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.