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!
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.
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 !
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
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
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
Step 7: Possible Battery Box Fitting?
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
Step 9: Completed !
Step 10: Calibration & Testing
Check => http://www.youtube.com/watch?v=-PtbP8STtwU <= for a short calibration video.
Step 11: In Use!
Step 12: EXTENSION: Solar PV - Battery Charging FLED Ammeter?
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
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...