Build a low-cost solar day lamp

Article By : Vijay Deshpande

This Design Idea presents inexpensive design features that expand the solar day lamp's usefulness for several applications.

The solar day lamp is a very simple and cost effective way of utilizing solar energy. It consists of one or more solar photovoltaic (PV) panels connected to an array of LEDs. Because it has no batteries, the system is low-cost and virtually maintenance free and still provides light to areas which receive little or no daylight from sunrise to sunset. More details on designing a basic solar day lamp system can be found in my previous Design Idea (Reference 1).

Since the PV panel is usually the major contributor to the solar day lamp’s total system cost, it’s desirable to make the most effective use of its output. One of the best strategies to maximize the panel’s utilization is to use its output to drive multiple lighting sources. Since the light sources’ combined power requirements may exceed the PV panel’s output the basic solar day lamp can make use of the following features:

• Each light incorporates a pyroelectric infrared (PIR) motion sensor to ensure that it is switched on only when it is needed.
• Further efficiency can be achieved with the addition of a light sensor, which adjusts the LED lamp to take advantage of any ambient light available.

Basic design

The circuit diagram shown in Figure 1 consists of four 10 W (peak) solar panels connected in series with the following operating specifications:

• PV array voltage developed at maximum power point = Vmp = 17.5 × 4 = 70V
• Forward voltage (Vf) of a typical 1W white LED = 3.0V
• Maximum number of LEDs in series that the PV output can support = 70/3 = 23.33 (rounded down to 23)
• Residual voltage = (23.33 – 23) × Vf = 0.33 × 3 = 1V
• Number of LED arrays: 3
• Current required by each array = single panel current = 0.193, 3 panels = 0.58 A
• Wire resistance = 1 Ω (assumed)
• System voltage drop (Vs) = residual voltage – wire drop = (70 – 23×3) – (1×0.58) = 0.42V
• Current limiting resistor value = voltage drop / ILED = 0.42/0.193 = 2.17 Ω (2.2 Ω used)

Figure 1 The 40 W solar day lamp design includes a resistance current limiter.

The circuit diagram shows three LED arrays A, B, and C that are fabricated using 24 LED strips, which are readily-available. Each of the array’s 24 1 W LED elements are mounted on a metal clad PCB. Each PCB array is mounted inside an aluminum channel, which acts as both a structural frame and a heat sink (Figure 2).

Figure 2 These photos show four PV panels (left) and three LED arrays fitted inside an aluminum channel (right).

Many commercially-available LED strings come with 24 LEDs, but our proposed design calls for only 23 LEDs. Therefore, a 2.2 Ω current limiting resistor is mounted in parallel with the 24th LED, as shown in Figure 3. This bypasses the 24th LED to effectively create a 23 LED string in series with the limiting resistor.

Figure 3 The current limiting resistor is connected in parallel with 24th LED.

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Solar day lamp with PIR sensor

It’s possible to get “more bang for your buck” from the solar day lamp system by adding a fourth LED array (D) and equipping it with occupancy sensors to switch off the light when there is nobody in the room. The circuit diagram of an LED lamp equipped with a PIR motion sensor is shown in Figure 4. Array D is normally OFF. This array is turned ON whenever the PIR sensor detects the presence of a user.

Figure 4 The solar day lamp design with a PIR sensor can detect the presence of a user.

Motion sensor design and construction

The power to array D is controlled by the NPN transistor Q1 (TIP31C) and driver transistor Q2 (BC546). This transistor is controlled by the digital output from the PIR sensor. Whenever the PIR motion detector senses a warm body’s presence, its output goes high (3.3V), turning Q1 and Q2 ON. Current limiting resistor R5 (5.6 Ω) was added to hold the current passing through array D to about 0.25 A.

The PIR sensor requires a 5V supply, and it draws about 1 mA of current. For this purpose IC1 (a LM431 adjustable precision Zener shunt regulator) is used. This regulator requires only 1 mA cathode current for regulation (at its lowest input voltage). Thus, the power dissipated in the regulator is quite low, in spite of being fed by a 70V input. The equation for determining the cathode voltage (K) is given by:

Vz = Vref (1 + R9/R8),

where the reference voltage at pin (R) Vref = 2.5 V.

