The LED in this circuit glows in proportion to the load current (see the figure). It was designed as a very compact substitute for an ammeter in the 12-V supply line of some astronomy equipment. The equipment includes low-power heating elements (dew preventers) whose operation is not visible. However, the LED brightens visibly when the heaters are switched on, giving a positive indication that they're connected and working.
Circuit analysis is very simple. The voltage across the 22-O resistor is the same as that across RSENSE. The current through the 22-O resistor is the same value that flows through the LED. Thus, with the values shown, the LED current is 0.05/22 of the load current. This ranges from a dim glow with 200 mA into the load up to full brightness (limited by the 680-O resistor) when the load is drawing 6.6 A.
If desired, a second LED with a fixed resistor could be included for comparison. Another option is to construct a bar-graph display by building several instances of this circuit with different resistors in place of the 22-O one, all sharing the same RSENSE.
The op amps must have inputs and outputs that can swing to the positive rail, plus the ability to take a supply voltage of somewhat more than 12 V. Texas Instruments' TLV273X series fills the bill.
