The most popular current w indicator in electronics is almost certainly the light-emitting diode (LED) which is available in a number of colors (red, green, blue, yellow, as well as dual colors) and variations (round, square, flashing).
Unfortunately, these virtually indestructible devices are often abused: terminals cut too short, current too high, burnt by a soldering iron, and many others. Most of these abuses may be prevented by the circuit described here: it enables the LED indication to be adapted to the circuit you are working on (flashing or not flashing, red, green, and so on).
The heart of the indicator is a dual-color diode D2. The manner in which this will light when the input of the indicator is actuated depends on the settings of S1- 1-3. Switch S1-4 enables the setting of the high or low level to which the input reacts Components R9, D3, and D4 protect the input from damage should the input voltage become too high.
An oscillator formed by ICia and 1C1t provides a flash frequency that is set by 131. Depending on the setting of switches Si 1-3, the oscillator signal is applied in a certain manner to the dual-colour LED via multiplexer 1C2. For convenience’s sake. the positions of switches S1-1-3 and the consequent actions of the LED are summarized in Table 1 and those of 51-4 and the status of the LED in Table 2. The indicator needs a supply of 8-25 V The current drawn from a supply of 25 V in normal operation is 30 mA.