Building the Box: Current Measurement

The Control Box

The "Control Box" of our node is what contains the bulk of the components; basically everything other than the solar panel and the battery.  So therefore, the box must contain all of the circuitry to output the correct voltages and plugs for phones, connect lights, etc. The way we envision it now, the box will need 3 main elements:

1. Current measurement device: measuring the current output of the solar charge controller

2. Voltage measurement device: measuring the charge of the battery

3. Interface for charging/lights: site where you would be able to plug in a phone or set up lights

      The box needs to be resistant to both dust and water so we are thinking of an entirely sealed container made from plastic because this is easy to make holes in and customize. With a complete seal there is no chance for air flow which is usually favored for heat dissipation. Given that Masaka, where we will deploy, has hottest temperatures floating around 25 - 30 °C for only a portion of the year this should not be a problem. The thing that will get the hottest are the phone chargers and those have been observed to only give off mild heat. The questions of what switches, what types of seals and which wiring to use is still up in the air but has been discussed. A later post should give all these details soon.

Today we prototyped the Current Measurement element.  

This piece (see the following circuit diagram) measures the current output and translates it into LEDs, so it's easy to tell the relative amount of power you're getting out of your panel, and make the critical decision of whether you should be using this time to charge phones/other devices or if this is an appropriate time to only charge the battery for later use.  There are 8 LEDs, one lit LED means one amp of current, two lit up means two amps, etc.
(NB: each IC chip  the image below has two MCP6002 opAmps in it.)

The circuit diagram below shows a similar image as in a previous post however this image has a few updates. Obviously in the ammeter built there were in fact 8 LEDs to give greater resolution. The diagram uses 3 dots to indicate that the a unit setup is repeated. The voltage regulator used, U1, gives an output of +5 V and the resistor values were chosen accordingly to give a reference voltage equal to the voltage drop across the ballast resistor (MP915). The calculations of all the resistor values are shown under the diagram in a series of tables.

The table below shows how each opAmp is able to get the correct reference voltage so that it turns on for the intended current. A summary table (top right) also shows how much current was to be used if all 8 LEDs were on. The current drain is not negligible but the the ammeter would only ever be operated temporarily by a push switch and so this should not be an issue in terms of energy loss.

One point of confusion: On one of our real multimeter ammeters we were measuring a current of 1.5 amps, whereas a measurement of voltage across the 0.1 ohm resistor indicated that the current was greater than that (and we saw between 2 and 3 LEDs light up).  We're still a little unsure about where the discrepancy is between these values. The value of the resistor was verified to be 0.1 Ohms and so it would be very hard to disagree with the amount of voltage being dropped across it. More lab testing in exact conditions will need to be done.

Coordinating with Voltage Measurement:

One thing we hope to do is coordinate the displayed LEDs for current and voltage measurements, so you can look at them and qualitatively (without calculating amps/volts/etc.) decide what it would take to fully charge your battery.  I.E.: construct a similar voltage measurement which has 8 LEDs (or could be more for higher resolution) such that 1 LED means 1/8th charge, 2 means 2/8th, etc.  Then, by coordinating correctly, pair the two measurement systems such that if you have 6 of 8 lights lit up for battery charge, and two lights lit up on current output, then you can say that in some fixed time (maybe an hour), the battery will be fully charged if you don't divert power to charge things and if the solar output remains relatively constant.  The readout would look similar to this:

This would hopefully be an extremely intuitive interface because the literal number of LED's (or literal length) from current output can be imagined to geometrically overlay the unlit LED's in the battery charge display. For this to work the way in which battery charge relates to terminal voltage will need to be researched. Obviously, in a perfect world, the voltage of a battery should not decrease because each c]m-ell has a precise voltage difference across it dependent on the element of the electrodes and the electrolyte. But I think that there still is a relationship between charge and voltage. Although it is definitely not the case that zero charge correlates to zero voltage.