If you have followed the previous 3 posts about this project you know that we are at the point where the rubber meets the road. I have now put the interface circuits, that were previously on bread boards, on to an Arduino shield board. The shield will make all of the necessary connections to the Arduino. The connections from the interface board to the rest of the circuit are being assembled so that I will be able to easily disconnect and remove the interface board for modification or service if necessary. Here is a picture of the completed interface board:
I have also improved the schematics for the interfaces and included the hookup wire color that is being used to assist me if I have to troubleshoot the unit in the future.
As mentioned before both of these circuits have been installed on the Arduino Shield board.
For the power hookup which brings 9 volts to the Arduino I have made up a polarized connector using board headers. The rest of the connections to the peripheral circuit are going to be made with Male-Female bread board wires which I have cut in half to produce a color coded connection (see schematics for colors used). While these wires are really quite small gauge they are more than adequate for this low current application. Half of the cut bread board wire is attached to the interface board and the other half is attached to the appropriate point on the peripheral circuits. I always put the female ends on the wires that may be powered when disconnected so that they can not short to other wires or components. Here are a couple pictures of the unit with and without the interface board installed.
With projects like this I like to layout the wiring harness so that I can open the unit and have access to the circuits without having to stress the wires or their connections. To do this I make sure that the wires all pass over the same edge so that the cover can be laid back as in this photo:
This point in building a project is always the most nerve wracking for me as I am about to turn the unit on for the first time. Though I have checked every connection there is always a chance that it may not work or worse yet there may be smoke and fire. If that happens it would force me into skipping sleeping tonight so that I can get to the bottom of the problem. Fortunately I have been lucky this time and everything comes on and works as planned. I have made a short video of the first tests that I performed on the unit using my power supply to provide the test voltage which I could decrease until it fell below the target voltage.
Now that I had a functional unit I decided to test it by putting a AA battery under load and seeing how long it would take for it to drop below 1 volt. The PDT has a set of banana jacks that allow an external electronic load to be attached to the device that is being tested.
I hooked up the external electronic load and set it to 300 mA. The Target voltage on the PDT was set at 1 volt and the AA battery was hooked to the circuit and the Start Button on the PDT was pushed. I walked away and returned several hours later. On return the complete light was on and the time on the PDT was waiting at the push of the TIME button. Sorry I have no photos or videos as I forgot in my excitement.
Now I wanted to run some experiments that were more predictable so I found the largest non-super capacitor that I had which happened to be 33,000 uF. My plan was to charge the capacitor to 30 volts and then time it through one time constant. I did not need the external electronic load this time as I would use the inherent 75K ohms input resistance of the PDT created by the voltage divider for the arduino's analog inputs. One time constant below 30 volts would be 11.3 volts. Here is a picture of the setup with the 33,000 uF capacitor.
You can see that when the picture was taken 1 minute and 53 seconds had elapsed and the capacitor was down to 28.15 volts. I ran this experiment 3 times and received the following results;
Test 1 Elapsed time 41:05 Calculated capacitance assuming input resistance of 75K = 32,900 uF
Test 2 Elapsed time 42:42 Calculated capacitance assuming input resistance of 75K = 34,000 uF
Test 3 Elapsed time 42:00 Calculated capacitance assuming input resistance of 75K = 33,600 uF
I wanted to produce a video to demonstrate this type of test so I chose a 350 uf capacitor and charged it to 16 volts. One time constant below 16 volts is 6.03 volts. This should take about 30 seconds which is more appropriate for a demo video. Just for back ground on this experiment and video I had to do it 4 times before I got it this bad. What I lack as a moderator I less than make up for as a videographer but here it is:
When I first felt that I needed a timer like the Process Duration Timer I was testing and comparing batteries from different manufacturers. To test batteries properly they need to be allowed to discharge at reasonable rates. The times involved made personally monitoring the process impossible. I built my first timer at that time but I have always found it clumsy to use and it lacked the range to test batteries over 12 volts. The PDT is my attempt to produce a more accurate unit with a greater range (up to 30 volts in this case). As you have seen it can also be used to time the process of capacitor discharge and with a simple interface it should be able to time any electronic or mechanical process that can be connected to a sensor or a switch. All that one has to do is sense the end of the process and pull the test voltage below the target voltage.
Thank you to my friends who have taken the time to read and make suggestions that have helped me make this a more successful project.
John
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