@▷ Light Activated Relay | Diagram for Schematic

Light Activated Relay

trigger
This is same circuit as above with the addition of a photo resistor to trigger the flip flop instead of a push button. The bias resistor in series with photo resistor was chosen so that sufficient voltage is present at the base of the 2N3904 to supply current to the circuit in ambient lighting conditions. The circuit should toggle when the photo resistor is hit by a flashlight beam or other fast changing light source. Slow changes in light intensity will have no effect unless the light gets too bright to maintain sufficient bias for the 2N3904.

From : http://ourworld.compuserve.com/homepages/Bill_Bowden/

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