Sundial. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Time is fleeting and we cannot control it. For millennia, man has been trying to stop time, but, alas, as a result, he only watches its course.

The sundial is the oldest instrument for keeping track of time. They have been used for centuries, and the charm that is inherent in the sundial, apparently, will never fade. The sundial we use today is structurally the same as it was in ancient times, and hasn't changed a bit in the last few millennia.

This article presents a completely new design based on the principle of a sundial. Like other homemade products, our design is completely autonomous and does not require external power to work.

The operation of a classic sundial

In a classic sundial, time is determined by the shadow of a gnomon or pin cast by the sun on a circle with numbers corresponding to the time of day (Fig. 1). The circle is oriented so that the shadow of the pin indicates the current time of day.

Ris.1

Our upgraded sundial works in a very similar way. Unlike traditional sundials with a fixed base, our clocks have a mechanism located on a turntable. The latter is connected to the fixed base of the clock with the help of an electric motor shaft. The table can be rotated by a low-speed motor around the circle at an angle of 360 °.

The motor is controlled by a complex electronic circuit. Unlike a classic sundial, the advantage of this circuit is that the electronics detect the position of the shadow and drive the motor to follow the sun.

Sun Tracking

The electronic circuit contains two photosensors (phototransistors Q1 and Q2) and two voltage comparators (IC1 and IC2) (Fig. 2). The photosensors are connected in series with resistors R1 and R2, forming a voltage divider, the signal from which is taken at the connection point and R2.

Ris.2

The reference voltage to the comparators is supplied from a divider formed by resistors R3, R4 and R5. Thus, a bridge circuit is obtained, one arm of which is formed by the elements Q1, R1, R2 and Q2, the other by resistors R3, R4 and R5. The second arm of the bridge has an unusual appearance, since the output signal here is not taken from a common point, as, for example, is done in the first arm of the bridge. Instead, two different voltages are taken from the terminals of resistor R4.

The potential at the top terminal of resistor R4 is higher than the potential at its bottom terminal. The higher voltage is applied to comparator IC1, the lower voltage is applied to IC2. Because of the difference in reference voltages, the comparators will operate at different input voltages.

Looking closely at the circuit, you can see the "cross" connection of the comparators, i.e. the negative input of IC1 is connected to the positive input of IC2. This leads to an unexpected effect.

To understand how the circuit works, let's apply voltage to its input. Assume that the input voltage is lower than the reference voltage of comparator IC2. Looking at comparator IC1, we see that its output will go high because the voltage at its non-inverting input is higher than at its inverting input. On the other hand, the output of IC2 will be negative because the voltage at its inverting input is greater than the input signal voltage.

As the input voltage increases, there comes a time when the voltage at the non-inverting input of IC2 becomes greater than the reference voltage taken from resistor R5. Comparator IC2 will switch and its output will be positive. However, comparator IC1 does not respond to this change in voltage, since the voltage at its input is one-third higher than the reference voltage of comparator IC2.

When the input signal exceeds the reference voltage of comparator IC2, its output will go negative. Note that the output voltages of both comparators are the same (positive) when the input voltage is between the upper and lower limits defined by resistor R4.

The change in input voltage depends on the intensity of light falling on the phototransistors. When more light falls on the phototransistor Q1 than on Q2, the input voltage is high. Conversely, when more light falls on Q2 than on Q1, the input voltage is low. When both phototransistors are illuminated equally, the signal takes on an average value between the two limits.

Drive unit

By connecting an electric motor between the outputs of the comparators, we could actually control its rotation using phototransistors. As shown earlier, both outputs are positive only when the phototransistors are equally illuminated. Turning off transistor Q1 causes comparator IC1 to switch, its output going low, while IC2's output remains high. The motor will start to rotate.

Dimming transistor Q2 has the opposite effect.

The output of IC2 is set low and IC1 remains high. The motor will also start to rotate, but in a different direction. In other words, the motor is controlled by illuminating the phototransistors. To eliminate the unstable operation of the motor near the zero point, a dead zone is created by applying various reference voltages to the comparators.

In fact, the comparator cannot directly control the electric motor. To increase the output power of the comparator, an IC3 chip is used, which controls the electric motor.

Structurally, our model is made in such a way (Fig. 3) that the gnomon (the central moving part of the device) shades one or another transistor depending on the position of the sun. The motor sets in motion and rotates the turntable until both transistors are equally illuminated, in other words, equally accurately directed towards the sun. Now, by the position of the gnomon, you can determine the time of day.

Ris.3

After carefully reading the above explanation, you probably noticed that there was no limit to the amount of light required to operate the device. As long as both photosensors receive the same amount of light, the entire device is at rest. As soon as one photosensor receives more light than the other, the motor will start to move.

This means that the sundial will follow the sun even if it is hidden in haze or clouds, which the classic sundial could not do. In fact, by adjusting the value and R2, you can even follow the movement of the moon in the night sky!

The sundial is powered by three nickel-cadmium batteries. Along with powering the motor, the batteries supply electricity to the electronic circuit. The batteries are charged from a small solar battery during the day. To prevent the batteries from discharging through the solar panel at night, a blocking diode is included in the circuit.

