publications PO Box 139 Collaroy, NSW 2097 Enclosed is my cheque/money order for $________ or please debit my  Visa Card      Mastercard Card No: ________________________________      Card Expiry Date ____/____ Signature ________________________ Noting our comments about safety, this warning label can be printed and glued to the top front of the case, as shown earlier. Name ____________________________ Address__________________________ __________________ P/code_______ May 2013  81