[Part 3] Electrical Systems Design

"In fact, I'm writing all these posts about the Peteometer after I finished this project. So I know that everything is functioning. The purpose of making these posts is, however, to document what I did and let other people know about my job so that they can share my knowledge, but most of all, I can learn from suggestions from my readers through email."

The electrical design precedes the completion of mechanical housing, but I think showing electrical setup after making the casing somehow makes more logical sense. Enough of digression, majority of the initial design stage was used to outline electrical components and corresponding wiring schematic. So it is pretty much a wait-for-the-parts-and-assemble-as-they-arrive plan. Nevertheless, I had to make an internal chassis which will serve as a base plate for housing all optical components, flow meter, the circuit board, wires, ports, and etc. The picture below shows some material that I picked up from scrap piles.


Before I took the picture, I already made few cuts on the stainless plate (not on the plastic pieces though, they were cut like that). The two sides of the plate are bent so that the plastic pieces can be mounted on the bent side of the end. At the time I took the picture, I only bent one side.


Before I bent the other side, I completed one side with holes and one slot (for phototransistor hook-up). Then I trimmed off the plastic pieces so that they are on the right length with the plate. Once everything made perfect ends, I moved onto bending the other side and making holes to complete it.

After such basic machining and setup, I physically marked a potential location for the flow meter on the plate and let my imagination take off in order to place necessary optical elements and marked all the possible locations for them. Machined all the parts, optical fixations, a base holder for the diode laser, ... then I made necessary holes so each part can be fixed to the plate. After around 4-5 hours of machining components, I got the following result .


Now notice that, at this stage, almost everything is complete: the display is fully wired up, a laser diode is mounted, an infrared laser source is mounted, two variable position phototransistors are secured and most importantly the flow meter is stably fixed to the internal chassis.


The plastic rim on the right side is not in one piece. This is done to allow wires to go around to the back side while the external casing does not touch the wires.Now we have to move on to the back side of this to show you the actual electrical circuit.


If you remember the circuit drawings on a white board (see part 1), this is it.

Display Connectors

Mechanical Relay with a Transistor and Capacitor (see Part1 for details)

Thermocouple Module and a Capacitor for Phototransistor 
(clock crystal is shown on the top right)

Parallax BS2pe 24-pin Microcontroller

Optional DB-9 Port for Computer Interface

This step has taken the most amount of time including the time looking for parts online or at a nearby Radioshack store. The wiring itself didn't take that long, probably around 2 hours total. Hardware debugging was surprisingly minimal. Maybe that's an indication of myself improving on this kind of things.

I have purchased a USB interface to avoid complications arising from using the DB-9 port shown above which is supposed to arrive today actually!  Oh, almost forgot it. The internal chassis is connected to the ground of the whole electrical system. It will maintain another electrical contact with the outer casings (see Part 2) eventually leading to the "common" ground through the Opti-Temp chiller chassis.

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