Super Light Sensor

This “Super Light
Sensor” responds to minute fluctuations in light level, auto-adjusting
over the range from about 200 lux up to 60,000 lux (ie, from a modestly
lit room to direct sunlight). It has lots of potential uses – eg,
detecting a car entering a driveway, a person moving in a room, or wind
rustling the leaves of a tree. At the same time, it has a high level of
rejection of natural light variations, such as sunrise, sunset and the
movement of clouds. While it is a “passive” system, it can also be used
as an “active” system – ie, used in conjunction with a light beam.

Its great advantage here is that, since it responds to fluctuations
in light level rather than the crossing of a specific light threshold,
it is much more flexible than other typical “active” systems. It can be
placed within the line-of-sight of almost any light source, including
“vague” ambient light, and simply switched on. As shown, the LDR
is wired as part of a voltage divider so that, between darkness and
full sunlight, its output at “X” varies between about one-quarter and
three-quarters of the supply voltage. A wide variety of sensors may be
used in place of the LDR, including photo-transistors, photo-diodes and infrared and ultraviolet devices.

Circuit diagram:

Super Light Sensor Circuit

Super Light Sensor Circuit Diagram

Fig.1: light level fluctuations are detected by LDR1 and the
resulting signal fed to comparator stage IC1. IC1 in turn triggers 7555
timer IC2 which is wired as a monostable and this drives transistor Q2
and a relay.

The signal from the sensor is fed to the inputs of comparator IC1
via two 150kO resistors. However, any signal fluctuations will be
slightly delayed on pin 3 compared to pin 2, due to the 220nF capacitor.
As a result, the pin 6 output of the comparator (IC1) switches low
during short-term signal fluctuations and this triggers monostable timer
IC2. IC2 in turn switches on transistor Q2 which activates Relay 1. It
also lights LED1 via a 1.5kO current-limiting resistor. Trimpot VR2
allows the monostable period to be adjusted between about 3s and 30s.

As with all such circuits, the Super Light Sensor may not work as
well under AC lighting as under natural lighting. If AC lighting does
prove a problem, a 16µF (16V) electrolytic capacitor can be connected
between the sensor output and ground to filter the signal to the
comparator. When pin 3 of IC2 goes high, FET
Q1 also turns on and pulls pin 2 of IC2 high. This transistor remains on
for a very short period after pin 3 goes low again due to the 100nF
capacitor on its gate. This “blanking” is done to allow the circuit time
to settle again after the relay disengages (and stops drawing current).

LDR placement:

The LDR should be installed inside a black tube, as shown here

the LDR should be installed inside a black tube, as shown here.

The “blanking” also makes it possible to run external circuits from
the same power supply as the Super Light Sensor, without upsetting the
circuit. The current consumption is less than 10mA on standby, so that
battery operation (eg, 8 x AA batteries) is feasible. After building the
circuit, switch on and wait for the circuit to settle. It’s then just a
matter of adjusting VR1 so that the circuit has good sensitivity
without false triggering. With some experimentation, it’s possible to
set the circuit to change seamlessly from natural to AC lighting. If
maximum sensitivity under natural lighting false triggers the circuit
under AC, then adjust VR1 to give maximum sensitivity under AC (and vice

In daylight, the Super Light Sensor will typically detect a single
finger moving at a distance of 3m, without the use of any lenses. It
will also detect a person crossing a path at a distance of more than
10m, again without lenses. And when used as an “active” system, it will
typically detect a person walking in front of an ordinary light source
(eg, a 60W incandescent light-bulb) at more than 10m. Note that these
ranges are achieved by placing the LDR (which
is used as the light sensor) in a black tube, as shown in Fig.2. A
single lens will double these distances, while the use of two lenses in
an “active” system will multiply the basic range by 6 or 7.

Author: Thomas Scarborough – Copyright: Silicon Chip Electronics Magazine

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