Gas station without pumps

2012 October 24

Thinking about PC boards and parts kits for circuits lab

On my way to work today, I was thinking a bit about redesigning the PC boards for the circuit lab.  I have 2 boards designed so far: an instrumentation amp protoboard and pressure-sensor breakout board.

I have to redesign the pressure-sensor board for two reasons: 1) it is mis-wired, so the screw terminals are labeled wrong (see Pressure sensor miswired), and 2) I’m going to change to a pressure sensor with a built-in barbed port, so that hoses can be directly connected (see Rethinking the pressure sensor lab). The new choice of pressure sensor has a different mounting

We’ll need to decide whether to make the pressure sensors lab equipment (which we’d have to check out and keep track of) or put them in the student parts kit for the course.  If I put them in the part kit, then the students can solder the breakout boards themselves, but it adds the cost of the sensor, the board, nuts and screws, a capacitor, and the screw terminals (about $13) to the parts kit.

The instrumentation amp protoboard is functional as it is (I wired up the instrumentation amp for the pressure sensor on it), but I’m not real happy with the design.  The core is ok, but I think now that the space for a barrel jack for a wall-wart is wasted space. Wall wart power is too low-quality for analog work, and I don’t want to put a regulator on the board. The students have a fine bench supply (Agilent E3631A), so we might as well use it.

I need to have a 4-pin connector for connecting to the pressure sensor (3 of which would be used for the EKG), 2 pins for connecting to the power supply, and 3 or 4 for connecting to the Arduino (Gnd, analog out, and Aref at least). I think that I’ll want to have more Vdd and GND points on the board, as the routing for those was trickier than I would have liked. It might be good to have a well-separated pin for connecting an oscilloscope ground. I’d like to add spaces for connecting up 1 or 2 transistors and another dual-op amp chip.  These changes would almost certainly increase the size of the board, raising the per board price from $1.40 to $2.60 (unless I buy many).  I need to think about what circuits we’ll have the students solder (versus building on a breadboard).  Currently, I’m leaning toward having them solder the EKG, the pressure-sensor amp, and the capacitive touch sensor.  The simple op-amp audio amp should be breadboarded, but I’m not sure about the variant with an output transistor for more power.  This means that each lab group would need 3 or 4 boards for the quarter, which adds another $5–10 for boards.  Screw terminals for connecting power and such to those boards adds another $6. Breadboarding is certainly cheaper, especially since the op amps and other parts could be reused from one project to the next.

I’m also leaning toward getting each student a large collection of resistors (like this collection of 10 each of 112 values for $12.90) and a smaller collection of capacitors.  I’ve not been able to find a cheap assortment of capacitors in different values (other than surplus assortments of random values, which is not of much use to us), so we may have to pick a small number of useful values, and buy the parts separately.

It looks like the parts and boards for the student kit including everything will come to far more than the $43 lab fee currently being charged to students in the usual circuits lab (which uses far fewer parts, and those mostly very cheap ones). If we require Arduinos as well, we’ll certainly far exceed that price. I don’t know if we’ll even be allowed to charge a lab fee.  The page about the current fees says “The fees shown below have been established using prior course history and have been approved by the Dean of Engineering and reviewed by the UCSC Student Fees Committee. The UCSC Miscellaneous Fees Advisory Committee recommended adoption of these fees and final approval was made by the Chancellor.”  I doubt we’ll have time to figure out all the parts we need in order to set a fee in time for a committee to meet and approve fees.  We may have to do direct sales of kits and parts to students, if that is permitted. It is going to be difficult even to get a parts list together in time to buy the parts, much less to figure out the prices, set the fee appropriately, and get approval for the fee.

 

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