Latching circuit with enable/disable latching mode.

[pmckeon]

I want to turn on an LED based on an external input, it will have two modes of operation. Latching whereby once triggered the LED remains in the ON state, and direct where the LED remains ON only so long as the input is active.

For the latching I was going to use a two transistor latching circuit, but am unsure how to modify the design to include a enable/disable switch for the latching mode. Alternatively if there were a different but equally simple circuit to achieve the same function?

 

[(*steve*)]

What is the nature of your input signal?

 

[pmckeon]

It will be an infrared phototransistor, effectively I want to use it to signal when an infrared light has been detected.

 

[(*steve*)]

There are many ways to do this, however one relatively simple way may be to use a Schmitt trigger and alter the bias point of your phototransistors' output to be either below the hysteresis range (so it will follow) or within it (so it will latch).

The same effect can be achieved by changing the lower bound of the hysteresis range. Third has the advantage of not affecting sensitivity.