With this circuit, the idea is that when the phototransistor detects a drop in the light, the LM339 (comparator) is able to cause a drop in voltage on the Trigger pin of the 555... the 555's mix of capacitors and resistors is designed to provide a 1 second (give or take) pulse. This pulse will be used later in the chapter when a small battery-powered alarm clock is added to the mix along with a latching relay. (It's a pretty specific relay, so I may have to order one if I can't find it local. Argh.)
In the video below, you'll see that when I shine the flashlight on the PT, the LED stays dark. It's not yet received a pulse from the 555. But as soon as I turn off the flashlight, I get the LED lighting up for about 1 second. You'll have to play around with the 500k trimmer until you're happy with the results. Decreasing the resistance on the Trimmer means it will take a lower amount of light to trigger the LED (if I'm understanding the circuit correctly).
|Variable voltage adapter - set to 6V|
I'm making a note to myself here that once I've got the latching relay in my possession (silly thing is back-ordered through Mouser), I'll need to place it closer to the bottom of the breadboard to allow for the final placement of a second 555 timer shown in Figure 7-12. I'm still trying to decide if I want to "finish" the experiment by wiring in an actual lamp with a 12VDC from AC Adapter -- I may skip that part if I'm satisfied with the final circuit and understand how it works.