Smart Solar-powered Street Light

This tutorial describes the design of an easy-to-make smart solar-powered street light using GreenPAK. It can be used to power any type of light efficiently and hassle-free using solar power and a battery.

A solar-powered street lamp is novel because it can be off-grid and still functioning in case of power outages. It also leaves power to be available for other purposes. The "smart" portion of the street light allows it to automatically illuminate when it detects the presence of a moving body. The resulting design is a truly energy efficient solar-powered smart street light.

This design can be scaled to larger power lights such as lighting used for infrastructure illumination to very small lights like the ones used to beautify and illuminate paths, landscapes and horticulture as well. The design of the GreenPAK will remain similar for all of these applications.

Step 1: Solar-powered Smart Street Light Function Overview

For this project Silego’s GreenPAK Configurable Mixed Signal Integrated Circuit (CMIC) has been used. The three key functions programmed into the chip will be:

• Motion Sensing

• Daylight Presence Sensing

• Battery Level Sensing

Motion Sensing

For power savings, the light is only allowed to turn on when it detects presence of any moving body. To accomplish this, a Passive Infra-Red (PIR) Sensor has been designed in. This PIR motion sensor has a single output pin along with pins for VCC and GND. A PIR sensor basically detects levels of Infrared Radiation. As soon as it detects a change in the levels of radiation, it sends out a signal notifying of the change. This sensor is one of the most commonly used for the purpose of motion detection.

Ambient Daylight Sensing

The solar panel itself is used to sense ambient light. A solar panel gives full output in full direct sunlight, and zero output with no light. These conditions can then be converted to produce appropriate levels to a digital input pin of the CMIC. By using a simple voltage divider circuit, the 18 volts generated by the solar panel can be converted to become a 3.3V signal that will represent a High-Level logic input for the SLG46140V CMIC.

Battery Level Monitoring and Charging

Battery level monitoring is also an important aspect of this system. In this design a Sealed Lead-Acid (SLA) battery has been used. SLA batteries are very versatile, dependable and cost- effective batteries. Charging methods of SLA batteries are much simpler compared to batteries with other chemistry. SLA batteries need to be charged at a constant current equal to 0.1C (here C = the capacity of a battery at fully charged scale, Ampere-hour) and a voltage of around 1.5 Volts to 2 Volts higher than the rated output voltage. Because of this, the charging circuitry can also be simpler. It is important to note that at full capacity a 12v SLA battery will give an open circuit voltage of around 13.2 volts. When discharged, the battery gives an open circuit voltage measuring a little less than 12v. But since our CMIC cannot measure such high voltages, these need to be converted down to acceptable ranges. Again, a simple voltage divider is employed to divide down the voltages to around 800mv to represent a discharged battery and around 1150mv to represent a completely charged battery. These voltage levels are then fed into the SLG46140V CMIC and compared using its Analog Comparators. As we will see in the next section, these comparators play a vital role in the overall project realization.

Step 2: Programming the GreenPAK IC

The GreenPAK IC can be easily programmed to control the smart solar street light by downloading the GreenPAK software to view the Smart Street Light Design File. Connect the GreenPAK development kit to the computer, pop an unprogrammed SLG46140V GreenPAK IC into the development kit socket and hit program. The IC will automatically be programmed to an IC that will control the smart solar street light.

Once the IC is programmed, you can keep the IC in the development kit socket for easier access to the pins, or for volume production, you can create a tiny PCB to access the chip.

With the GreenPAK IC now programmed to control the smart street light, you can skip to Step 4.

If you would like to better understand and modify the inner circuitry of the Smart Street Light Design File, Step 3 will give you an overview of how the GreenPAK smart street design file was programmed.

Step 3: GreenPAK Design Details

The GreenPAK schematic to implement this design is shown in Fig. 5.

Description of Pins Used:

• PIN3: Digital Input pin for detection of presence of ambient light.

