Controls the ignition of lamps or other electrical loads
when the brightness of the environment falls below an adjustable
threshold.
There are circuits that never get old and withstand time,
progress, and emergence and spread of a variety of microcontrollers and
boards, perhaps because there is always a need of something simple,
cheap and that gets the job done.
Among those there’s the twilight switch, that will never
get old, at least until there will be the need for something to turn
the lights on and off based on ambient light.
A lot of twilight switches are available on the market,
what to invent then? This time we tried to act on size and we propose a
project that, despite being a classic, adapts to one of the needs of
modern times: miniaturization. In fact, what we present is a magnificent
twilight switch a with relay output to control in low voltage loads (to
handle loads operating at 220 VAC just use an adequate capacity relay
to control the exchange) whose printed circuit board, complete with all
components, measures just 29x29x15 mm!
Diagram
The diagram is very simple: an operational amplifier
mounted as a comparator and a photoresistor we use to detect the level
of lighting in the environment. To complete the circuit you’ll find also
the actuator, which in our case is a small relay. As said, to detect
the ambience illumination we use of a photoresistor dubbed FR1, which
has maximum resistance in the dark (about 1 Mohm) and the minimum (some
hundreds of ohms) at the exposure to a strong intensity light: this
allows to detect the level of illumination of the environment on the
basis of the value taken by the resistive component. To do this we
insert a photoresistor in a voltage divider, doing so by referring to
the voltage obtained at the output from the latter.
By using a divider we can use a comparator with a defined
voltage threshold corresponding to a certain brightness value: in
correspondence of the threshold the will be energized. The inclusion of a
trimmer in the comparator network leaves us free to define the
brightness level at which the relay must be activated.
Let’s see the operation in detail, assuming we start from a
total darkness condition, with the FR1 resistance much higher than that
of R3 and R5, and then the voltage present between the node formed by
it with R3 and R6 being approximately equal to that which is detected
downstream of the D1 diode and thus the same that feeds the U1.
When the light in the environment increases, the voltage
brought from R6 and D3 to pin 5 of the U1 begins to lower, due to the
fact that the resistance of the photoresistor starts to fall
progressively, in relation with the intensity of the light that hits the
sensitive surface. At some point in this process, the non-inverting
input is at a lower potential than that brought on the inverting one
from the RV1 trimmer and the comparator inverts the state of its output,
which switches to low level and leaves the transistor T1 inhibited.
Now, the LED turns off and the movable load of the relay
falls. If the brightness of the environment drops back, pin 7 of the U1
gets back to high and the relay is energized again (also the LED turns
back).
The point at which the relay is energized and the LED
lights up is regulated by RV1 trimmer. By bringing the cursor of this
component to ground reduces you reduce the voltage at which the
comparator returns to rest: plenty of light it is required to deactivate
the relay. On the contrary, moving towards the D1 diode cathode, the
voltage that pin 5 must reach grows and to trigger the relay you must
submit to the photoresistor higher resistance values (and therefore a
darker ambience).
Looking at the comparator circuit you can notice that the
D3 serves to bring R6 potential to the operational, avoiding that C3 to
discharge through it. D3 is inserted to achieve, together with the C3
capacitor, a kind of anti-commuting network indispensable to avoid the
comparator to switch at the occurrence of a very short light variation
(due for example to overflight of a bird or the movement of a person or a
car). Both in the transition from dark to light and vice versa, the
relay will start to ring because the comparator switches repeatedly
since the resistance value taken from the photoresistor oscillates in
the neighborhood of the one which determines the switching. The latter
situation could also be avoided by retroacting U1 in positive, thus
realizing a circuit hysteresis (with two different switching
thresholds): in this case we opted for the normal comparator, filtering
the voltage supplied by the divider that comprises the photoresistor by
means of a RC network.
Having said that we should only look at the power supply
circuit, consisting of the D1 diode (which protects the input terminals
from reverse polarity) is located downstream of the power supply and the
C1 and C2 capacitors (the purpose of which is to filter the power
supply, especially if taken from a power line.
The circuit requires DC voltage to work, better if
stabilized (otherwise the comparator may oscillate in the vicinity of
the threshold voltage, despite the network RC filter), at values between
9 and 12 volt. The required current is of the order of 40 milliamps,
thanks also to the adoption of a sub-miniature relay whose coil absorbs
very little (about 15 mA).
A final detail concerns the D2 diode antiparallelly placed respect to the RL1 coil and therefore interdicted in normal conditions; this component is used when the transistor, by interdicting, interrupts the current in the coil of the relay while, due to the inductive nature of the inductances, the same reacts by generating an reverse extravoltage. This tension, if not suppressed by the fact that the diode, inversely polarized, short circuits it, would damage the collector junction of T1.
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