Mini DMM Enhancement

Shoppers in the Australian/NZ  Jaycar electronics stores may have recently spotted their cute new orange QM-1502 mini DMM (digital multi-meter) and wondered if such throw away priced “toys” can be serious! At it’s bargain ~Aust $5 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 analogue meter. For schools and hobbyist needs they look extremely appealing – you can never have too many multi-meters of course! But, at even just $5, are they worth a punt?

Although superior meters abound in the $40 range (and should certainly be considered for those serious about electricity), this little DMM merits a closer look for new comers, schools and hobbyists.

With dimensions of 94(H) x 46(W) x 26(D)mm it’s almost laughably tiny (the footprint being similar to a bank card), but range settings are very lucid and the LCD display is bright and clear. Build is surprisingly good, with a sturdy switch action, and current drain an astoundingly low ¼ mA from the small cylindrical A23 12V (8 series button cells) battery. A full set of normal electrical readings, including transistor testing, are featured. The (un-fused) 10A DC current range remains usefully connected even with the meter off- handy perhaps for occasional monitoring of a PV solar array or battery charger.

Aside from skinflint users,hobbyists,schools & inbuilt applications these mini DMMs are small enough to pop into a small tool kit as well. Note the croc. clip connectors here - these nicely suit as more rugged and versatile mini meter leads.   Not having a meter handy to check the likes of simple continuity or battery voltages can be agonising. Every e-toolkit should include such a mini DMM, even more so if tweaks are attended to! 

Here's a summary  of  enhancements this Instructable considers worthwhile -
1. Superior meter leads.
2. Stray light masking.
3. External switching.
4. 9V powering.
5. Boxing and labelling.
6. "FETlington" auto power off.
7. Battery state test points.
8. Setting pointer indicator

More ambitious enhancements require internal circuitry hacks, & are still being considered. Refer -
TEMP.    => www.instructables.com/id/A-simple-tip-to-add-a-thermometer-to-your-digital-/  
SERIAL => www.instructables.com/id/DMM-Piggyback-add-RS-232/

The 12 V supply voltage is not critical and the DMM runs well from a fresh 9V battery, with the "low batt." symbol only showing at ~7V.  Demand current, which "surged" to ~2 mA on low resistance and diode test ranges, otherwise remains near 250 µA (¼mA).

Aside from skinflint, schools and toolbox spare appeal, the DMM also suits use as an inbuilt panel meter. That’s right – just switch it to the setting you want, internally connect leads and supply and build it into the project! Note that it’s not however possible to power a meter from the same energizing circuit that you are monitoring.

Several weak points emerged however. The meter insertion resistance is only ~1 Meg. Ohm (most DDMs now are far better), and the supplied probes are low quality- do not trust them for mains work!  The leads can readily be upgraded however, perhaps simply with sturdier crocodile clip types.
 

NOTE #1: The meter was found to be slightly light sensitive, with readings being most influenced when in very bright sunshine. Although perhaps minor (for the low price!) this is likely to be of some annoyance in outdoor or solar work - or for indoor users near windows.   Investigation showed the light coming thru'  the meter's transluscent case and striking the COB (Chip On Board)  IC was the culprit, & a strip of black electricians tape over the IC masked off stray sunlight nicely. This tape has a further benefit of securing the supply leads slightly when switch options are later worked on.

NOTE #2: The DMM’s variable resistor is used  for calibration, so avoid altering it’s factory setting! Re-calibration ( which can be done with a 2nd known good meter or electrical standards of course) may be needed if this is idly turned...

NOTE #3: The 500mA circuit fuse is spot soldered to the board, and is thus a potential annoyance for an end user to change. These fuses usually blow when small current settings are innocently selected with the DMM in parallel with the power supply! Naturally an external fuse could be added, or even a polyfuse, but as the unfused 10A range is capable of 0.01A ( = 10mA ) resolution, this higher current setting may well be better suited for typical educational investigations. Avoid hence the very small current ranges unless the user is aware of just-what-they-are-doing !

The meters most annoying attribute however is the lack of auto power. Combined with the low capacity 12 V A23 battery, this means it’s likely be plagued by flat batteries.  A23 (often also seen as 23A)  are typically used for car remotes, and can be annoyingly elusive in general battery outlets.  Being specialised hikes their price (~$5) – which may exceed the cost of the meter!

