omnivore LED - not fussy about voltages

Ordinary LEDs have a rather monot- onous diet: they will only "swallow" DC current with the right polarity, in which case a series resistor cuts down the current appetite to a moderate 10 ... 30 mA. This type of provision has a drawback in that the value of the series resistor must be calculated for each separate supply voltage, and that fluctuations in the supply signal can only be handled within a limited range. Substituting a FET for the series resistor affords a number of advantages. When the gate and source are linked, the transistor forms a current source without the need for any additional components. In the type used here, the BF 256C, the con- stant current is between roughly 11 and 15 mA, with a wide supply range of 5 ... 30 V. A universal silicon diode (DUS), such as the well known 1 N4148, will provide polarity protec- tion when connected in series with the LED. As a result, the "Omnivore" LED can be driven with AC voltages in the 5 ... 20 V (= 7 ... 30 V) as well. At the n