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Tuesday, May 14, 2013

Precision LUX Detector

Here is a design I made for a friend who wanted an white LED to turn on when it was dusk, but also wanted so adjustment via a pot. So off to pick a photocell, I picked a VT9ØN3 that was used in many low light applications.

turns out there are as couple of types of photocells the Ø material and the 3 material

Type Ø Material
This is a general purpose material. Its characteristics include a good temperature coefficient and fast response time, especially at very low light levels. Cells of this type have relatively low dark history. Type Ø material is often used in lighting controls such as nightlights, and security lighting

Type 3 Material
This is a high speed material with a spectral response closely approximating the human eye. This material is well suited for switching from one light level to another and offers our best temperature stability and response time. This material is often used in cameras and industrial controls.

Once ordered we tested the photocell characteristic curve and found the following:

Now the circuit we chose was a op-amp comparator type, this was a great circuit because we used a JFET input opamp the TL072 and current consumption was less than 1ma (our target current) and have a very crisp trigger level unlike using a single transistor or FET. We used a 25 turn precision pot to get the accuracy down to +/- 1 LUX at the set-point. In-fact a voltage of 1.625 at pin 3 (reference) was ideal for dusk at 7pm local, the photocell read about 130k ohms indoors at dusk shades open with 5.25 sqft total window area facing east+west (2.625 sqft ea) approximately 3 to 4 LUX

The protocircuit below:

Video of the circuit working:

And finally PCB