As alkaline A23 typically have a 55mAh capacity, then (even at the meter’s tiny ¼ mA drain) only some scores of operation hours 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 meters on Friday will likely see them 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 the DMM's meager supply needs it makes no sense to leave it on wastefully on when battery replacement costs are so high and replacement frustration a possibility!

Diverse supply workarounds were considered, with an eye to cost effectiveness –it’s unjustified spending far more than the ~$5 meter cost on enhancements of course! Space inside the meter case is tight, but suits 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 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 100s of µA), a still powered meter would then readily be noted in a dark storage cupboard. Most switches however are rather too large and/or costly...

External 9V battery supply: Alkaline types of ~500mAh capacity (of which perhaps half will be available before the supply falls below 7V), are both cheaper than A23 and widely available. They won’t fit inside the QM-1502 case (even with hacking!), but could be readily mounted externally, perhaps along with a series supply switch. For many users this approach may be quite appealing!.

Here's the DMM inside a clip top snack box, that just nicely hold the meter,9V battery and an external power switch (which may be either switch on/switch off or momentary. 

The clear box lid can have a paper insert fitted within so that ONLY the LCD is visible, which may greatly ease confusion for the inexperienced. For schools use the meters could use colour coded inserts ( with switched settings organised ahead of time by a lab. tech etc) for further end user friendliness.

An ammeter can give valuable system insights- this one has been organised for 20W PV charging of a bank of 12V SLA batteries. End users (who were NOT technical) needed a simple way to occasionally check that it remained working OK. The label can be printed, trimmed to size (with LCD cutout) and simply slipped under the top cover to both inform and mask unwanted meter settings.

"Backyard" photovoltaic solar setups increasingly abound, but they usually run "blind" with charging performance often unknown.  Simple LED indicators,although better than nothing (see perhaps https://www.instructables.com/id/Single-LED-ammeter-FLED-based/ ), may not be revealing enough for keen investigations. Full current flow monitoring (both INTO & OUT OF a solar setup) can be near essential in many setups, and a cheap DMM set to 10A suits well.  A LED ( c/w suitable dropping resistor) could be wired in parallel with the on/off switch for night viewing too. Backlit DMMs are not a cheap off the shelf item!

Under  charge  it can reveal slight changes in panel performance due to air quality, cloud cover, aging, placement, inclination or seasons and also drop offs arising from such tedious issues as leaf/dirt/bird droppings or corroded & suspect connections.  Naturally panel damage (or even theft - it happens!) will be quickly spotted too. The relative merits of assorted batteries and panels that may become available can be  investigated as well.

Discharge  performance checks of new devices (or alerts to excessive or un-authorized loads) can be quickly noted, while longer term battery degradation can also be monitored.

Here the relative performance of different 20W PVs ( polyX/mono X) is being evaluated  while charging 12V deep cycle batteries. Both PVs delivered ~1.2 Amps under bright sunshine, with little difference during the (very sunny) day .  The encased DIY ammeters are cheap enough to have on each panel, thus saving tedious swap overs. Of course the likes of a US$49 "Watt's Up" energy meter ( www.rcuniverse.com/magazine/article_display.cfm?article_id=468 ) would offer superior insights. but ...

Even casual shading (here shown with a leafy branch) caused the bright sun current output to drastically fall to near half -  the very sort of situation a non technical user needs altering to !

Other approaches considered and explored   (excuse the very messy workbench!)    -

Solar Power: Although using a few tiny PVs 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.

 Orientation switch: Mercury position switches (Jaycar SM1044) are relatively costly and may annoy users when the meter is in unexpected working positions. 

 Auto power off - microcontroller or IC: A popular PICAXE-08M microcontroller can esily shut down totally  after some minutes, but a SLEEPing PICAXE will still draw 10s of µA, which over time will still drain batteries. Such an approach is rather an over kill anyway, as of course a micro can do far more! Additionally the IC and extra components cost will likely exceed that of the DMM.

 “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.

 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 ~7V "low battery". This is readily verified by  Q = I x t = V x C, when a ¼ mA drain at 12V should fall in one time constant (tau) to 1/e (37%) of the original voltage (12V x .37 = ~ 4V. Here hence one time constant = 12x4700x10^-6/(250x10^-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. Such a hold up time will be too short for most users, and can only be extended with larger value capacitors (10,000s µF),or even super-caps, either of which will be bulky and perhaps costly. 

 Auto-power-off- FETlington capacitor discharge:  John Crichton’s recent “Circuit Notebook” 2N7000 time-out switch (Jan. 2013 Silicon Chip) showed most promise, and -after exploration- it's been the approach adopted!  

