Tuesday, April 13, 2010

Saline Level monitoring using LDR

Hi folks, we are looking into one of my project "EMAS - Wi Tech", In our previous post we saw some introduction about it. Now we are going to dig further on the different sensor circuits used. The first one which we would see is how we monitored the saline level using an LDR, Just go through the below para for detials on the circuit what we used.

The LDR sensor circuit consists of a comparator, an LDR and a pair of multi-turn preset resistor (10k ohm). The LDR is connected in series with a preset resistor forms a voltage divider configuration. When no laser beam falls on the LDR its resistance will be very high compared to 10k ohm. So by voltage divider rule small voltage will be available across the preset resistor. But when laser beam falls on the LDR its resistance becomes almost zero. So voltage across the preset will become nearly 5V. This change in voltage across the preset can be detected using a comparator. Comparator is constructed using UA741.
The threshold voltage can set using another 10k ohm preset connected to the inverting input of the opamp.
In normal condition output of comparator remains at logic zero because the laser beam is distracted by the saline present in the bottle, but when saline level reduces, the distraction is lost and the laser beam falls on the LDR, because of this comparator output changes to logic one. This change is used by microcontroller to detect the quantity in the saline bottle.

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