Friday, July 11, 2014

Experiment 7 Part 1 (Chapter 7) -- The Sun Goes Down...

The first circuit for Experiment 7 is fairly easy to wire up -- it's the one found in Figure 7-1 and consists of a 555 timer, LM339 comparator, a phototransistor, and a collection of capacitors and resistors. One thing this circuit does also call for is a 7806 6V voltage regulator -- I didn't have one, but fortunately I did have a variable adapter that lets me select the voltage using a switch on the front. I set it to 6V, disabled the 5V regulated power near the top of my breadboard (basically removing the wires connecting it all to GND and power supply lines), and verified with the multimeter that it was providing 6V. Everything is good.

I must be getting better at translating schematics to a breadboard because this one worked the first time I powered it up. My wiring method is simply working from the top down... and when I encounter a chip, I work counterclockwise and double-check all wires going into and out of the pins. Finally, when the circuit is finished, I count the number of connections to power and the number of connections to GND and verify I have matching numbers on my breadboard.

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
It works... so now I need to move forward and integrate the latching relay, part # DS1E-SL2-DC3V or equivalent. And trust me -- grab that relay wherever you can find it... it's been very difficult to track one down.

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.

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