Due to its straightforward and relatively cheaper design the system can be suitably used for village street lighting or in other similar remote areas, nonetheless this by no means restricts it from being used in cities also.
The main salient features of this system are:
1) Voltage controlled Charging
2) Current Controlled LED Operation
3) No Relays used, all Solid-State Design
4) Low Critical Voltage Load Cut-off
5) Low Voltage and Critical Voltage Indicators
6) Full Charge cut-off is not included for simplicity sake and because the charging is restricted to a controlled level which will never allow the battery to over-charge.
7) Use of popular ICs like LM338 and transistors like BC547 ensure hassle free procurement
8) Day night sensing stage ensuring automatic switch OFF at dusk and switch ON at dawn.
The entire circuit design of the proposed simple LED street light system is illustrated below:
The circuit stage comprising T1, T2, and P1 are configured into a simple low battery sensor, indicator circuit
An exactly identical stage can also be seen just below, using T3, T4 and the associated parts, which form another low voltage detector stage.
The T1, T2 stage detects the battery voltage when it drops to 13V by illuminating the attached LED at the collector of T2, while the T3, T4 stage detects the battery voltage when it reaches below 11V, and indicates the situation by illuminating the LED associated with the collector of T4.
P1 is used for adjusting the T1/T2 stage such that the T2 LED just illuminates at 12V, similarly P2 is adjusted to make the T4 LED begin illuminating at voltages below 11V.
IC1 LM338 is configured as a simple regulated voltage power supply for regulating the solar panel voltage to a precise 14V, this is done by adjusting the preset P3 appropriately.
This output from IC1 is used for charging the street lamp battery during day time and peak sunshine.
IC2 is another LM338 IC, wired in a current controller mode, its input pin is connected with the battery positive while the output is connected with the LED module.
IC2 restricts the current level from the battery and supplies the right amount of current to the LED module so that it is able operate safely during night time back up mode.
T5 is a power transistor which acts like a switch and is triggered by the critical low battery stage, whenever the battery voltage tends to reach the critical level.
Whenever this happens the base of T5 is instantly grounded by T4, shutting it off instantly. With T5 shut off, the LED module is enable to illuminate and therefore it is also shut off.
This condition prevents and safeguards the battery from getting overly discharged and damaged. In such situations the battery might need an external charging from mains using a 24V, power supply applied across the solar panel supply lines, across the cathode of D1 and ground.
The current from this supply could be specified at around 20% of battery AH, and the battery may be charged until both the LEDs stop glowing.
The T6 transistor along with its base resistors is positioned to detect the supply from the solar panel and ensure that the LED module remains disabled as long as a reasonable amount of supply is available from the panel, or in other words T6 keeps the LED module shut off until its dark enough for the LED module and then is switched ON. The opposite happen at dawn when the LED module is automatically switched OFF. R12, R13 should be carefully adjusted or selected to determine the desired thresholds for the LED module's ON/OFF cycles
How to Build
To complete this simple street light system successfully, the explained stages must be built separately and verified separately before integrating them together.
First assemble the T1, T2 stage along with R1, R2, R3, R4, P1 and the LED.
Next, using a variable power supply, apply a precise 13V to this T1, T2 stage, and adjust P1 such that the LED just illuminates, increase the supply a bit to say 13.5V and the LED should shut off. This test will confirm the correct working of this low voltage indicator stage.
Identically make the T3/T4 stage and set P2 in a similar fashion to enable the LED to glow at 11V which becomes the critical level setting for the stage.
After this you can go ahead with the IC1 stage, and adjust the voltage across its "body" and ground to 14V by adjusting P3 to the correct extent. This should be again done by feeding a 20V or 24V supply across its input pin and ground line.
The IC2 stage can be built as shown and will not require any setting up procedure except the selection of R11 which may be done using the formula as expressed in this universal current limiter article
R1, R2, R3 R4, R5, R6, R7 R8, R9, R12 = 10k, 1/4 WATT
P1, P2, P3 = 10K PRESETS
R10 = 240 OHMS 1/4 WATT
R13 = 22K
D1, D3 = 6A4 DIODE
D2, D4 = 1N4007
T1, T2, T3, T4 = BC547
T5 = TIP142
R11 = SEE TEXT
IC1, IC2 = LM338 IC TO3 package
LED Module = Made by connecting 24nos 1 WATT LEDs in series and parallel connections
Battery = 12V SMF, 40 AH
Solar Panel = 20/24V, 7 Amp
Making th 24 watt LED Module
The 24 watt LED module for the above simple solar street light system could be built simply by joining 24 nos 1 watt LEDs as shown in the following image: