176: Sunday 13th February 2011

The circuit for the Amps x 100 gauge is finally built. I had a few problems along the way, but got round them. I didn't realise the current sensor output voltage was with respect to the current sensor 2.5v reference. I had to remove the ground to the low end reference voltage and apply the 2.5v reference. Using the calculations on the current sensor data sheet the maximum current reading would produce an output voltage of 4.375v and that is 1.875v above the 2.5v reference. With no current being sensed, the output voltage is 2.5v. I also worked out that I should see about 3.67v when the current is about 500A. I calibrated the display driver by feeding 2.5v in the reference, 4.375v in the input connection and then adjusting the multi-turn pot until 9 LEDs were lit on the temporary display. I shall change the connector strip on the gauge ribbon cable as I am using a different type now and the old one will just fall straight out. The new connector strips are much more secure. Once I have it set up on the bench I can illuminate the ultra bright blue LEDs on the instruments so I can get some photos. It would be difficult to do this once I have fitted it on the Probatron as I would need to pull current and that would require me to be driving and taking photos at the same time. Even if I get someone else to take the photos then they would only be vague as they would need to be done over my shoulder as I am driving. If I do this on the bench then I can get a much better shot without any stress. I had to add a 5v supply regulator to the circuit to supply the sensor and I had not planned for this on my board layout design, but I did leave some extra strips all the way round and I could mount this on them, just had to add a link and put one track break on the back of the vero board and that was working fine. I am looking forward to fitting this tomorrow when I get home from work. I have been putting this job off since before Christmas. I hope then to get the matrix display built. I have been discussing this with Tim Catellier ( http://evz3.blogspot.com/ ) and I have decided that when I design the display I want to build an algorithm into the PIC chip to keep adding the accumulated current draw and keep a record of the Amp hours used. I can then over time see how the battery pack is performing. Since the Amp hours is a proper measure of the battery capacity, being able to measure this should tell me more accurately when I am all done and to find a charger. In case you are wondering, the box is just a DV tape box with some insulation tape wrapped round to keep is shut. Notice the hole I drilled very carefully where the adjustment screw is, so I can now leave it "sealed up" and still adjust it. The last gauge I made had to be adjusted with the lid off the project box I had it in. This is much better. Also if I mount this box so that it can be accessed without having to remove the instruments each time I want to "tweak" it then that would be just great.

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