Today solar panels and PV cells have become very popular and in the near future we would possibly see everyone of us using it in some or the other way in our life. One important use of these devices has been in the field of street lighting.
The following article discusses one such interesting circuit which comprehensively explains the making of a 40 watt fully automatic solar street light system circuit (exclusively designed by me).
The circuit which has been discussed here has most of the standard specifications included with it, the following data explains it more elaborately:
LED Lamp Specifications
Voltage: 12 volts (12V/26AH Battery)
Current Consumption: 3.2 Amps @12 volts,
Power Consumption: 39 watts by 39nos of 1 watt LEDs
Light Intensity: Approximately around 2000 lm(lumens)
Input: 32 volts from a solar panel specified with around 32 volts open circuit voltage, and short circuit current of 5 to 7 Amps.
Output: Max. 14.3 volts, current limited to 4.4 Amps
Battery Full - Cut OFF at 14.3 volts (set by P2).
Low Battery - Cut OFF at 11.04 volts (set by P1).
Battery charged at C/5 rate with float voltage restricted to 13.4 volts after “battery full cut OFF”.
Automatic Day/Night Switching with LDR Sensor (set by selecting R10 appropriately).
In this first part of the article we will study the solar charger/controller stage and the corresponding over/low voltage cut-off circuit, and also the automatic day/night cut-off section.
R1, R3,R4, R12 = 10k
R5 = 240 OHMS
P1,P2 =10K preset
P3 = 10k pot or preset
R10 = 470K,
R11 = 100K
R8=10 OHMS 2 WATT
T1----T4 = BC547
A1/A2 = 1/2 IC324
ALL ZENER DIODES = 4.7V, 1/2 WATT
D1---D3,D6 = 1N4007
D4,D5 = 6AMP DIODES
IC2 = IC555
IC1 = LM338
RELAYS = 12V,400 OHMS, SPDT
BATTERY = 12V, 26AH
SOLAR PANEL = 21V OPEN CIRCUIT, 7AMP @SHORT CIRCUIT.
Solar Charger/Controller, High/Low Battery Cut OFF and Ambient Light Detector Circuit Stages:
CAUTION: A charge controller is a must for any street light system. You may find other designs on the internet without this feature, simply ignore them. Those can be dangerous for the battery!
Referring to the 40 watt street light circuit diagram above, the panel voltage is regulated and stabilized to the required 14.4 volts by the IC LM 338.
P3 is used for setting the output voltage to exactly 14.3 volts or somewhere near to it.
R6 and R7 forms the current limiting components and must be calculated appropriately as discussed in this solar panel voltage regulator circuit.
The stabilized voltage is next applied to the voltage/charge control and the associated stages.
Two opamps A1 and A2 are wired with converse configurations, meaning the output of A1 becomes high when a predetermined over voltage value is detected, while the output of A2 goes high on detection of a predetermined low voltage threshold.
The above high and low voltage thresholds are appropriately set by the preset P2 and P1 respectively.
Transistors T1 and T2 respond accordingly to the above outputs from the opamps and activates the respective relay for controlling the charge levels of the connected battery with respect to the given parameters.
The relay connected to T1 specifically controls the overcharge limit of the battery.
The relay connected to T3 is responsible for holding the voltage to the LED lamp stage. As long as the battery voltage is above the low voltage threshold and as long as no ambient light is present around the system, this relay keeps the lamp switched ON, the LED module is instantly switched OFF in case the stipulated conditions are not fulfilled.
IC1 along with the associated parts forms the light detector circuit, its output goes high in the presence of ambient light and vice versa.
Assume it's day time and a partially discharged battery at 11.8V is connected to the relevant points, also assume the high voltage cut off to be set at 14.4V. On power switch ON (either from the solar panel or an external DC source), the battery starts charging via the N/C contacts of the relay.
Since it's day, the output of IC1 is high, which switches ON T3. The relay connected to T3 holds the battery voltage and inhibits it from reaching the LED module and the lamp remains switched OFF.
Once the battery gets fully charged, A1's output goes high switching ON T1 and the associated relay.
This disconnects the battery from the charging voltage.
The above situation latches ON with the help of the feedback voltage from the N/O contacts of the above relay to the base of T1.
The latch persists until the low voltage condition is reached, when T2 switches ON, grounding T1's base biasing and reverting the top relay into the charging mode.
This concludes our battery high/Low controller and the light sensor stages of the proposed 40 watt automatic solar street light system circuit.
The following discussion explains the making procedure of the PWM controlled LED module circuit.
The circuit shown below represents the LED lamp module consisting of 39 nos. 1 watt/350 mA high bright power LEDs.The whole array is made by connecting 13 number of series connections in parallel, consisting of 3 LEDs in each series.
How it Works
The above arrangement of LEDs is pretty standard in its configuration and does not focus much importance.
The actual crucial part of this circuit is the IC 555 section, which is configured in its typical astable multivibrator mode.
In this mode the output pin#3 of the IC generates definite PWM wave-forms which can be adjusted by setting the duty cycle of the IC appropriately.
The duty cycle of this configuration is adjusted by setting P1 as per ones preference.
Since the setting of P1 also decides the illumination level of the LEDs, should be done carefully to produce the most optimal results from the LEDs. P1 also becomes the dimming control of the LED module.
The inclusion of the PWM design here plays the key role as it drastically reduces the power consumption of the connected LEDs.
If the LED module would be connected directly to the battery without the IC 555 stage, the LEDs would have consumed the full specified 36 watts.
With the PWM driver in operation, the LED module now consumes about 1/3rd power only, that is around 12 watts yet extracts the maximum specified illumination from the LEDs.
This happens because, due to the fed PWM pulses the transistor T1 remains ON only for 1/3rd of the normal time period, switching the LEDs for the same shorter length of time, however due to persistence of vision, we find the LEDs to be ON all the time.
The high frequency of the astable makes the illumination very stable and no vibration can be detected even while our vision is in motion.
This module is integrated with the previously discussed solar controller board.
The positive and the negative of the shown circuit needs to be simply connected to the relevant points over the solar controller board.
This concludes the whole explanation of the proposed 40 watt automatic solar LED street lamp circuit project.
If you have any questions, you may express them through your comments.
UPDATE: The above theory of seeing high illumination with lower consumption due to persistence of vision is incorrect. So sadly this PWM controller only works as a brightness controller and nothing more!
Circuit diagram for the street light LED PWM controller
R1 = 100K
P1 = 100K pot
C1 = 680pF
C2 = 0.01uF
R2 = 4K7
T1 = TIP122
R3----R14 = 10 Ohms, 2watt
LEDs = 1 watt, 350 mA, cool white
IC1 = IC555
In the final prototype the LEDs were mounted on special aluminum based heatsink type PCB, it is strongly recommended, without which the LED life would deteriorate.