• PIN4: Digital Input pin for detection of presence of motion or an object.

• PIN10: Analog Input Pin for battery level monitoring.

• PIN12: Digital Output with 1x Push-Pull Output Mode for controlling the LED.

• PIN14: Digital Output with 1x Push-Pull Output Mode for controlling the charge flow into the battery.

3 – Bit AND Gate:

The 3-bit AND Gate in this design ensures the light is turned ON only when all the conditions are met. Conditions such as Detection of Motion in the vicinity, Ambient Sunlight presence & Desired Battery Level are the three bits that determine the output of the AND Gate.

Battery Level Monitoring:

Two analog comparators (ACMP0 and ACMP1) monitor the battery voltage. As described earlier, two voltage levels of 800mV and 1150mV have been used to determine the state of the battery. If the measured battery voltage drops to 800mV or less, the comparator (ACMP0) gives an output of zero. This output is fed into the 3-input AND Gate which in turn gives an output of zero as well turning the light OFF on detection of a low battery voltage level.

The High Level voltage is measured during charging and the divided down voltage taken as input to Pin10 of the CMIC is fed into the comparator (ACMP1). As soon the voltage level reaches or exceeds the reference voltage set on the inverting input (1150mV in this case), the comparator outputs a high. As soon as our desired level is reached we need to cutoff battery charging, hence a Low-level Output is required and a simple Inverter is employed for this purpose.

Input from the Solar Cell:

As described earlier, when there is no ambient light present, the solar cell gives an output of zero/Digital-Low signal. Since absence of sunlight is one of the conditions for turning our Street Lamp ON, we need to convert it into a Digital- High so that our AND Gate also outputs a HIGH. Hence, an Inverter is used here as well for that purpose.

Usage of Counter as means of extending Output Period: In the above design a Counter(CNT0/DLY0) is also used to produce a certain amount of delay in turning off the signal at PIN12 of the CMIC. This creates a desired delay that avoids rapid output switching. (see Fig. 6)

Usage of Counter as means of extending Output Period:

In the above design a Counter (CNT0/DLY0) is also used to produce a certain amount of delay in turning off the signal at PIN12 of the CMIC. This creates a desired delay that avoids rapid output switching. (see Fig. 6)

Step 4: Description of External Circuits

This section describes the use of external circuitry that was needed to drive larger loads such as the 10W LED lamp, and the battery charging. To create the most efficient and energy saving circuits, MOSFETs have been used as opposed to normal BJTs. This results in faster switching time as well improved/reduced power consumption.

Battery Charging Control:

An IRLZ44N HEXFET POWER MOSFET (similar MOSFETs such as FQP30N06L can also be used) has been used which is an N-channel Enhancement Type MOSFET.

This MOSFET is specifically designed to operate with gate voltage levels (3V and 5V) that can be easily generated by small controllers and integrated circuits.

The datasheet of the MOSFET describes Threshold Gate to Source Voltage (Vgs(th)) of around 3 volts for a Drain Current ID of around 1 Amperes. This can be easily achieved by our small CMIC according to the electrical specification table for Pin 12 (similar to PIN14).

LED Lamp Control:

Similar to the charging control circuit, this part of our project also uses an IRLZ44N HEXFET MOSFET for switching a 10 Watt LED which is the main source of illumination. Since we want to control a 10 Watt LED with a 12V battery, we need to be able to supply a current of approx. 0.8 Amp. The circuit is shown in figure 8.

These circuits can be wired up on a breadboard or via deadbug circuit wiring.

Step 5: Conclusion

This tutorial demonstrated the design of a smart street lamp using the SLG46140V GreenPAK CMIC as the main controlling element. The small sized IC proved to be immensely capable at performing the task at hand while also minimizing power consumption. The provision of proper design tools also aided very effectively in the process of realization of this project. This project can also be enhanced by introducing other circuitry such as detection of dust on the Solar Panel, or an external over-ride switch to latch the LED output.