High gain Darlington bipolar based auto-power off circuits exist, but the ever popular (and cheap) 2N7000 (N-Channel enhancement mode FET compound pair “FETlington”) is superior, as -being a FET- it has negligible insulated gate current. A major practical benefit of such gate supply switching is that only low value (10s-100s µF range) electrolytics are then needed, which are cheaper and better fit in the DMMs case than larger types.

 

 A breadboard trial with the 2N7000 verified that a momentary push on the tactile switch, with a 100 µF electrolytic, paralleled with a 3.9 MOhm across the gate, held the meter on for about 10 minutes before the low battery symbol appeared. Shortly after this the meter then powered off, at which stage the battery drain dropped from the usual ¼ mA to well under 1µA. This ~10 minute hold on period should be enough for most users, but could be readily altered with different value discharge resistors.

An A23 battery, with it’s ¼ mA drain could stretch to perhaps 100s of such test sessions. If an external 9V battery was instead used then shelf life of some years could be approached. Naturally diligent users could still make quick checks and then switch the rotary switch to the normal off position!

Veroboard is considered with some dismay these days, but vero offcuts are often VERY convenient for assembly of small simple circuits.. A small fine tooth saw can slice off a suitable portion (here 5 holes x 5 holes -there's space aplenty so it could of course be longer!). Smooth the off cut with sandpaper & perhaps buff up the Vero copper tracks to make soldering easier.

A wafer of similar 1/10th inch spacing Kiwi Patch Board was instead in the prototype,as it's upper silk screened layer allowed the off cut's copper under side tracks to be much more readily visualised.  

Footnote:KPBs (Kiwi Patch Boards) are a NZ invention, intended to allow "paint by number" lift over  parts transfer from their initial taming on a  solderless breadboard.  Using a KPB makes the transfer MUCH easier that reconverting to suit vero board, and the soldered Kiwi Board version,being greeny fibreglass rather brown phenolic, is stronger & also looks quite professional.  Refer =>http://www.kei.co.nz/A000273_KiwiPatch%20Data%20Sheet.htm  and http://www.surplustronics.co.nz/shop/product-KIWIPATCH.html

Prototype construction shown here was conveniently made  on a small 5 hole x 5 hole Kiwi Board offcut, with the supply wiring adjusted as shown in the diagram.  Prevent possible shorts to existing meter components with a small cardboard cover  placed over the additional circuitry.

Before soldering to the tactile switch contacts, check continuity with (what else!) a DMM to correctly Identify the pair (of the 4) that switch ON when the  button is pushed. Croc. clip leads fit the DMM sockets very well and offer handy holding versatility for measuring such small components.

4 small holes can be drilled to mount the small momentary tactile switch. The top RH corner position for these seemed the most logical.   After soldering to the correct 2 contacts, all 4 pins can be bent over inside the case to neatly secure the switch..

 

While a drill is at hand two further enhancements can make the meter more user friendly.   Drilling two small holes on the case rear under the battery terminals conveniently allows the supply voltage to be measured without meter disassembly. Switch the DMM to DC voltage and touch just the red positive probe through the holes to make contact with each exposed battery terminal. Adding the voltages shown (typically upper -8.8V and lower +3V) conveniently gives the A23 supply voltage – here 8.8 +3.0=11.8V.  

This convenient  but stange voltage situation arises from the meter's probable use of an Intersil ICL7106 - it's under the black "blob".   ICL7106 based multimeters have the COM socket at 3V lower potential than the +ve terminal of the battery inside. This allows negative voltages to be measured  too. Refer  https://www.instructables.com/id/A-simple-tip-to-add-a-thermometer-to-your-digital-/  for practical insights.

The final tweak involves neatly marking the setting arrow more boldly with a spirit based pen to ensure ensure the correct range has indeed been selected!

      Here's the converted meter- a small neat "Auto power off" label can perhaps also be attached.   

Summary:  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 keen “hands on” understanding of RC discharge and FET action should arise, and organizing the few components to fit the DMMs interior may help new comers develop skills with compact circuitry. 

Parts (all available Jaycar - or "junk box" !):    Resistor and capacitor values not that critical

* 2N7000  N-channel enhancement mode  "FETlington" MOSFET.
* 3.9M Ohm and 1k Ohm resistors.
* Tactile switch (a short actuator type seemed "best").
* 100 µF 16 V electrolytic.
* Vero board/Kiwi board offcuts.
* Short lengths of hookup wire