Single LED "ammeter"- FLED Based.

I've had my arm twisted to rustle up a design for a very compact, cost sensitive (<  ~ US$1) educational low current (initially 100 µA-1A) ammeter. Here's one approach of several trialled over Easter 2010- further details may be later added.

The brief was to have a single LED that can be "read" by the stressed teacher at a glance across a crowded class room- with flash rate & intensity related to current demand. Moving coil movements are now history of course, & dedicated DMMs are too large, or are always blowing fuses when students connect incorrectly. Although a popular micro (such as the esteemed PICAXE 08M) would lap up such an application, none were used at this stage in the interests of simplicity & economy.

Step 1: Yesteryear!

It's a funny old thing in fact that simple DIY ammeters are now elusive, especially since (IMHO) every power supply should have some sort of ammeter!

Back in the "good old days" they festooned most items, even on many cars and motorbikes,& it used to be standard to roll your own with surplus moving coil meters. It was a basic electrotech. skill to add extra R's or shunts to suitably persuade basic meters to read otherwise. Some types were very small single hole mount circular designs, & could be rescued from cassette recorders/radios etc where they were used for signal or battery strength.

I'd pondered modern approaches, perhaps using Hall Effect devices for sensing the current flow, but their overall cost looked an order of magnitude greater than the FLED engine utilised here.

Step 2:

The idea behind this FLED based meter relates to classic PNP transistor action - here shown acting as a day/night switch for a solar charged LED nightlight setup. It's rated at ~1Watt, & with modern white LEDs ( perhaps just a single bright  "LumiLED")  this could be quite enough for illuminating a backyard. steps or pathway etc.

Day time charging serves to bias the PNP transistor base (B) to prevent main current  flow, but after dark the weak but firm pull down overrides this & turns the PNP on. Naturally then the entire load becomes powered - it's important of course that the semiconductors can handle this !

Step 3:

Such PNP load control circuitry can be arranged to instead show current flow. The basic design arose via Dave Johnson of www.discovercircuits.com in fact (refer below or  www.manuka.orcon.net.nz/lampmonitor.gif ). This simpler technique suits an indication that  circuitry  IS still working, & perhaps (if battery charging) actually performing as expected. Naturally a slight voltage drop ( ~0.7V for Si or ~0.3V for Schottky) occurs across the diode, and supply critical circuitry may have to be feed a slightly higher raw supply in compensation.

The idea could lend itself to all manner of applications- perhaps also mains based plug-paks etc. It's beauty relates to being compact,rugged & line powered, and being visible at a distance by even a child, invalid or non technical person.

Regular LED brightness changes may however be hard to judge (especially outdoors), so the merits of a flashing LED (FLED) were explored.

Step 4: FLED Details

FLEDs are LEDs that come  with an inbuilt "black dot" flasher IC already. This normally flashes the LED at ~ 2 Hz & also allows operation over a wide voltage range (~3V-12V).  Technical details of this IC are annoyingly elusive...  FLEDs have been widely used in "Beam" solar engines, since they are essentially out of circuit until they flash, at which moment they can be used to trigger such other  devices as small motors- or even perhaps another series LED.

Extensive breadboarding (including attempts to persaude them with capacitors & external lights etc!) revealed  a promising action with another LED in parallel with the FLED.  When the pair were controlled by a PNP transistor, the FLED ( even before it began to flash) noticeably influenced  the normal LED's illumination. A much wider response flashing/intensity range of this normal LED eventuated - seemingly due to the sudden reduction in parallel load when the FLED attempted to flash. 

Step 5: FLED Application

Experimentation with simple discrete components eventually yielded the flashing LED (FLED) based circuit shown. Although the basic PNP driven LED indicator idea is well known, the paralleled FLED extension used here is original. Copyleft- but credit for my tinkering is appreciated !

NOTE: The FLED's winking here is masked, & instead of being directly viewed it's used  instead to very usefully influence a normal paralleled  LED's flash/pulse behaviour. This visible LED's output ranges from a weak wink when around 1 mA drain, merging to a distinct pulse at 5mA, & then becoming urgently faster & increasingly brighter at 50mA & up. With experience the approximate current can readily be estimated at a glance- a teacher's dream!

Flat batteries,or no go circuits, of course will not give any LED illumination at all. 

Step 6: 3x7 Veroboard Layout

Layout is not critical, but must be compact if eventually fitted inside a 3 x AA battery box. A 3 x 7 Veroboard strip will mount all components nicely. The FLED itself, although an "engine" to the design, was masked  as it's flash intensity & rate had nothing like as wide a dynamic range as a paralled normal LED. Since FLEDs usually only come in 5mm version, to save space it can even be sanded down for a lower profile mount under the visble external LED.   