Watch design

The sundial is made from a sheet of acrylic plastic such as Plexiglas. First, cut out a circle of plastic with a diameter of 26 cm. Remove a disk with a diameter of 21 cm from its central part. Be careful not to split the remaining ring: it will serve as a dial, and the smaller circle will serve as a "moving table".

Then cut a square with a side of 17 cm from a sheet of plastic. Cut it diagonally into two isosceles triangles that will serve as the sides of our gnomon.

To prevent light penetrating through the transparent plastic sides of the photodetectors, they must be painted, preferably from the inside. Painting from the inside allows you to maintain the gloss of the plastic, while creating a sense of its depth, and increase the life of the paint. Opaque dye of any color is suitable for coloring.

Finally, cut out a plastic plate 24 cm long and 6 cm wide on which to place the solar panel. Connect a battery of nine solar cells sized 2,5x5,3 cm2 in series and arrange along the length of the plate (battery length is 22,5 cm). The total battery output voltage should be 4 V at 100 mA. Using this information, if necessary, you can change the structural dimensions of the battery.

Now you need to fix the motor (with the axis down) to rotate the movable table with a diameter of 21 cm. The axis of the motor is passed through a hole drilled in the center of the table, and the motor itself is fixed to the table with two screws or glue.

Before continuing work, a hole with a diameter of 6 mm must be drilled in each triangle. Draw a mental line between the base of the right triangle and the top of the right angle. This line is the height of the triangle, if we take the hypotenuse as the base. The hole is drilled at a distance of approximately 5 cm from the top at an angle of 45 ° to the plane of the triangle towards its base (hypotenuse). Once the PCB assembly is completed, phototransistors will be fixed in these holes.

PCB design

On the printed circuit board is the electronic part of the sundial circuit. The pattern of the PCB conductors is shown in fig. 4, the placement of parts on the board - in fig. 5. All elements must be soldered at the appropriate points on the board, with the exception of phototransistors.

Ris.4

Ris.5

Phototransistors are placed last. Phototransistor Q1 is soldered on one side of the PCB and Q2 on the other. Leave the full length of the transistor leads, do not shorten them. Now solder the motor and battery leads to the PCB.

At this stage, it is necessary to conduct a preliminary check of the operability of the circuit. Carefully bend the leads of the phototransistors so that the transistors look in one direction. If the circuit is balanced exactly, the device must be stationary. When alternately closing the phototransistors, the motor must rotate in opposite directions.

If the motor continues to rotate with the exact direction to the light source, then the characteristics of the phototransistors do not match. If the difference is small, then it can be eliminated by choosing the values ​​of the resistors and R2. You can check the balancing of the bridge by connecting a voltmeter to the connection point of the resistors. With a large imbalance, it is necessary to select phototransistors with similar characteristics.

Now it's time for the final assembly of the sundial. Glue one phototransistor into the 6 mm diameter holes drilled in the triangular sidewalls. It is necessary to carefully fix the triangular sidewalls on the turntable, then the phototransistors will be directed at an angle of 45 ° to the horizon.

Glue the painted triangular sides to the turntable with acrylic adhesive. They must be placed parallel to each other at an equal distance from the edges of the table, this distance depends on the size of the motor used.

Solar battery

Carefully, so as not to melt the plastic, solder the conductors coming from the solar panel to the printed circuit board. Then glue the plate with the solar panel placed on it to the long sides of the triangular sidewalls. You will see that the edges of the plate protrude from the sides of the triangular plates by approximately 6 mm. It's made on purpose.

The protruding edge casts a shadow on the sidewall of the gnomon and slightly obscures the phototransistor. In order to avoid translucence of the plate in these places, paint over the edges with opaque paint.

It is necessary to avoid getting paint on the parts to be glued together. It is better to paint over these areas after gluing.

If the installation is done correctly, the motor will rotate the turntable according to the shading of the photo sensors. When rotating the platform in the opposite direction, swap the motor leads.

Finally, to protect the gnomon from rain and moisture, cover the remaining open side with a 17x5 cm2 strip of plastic. This part also needs to be painted over to avoid unwanted light.

Finishing

In order for the clock to start working, it is necessary to attach the motor shaft to the supporting base. They can be a piece of wood, metal, stone or other material into which a metal sleeve with a hole for the motor shaft is inserted and glued.

A large plastic ring, cut from plastic during the manufacture of the turntable, is located around the sundial and serves to indicate the time. It is also attached to the outer base.

A sundial looks good if you first paint the circle with gold or copper paint, and then attach 13 Roman numerals to it. Start with the number VI (6) and place the numbers on a semicircle, moving clockwise until you again reach the number VI (6).

Both numbers VI (6) are located opposite each other (at an angle of 180 °), and the Roman numeral XII (12), corresponding to noon, is at a right angle (90 °) to both numbers VI. In fact, the clock face is compressed to a semicircle, the other half remains clean (night hours).

To set the sundial, simply rotate the circle until the pointer shows the correct time, then lock it. When the sun moves across the sky, the gnomon will follow it.

Time Correction

According to the seasonal change in the position of the sun in the sky, there is a slight difference between the true and the time shown. The error can be corrected by calculations using the data in the table.

Now you have a modern sundial with a traditional look.

Author: Byers T.

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