Light+Level


 * Light Levels**
 * March 3**

It has been decided that this instrument will be built using a wheatstone bridge incorporating a LDR and the instrumentation amplifier detailed in the diary.

Having built the circuit, the next step will be to plot the input against the output to determine instrument sensitivity and deviation from linearity.


 * March 4 - Test 1.**

I connected the amplifier circuit to a +/- 9V supply and the bridge to a +9V supply. This value was picked to approximate working from battery power.



The test was set up as seen above. The anglepoise lamp was used to give varying light levels. The lux levels were measured using a CEM DT-8820 Environment Meter. The output voltage of the circuit was measured using a calibrated Fluke 179 multimeter. A range of measurements were taken and the results were shown below.

As can be seen the relationship between the values is logarithmic.


 * March 5 - Test 2.**

The circuit was adjusted by putting two LDR's in series in the bridge and changing the gain adjusting put to 100k. The test was repeated and the results can be seen below.



Although there is a slight improvement, it is far from where it needs to be. Back to the drawing board!

April 20

In consultation with my tutor, it has been decided to use this sensor to time the amount of daylight. This will involve finding a value of resistance for sunrise and sunset, then using the PLC, time the amount of daylight and give a value each evening. The code below should achieve this. A random value for the input signal has been written into the code pending acquiring the actual value required.



Although one can only see a data register for hours, the minutes will automatically be placed in D12, there is no additional programming necessary. Additional code for the other instruments will have to be written here also.