App.Note - Self-powered Indoor Light Sensor
Application Note AN011
Self-powered Indoor Light Sensor
1. Intention of this Application Note
To present a simple concept for a self-powered indoor light sensor based on the EnOcean
STM 100.
Vc = e.g. Vref from
VBT4480, very
CdS photo resistor,
(reaction time up
to 100 ms!) not
RoHS compliant
Fig.: Indoor light sensing concept
2. Challenge
Even if not recognized by the human eye, artificial indoor light flickers with a cycle time
of about 20ms due to the 50/60Hz frequency of the AC mains power. For energy saving
reasons the STM 100 operating and measurement time is about 2ms. This could result in
permanently different measurement values depending on the sampling time within the
mains period.
A simple method for avoidance is measuring via a CdS based photo resistor. Due to its
inertial behaviour this component averages the 50/60 Hz light fluctuations. In addition,
this component has a linear characteristic. Unfortunately a Cds photo resistor is not
RoHS compliant.
That’s why a silicon based photo diode has to be used. Photo sensors are RoHS compliant
but unfortunately they are too fast for this application (response time range is some ns
to µs).
EnOcean GmbH – RF Technology Without Batteries
Bach/Anders, Jan 2007, subject to modifications
Kolpingring 18 a – 82041 Oberhaching – Germany l phone +49 89 6734 689 –77 l [email protected] l
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Application Note AN011
Self-powered Indoor Light Sensor
3. Solution Concept
The circuitry concept showed in the figure uses a photo diode operated in the photovoltaic mode. The voltage at the diode pins correlates to the illumination and is
permanently available (voltage generator) to the STM 100. The 50/60 Hz fluctuations can
be simply averaged by a RC element (Tau > 50ms).
Photo-voltaic operation has a better temperature characteristic than the usually used
photoconductive operation. If needed, the non linear component characteristic in the
higher brightness range must be linearized on the receiver side.
4. Important Note
The information contained in this document has been reviewed with great care and is
believed to be accurate. EnOcean assumes no responsibility for its use, for any problems
or damage, which may result from errors or omissions. EnOcean reserves the right to
make corrections, modifications, enhancements, improvements and other changes to this
information at any time without notice. EnOcean assumes no liability arising from the
application or use of any product or circuit described herein, for its applications
assistance or customer product design. Customers are responsible for their products and
applications using EnOcean components. To minimize the risks associated with customer
products and applications, customers should provide adequate design and operating
safeguards. This information neither states nor implies warranty of any kind, including
fitness for any particular application. Information published by EnOcean regarding thirdparty products or services does not constitute a license from EnOcean to use such
products or services or a warranty or endorsement thereof. EnOcean does NOT
recommend the use of its products in critical life support applications or security systems
without explicitly written approval of the president of the EnOcean.
EnOcean GmbH – RF Technology Without Batteries
Bach/Anders, Jan 2007, subject to modifications
Kolpingring 18 a – 82041 Oberhaching – Germany l phone +49 89 6734 689 –77 l [email protected] l
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