Daniel Kramnik’s Project Log

Introduction

This stick out was built for the Water Quality Science Olympiad consequence. The rules manual of arms just said “ build a salinometer ”, so this is what I came up with. The obvious choice, of naturally, is a childlike mechanical hydrometer with possibly +/-5 % brininess accuracy in the best subject. This electronic solution ( hehe ! ) has a theoretical accuracy of +/-0.0014 % brininess, but some serial dilution is required to bring the sample into its roll .
build up time : ~1.5 weeks
Sidenote : always since this foliate started making it direction around the ‘ net, I ’ ve been accused of getting excessive “ aid ” from parents/adults and of winning by overspending on the undertaking. Neither of these claims are true. I know how to use Eagle and etch my own tour boards because I learned how to for my DRSSTC project, I can design and understand opamp circuits because I read the match sections in The Art of Electronics, and I spent no more than $ 60 on this project, and entirely this much because I insisted on buying the parts in-store and not online due to prison term constraints .
Thanks for read, please don ’ metric ton be hostile !

-Daniel
Completed view :

Design

The integral circuit consisted of respective parts : an opamp-based salinity-measuring racing circuit, a linear positive/negative power issue, a digital voltmeter, and a peltier device temperature control .
In holy order to save time debugging ( inevitably ) flawed boodle or perfboarded circuits, I designed custom-made circuit boards in Eagle CAD and etched them at home with ferric chloride. I won ’ thyroxine post my .sch/.brd files as that would make it besides easy to copy this project for Science Olympiad, but I will partake conventional effigy files and board double files ( if you already know how to reCAD them for etching in Eagle, chances are, you ’ re already beyond this guide : ) ) .
1. Opamp racing circuit :
The first opamp is a Wien bridge oscillator, the second gear is a non-inverting amplifier with profit set by the AC electric resistance between the probe ’ s tips, the third is a broad wave rectifier with a smoothing blabbermouthed point detector, and the fourth is a derived function amplifier that measures the potential between the DC poles of the rectifier. Due to time constraints, I copied the opamp tour from hypertext transfer protocol : //www.octiva.net/projects/ppm/ in my plan, failing to notice that it has several flaws : the non-inverting derive stage defaults to G = 1 with high electric resistance on the probe, so the range of the instrument is sternly limited ( this could well be fixed by precisely making it an invert stage – we don ’ metric ton worry if our AC signal is 180 degrees out of phase once we rectify it ) and the entire wave rectifier in the third base opamp stage could be implemented in a better way that references it to ground, making the fourthly stage and some of the biasing pots unnecessary .
conventional :

Board layout :
( Edges are cropped out when print, I barely decided to keep Eagle ’ mho default size )

2. linear regulator :
Nothing extra hera, just some 78xx/79xx ICs to regulate +/-18V from the four 9V batteries down to +/-12V and +/- 5V. It has more capacitors than it needs ( they could be omitted in a low budget or low outer space position ), but I suppose it ’ mho well drill. There is one error in this conventional : the switch should be SPDT and should connect the plus and veto inputs to the regulator, not the ground remark. With the switch only connecting the ground input, there is some feedthrough in the ICs and might is wasted and weird voltages are supplied to the ICs- with the switch turned off ! If anyone knows why this is occurring, I ’ d love to hear your explanation !
conventional :

Board layout :
( I couldn ’ t find any 79xx in a erect box in Eagle, so I used the horizontal one alternatively – that ’ s why it looks like the negative regulators are battling it out across the tour board with the filter caps )

3. Digital voltmeter :
A standard execution of the ICL7107 with range 0 – 20V ; the LED displays and IC are mounted on face-to-face sides, technically making this a 2-sided board ( but not in truth, so preceptor ’ metric ton be intimidated ) !
conventional :

Board layout :

4. temperature restrainer :
This is a comparator racing circuit I designed to turn a high current heating element ( with my front-runner gate drive IC from my Tesla handbuild ! ) on when a thermistor senses that the temperature of the sample is below a certain set point and turn it off when it senses that the temperature is above that point. I didn ’ deoxythymidine monophosphate add any hysteresis or PID control, with current reversing through the peltier rather than merely shutting off, due to time constraints and because I didn ’ deoxythymidine monophosphate feel it was absolutely necessary for the racing circuit to work. In fact, this entire part can be omitted completely if an ultra-precise measurement is not necessity because the conduction of saline solution solution is not highly temperature-dependent .
conventional :

Board layout :

Construction

The racing circuit board layouts were printed on slick composition with a laserjet printer ( which character of glossy newspaper is my privy : p ) :

following, they were ironed onto blank single-sided circuit board corporeal :

After that, I etched them in a bathe of ferric chloride using a VWR Orbital Shaker ; ask around local anesthetic colleges and universities, they much have stuff to throw off ( this particular one came from MIT ) :

immediately after etching, the boards were tinned to prevent oxidation. It ’ mho besides good commit in power circuits ( which these are not ) that I inherited from Tesla gyrate construct :

The boards were then drilled with an 0.035″ standard size PCB drill morsel on my bore press ( yay harborfreight ! ) :

ultimately, the boards were populated :
Opamp Circuit :

linear regulator :

Digital Voltmeter :

temperature restrainer :

The integral thing was assembled on a orthogonal objet d’art of acrylic fiber and the circuit boards were secured with bolts via 8-32 tapdance holes :

Testing

Confirming that the DVM works by measuring a power add :

Tuning the amplitude of the Wien bridge oscillator ( we want the lowest amplitude that will produce a clean sine beckon in order to maximize the stove ) :

Results

We won foremost station at regionals ! ( Although I ’ thousand told that may have been more a product of the team ’ mho written mark than salinometer score, as there was some mistake in the serial dilution ; I didn ’ t go due to Harvard Extension finals ) In any case, an improved interpretation of this device will be making an appearance at the department of state rival in March .

Updates!

Water quality took moment place at the MA submit science olympic games on Saturday, March 18 ! ( Again, the salinometer was lone function of their grudge though )

I changed the gain-setting resistor in the non-inverting amplifier to 200 ohms to adjust the image of the musical instrument from 0 – a few percentage brininess to 0 – roughly 20 percentage salt ; much, it can measure to about +/-0.25 % brininess – still reasonably dear, and doesn ’ t require any series dilution .
I besides switched to goldplate electrodes made from battery connectors for the probe ends to eliminate problems associated with corrosion :

reservoir : https://oanhthai.com
Category : Tutorial

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