Substituting the values of R8, R9, and Vref in the equation, we get:

Vz = 2.5 (1 + 3300/3300) = 5V

When the user moves beyond the PIR’s sensing range, its output goes low (after the time out period). Figure 5 shows the assembled PCB.

Figure 5 The assembled PCB has a PIR sensor, LM431 IC, and driver transistors.

The motion sensor in operation

When the motion detector is activated, the output of the PV panel is divided between four LED arrays, a condition which exceeds the PV panel’s maximum output. To ensure the user gets sufficient illumination from array D, it has only 22 LEDs connected in series. The resulting lower on resistance causes it to draw slightly more current than the other arrays. Since the PV array’s output is limited, the current passing through arrays A, B, and C is reduced when the motion sensor is activated causing the lights to run at less than their full rated brightness while array D remains brighter. This is illustrated in Figure 3, where the white jumper wire bypasses the 23rd LED, not the 24th.

The left hand photo in Figure 6 shows LED arrays A, B, and C in their ON state and array D turned OFF. The photo on the right shows all four LED arrays powered up.

Figure 6 The photo on the left shows three arrays turned on and four arrays are powered on in the photo on the right.

Testing

Tests were carried out at three different sunlight intensities. The results are presented in Table 1. The data collected during near full power conditions are highlighted in green, performance data collected at half-power conditions is highlighted in blue, and data collected during operation at roughly 25% power conditions are highlighted in grey.

Table 1 Test results

Under full sunlight conditions, whenever array D turns ON, LED array A’s light output is reduced by about 23%. However, under very low sunlight conditions, array A’s output is reduced by only about 37%. The data confirms that under all sunlight conditions, array D will receive sufficient power to provide the user with enough light. The test results also show that there is a relatively small change in the total power whether three or four arrays are ON.

Notes:

1. Current and power through B and C arrays will be same as that in array A.
2. The above table is only a guideline. It may be necessary to slightly tweak the value of R5 to get your desired current through LED array D.

Applications

Homes

One simple application of such a system is in homes. If arrays A, B, and C are installed in rooms and halls of the house, array D can be installed in the bathroom along with a PIR sensor. So, whenever a user enters the bathroom, array D turns ON. When the user leaves the bathroom, array D will be switched off, following the timeout delay. If a user has more than one bathroom, or other infrequently-used rooms, they can install a type-D (motion-sensing) array in each one.

Warehouses

Warehouses require a certain amount of ambient light for safe movement of people and goods. Then, when a user goes to a particular rack, a brighter light level is required to read labels, pick or place goods, and other tasks. In this application, a motion-sensing LED array provides the needed illumination when a user approaches and then returns the area to ambient intensity when they move away from the rack.

In this application, a small change is required in array D. In the schematic shown in Figure 4, array D is normally OFF. When user presence is detected, array D turns ON. A modified schematic for warehouse-type applications is shown in Figure 7.

Figure 7 In this circuit diagram, array D is modified for warehouse-type applications.

In this design, array D′ has 23 LEDs (L70-L92) and is connected to the PV panels through R4. Hence, it is normally ON and provides the same light output as the non-motion-sensing arrays because LED L92 and R4 are bypassed through R5 and Q1. Whenever user presence is sensed and the PIR output goes high, Q1 turns ON. In this condition, only 22 LEDs (L70-L91) are ON. In this condition, array D′ draws more current and becomes brighter. Once the user moves away, sensor output goes low after the programmed time out and array D′ returns to its normal light output.

Here are the basic parameters for a 400 W warehouse lighting system:

• PV panel power rating (peak) = 100 W
• Number of panels connected in series = 4
• Number of type A arrays (without sensor) = 20
• Number of type D′ arrays (with PIR sensor) = 10*

*Note: There is no restriction on the number of type D′ arrays the system can have. If desired, type D′ arrays can be installed on nearly every rack. This is, however, based on the assumption that, at any given instant of time, all type D′ arrays will not get activated. There is no harm in increasing the number of type D′ arrays in a system, but the total number should be limited to 30.

These examples show how the basic solar day lamp design can be modified to add functionality that makes it useful in a much wider series of roles in residential and commercial environments. Part 2 of this series will describe a third variant of the design, which introduces a prioritized lighting function.