Step 7: Possible Battery Box Fitting?

3 x AA cell switched battery boxes are  extremely suitable for powering todays electronic devices,which are increasingly very tolerant of the supply voltage. Almost any value between 3 & 5½V will do fine for many circuits, although of course SOME are still sticklers for 5V. The down side of this FLED engined circuit relates to the supply voltage drop (of ~ 0.8 V ) across the series Si diode. Most hobbyist and schools circuitry will not object, but critical circuits may "protest".

Note -FLEDs are tolerant of a wide supply voltage, BUT if a red LED is used it'll of course need a suitable dropping resistor, as when run from 3 x AA it'll be over driven. White LEDs (which can be run directly from 3-4 Volts) were eventually used in the final circuit anyway, as these were found to give more striking flash/intensity effects than a red, & did not need an extra dropper R.

Step 8: Battery Box Housing

If neatly assembled on 3x7 Veroboard strip,the simple line powered LED ammeter circuit can be compacted  for housing in the spare internal space at the top of the battery box. Things will be tight, BUT if assembled carefully they'll fit nicely! Note the sanded down low profile FLED - the  red LED behind it was part of the initial trial & was eventually changed for a white type.

Step 9: Completed !

Here's the completed circuitry neatly positioned. Note the small cardboard insulator, used to prevent any possible shorts between the grounded Vero track & nearby battery terminal!

Step 10: Calibration & Testing

Calibration &  testing can readily be done with a resistance wheel, but avoid burning out the low value R's on higher currents. At 330 Ohms  load a current of approx. I=V/R = 3.8/330 or ~11 mA will flow, falling to only ~ ½mA ( & just visible on the LED) at 6.8k. LED flashing & intensity behaviour can readily hence be related to currents below 1mA ( or briefly !) as high as ~ 500 mA.  A  label glued to the battery box makes a handy reference - the initial one used here was better suited to the red LED & was changed for the final white LED version.

Check => http://www.youtube.com/watch?v=-PtbP8STtwU    <=  for a short calibration  video.

Step 11: In Use!

Here's a typical classroom/hobbyist's load - a small low voltage filament Christmas lamp (drawing ~100mA). All manner of  common circuitry can be powered & monitored, with the drain "sleeping" circuits (such as microcontroller driven data loggers or wireless  transmitters) very distinctive when they briefly awaken for readings/signal transmission. Small motors are especially interesting, as the current increase when they stall is readily noted.   

Step 12: EXTENSION: Solar PV - Battery Charging FLED Ammeter?

Extension:  A common  " is my $%*#@ panel working" need these days relates to backyard solar PV battery charging,perhaps for a simple shed lamp.

Many simple solar chargers (especially those in lesser developed regions) have no charging current ammeter, and you may only find out about broken leads/dirty terminals/low electrolyte levels/shot batteries/stolen panels(!) when the power fails- after dark !  It hence can be a crucial need to quickly reassure that batteries ARE IN FACT BEING CHARGED and that an expected level of charging current IS flowing in sunshine.

Although DMMs (Digital Multi Meters) are now cheap,and could probably be built into a solar system, they need instructions in their use & require their own battery. Many go auto-power-off after some minutes too. However a LED/FLED  indicator  could be cheap & easy to understand.

Step 13: Simpler Charge Indicator

Just a simple LED approach may still have merit of course in such a solar battery charging setup. As a blocking diode should be used in the supply lead anyway (to prevent battery drain back thru' the PV at night),it's inclusion can be usefully exploited.

Current flow indication can be VERY handy for problem solving, perhaps to verify & monitor roof mounted panel performance. Dirt, salt spray, leaves, bird droppings, damage or even outright theft can of course be hard to spot from below.   PVs are fortunately getting much cheaper & panel theft is now not so common, although copper wire "salvage" can certainly tempt cash strapped rascals ...

Note: The 3 x AA ( ~4½ V)  circuit used initially will of course need  tweaking for higher supply voltages, as although FLEDs usually work OK between 3 & 12 V, normal LEDs will need at least a decent dropping resistor (~1000 Ohms ?) on the likes of 12V. The 33 Ohms shown in the simulated version above is probably unrealistically low & should be higher in a working circuit !

EXPERIMENT & ENJOY - & feel free to extend the idea, perhaps to a charge/discharge  bi-colour LED(s) application?!   FLEDs are a bit of a technical unknown IMHO, yet they may offer all manner of  simple solutions to otherwise devious problems...