This circuit was found in an 80's EDN. The circuit uses a current common CMOS, the 4093 powering an LED. Trigger adjustment is done in a trimpot.
Found in the American 73 magazine for radio amateurs, this circuit can test common quartz...
This circuit for the automatic activation of a small lamp (or high brightness LED) was found in...
This circuit provides a 5 V negative output with a maximum current of 1 A. The circuit is from the...