The following LED emergency light with battery over charge protection feature circuit was designed by me in response to the request sent by PP.
Main Features
In this article I have explained an LED emergency light circuit with advanced features such as,
- over charge battery cut off,
- day time auto-disable,
- and need less to say that the circuit switches ON the LEDs automatically when AC mains fails and reverts to charging mode when power is restored.
- The good thing about this circuit is that it incorporates ordinary, cheap components which can be easily procured from the local market.
Circuit Operation
Let's try to understand the circuit functioning with the help of the following points:
IC1 which is our very own IC555 has been set as a comparator. During day time, the light over LDR keeps the LDR resistance low such that the potential at pin #2 of the IC is kept well over 1/3Vcc. This situation ensures that the output of the IC at pin #3 stays at logic high.
The logic high at pin#3 of the IC keeps T1 switched ON, which consequently keeps T2 switched OFF.
With T2 switched OFF, the LED array remains inhibited from the ground connection and therefore the whole white LED array also stays shut off.
Another factor that keeps T1 switched ON and T2 switched OFF, is the voltage from the transformer power supply stage.
This function is implemented via the resistor R9. This also means that as long as mains AC is available, T2 is restricted from conducting and therefore the LEDs cannot light up.
Now suppose the mains power to the transformer fails, and assume that this happens during night or complete darkness, pin#3 of IC555 reverts to zero and also there's no voltage from the power supply, means T1 has absolutely no base bias and therefore has to switch OFF.
This instantly prompts T2 to switch ON and consequently the entire LED array also switches ON, providing the required emergency illumination to the surrounding.
MAKE SURE THAT THE LIGHT FROM THE LED DOES NOT FALL OVER THE LDR, WHICH MIGHT TRIGGER A RAPID UNDESIRABLE SWITCHING OF THE LEDS.
The battery charging section consists of T3, T4 and the associated parts. P1 is set such that it switches ON T3 when the battery voltage reaches just above 14 volts.
The moment this happens, T4 switches OFF, cutting of the negative supply to the battery and restricting any further charging of the battery.
Diode D2 ensures that the battery receives the negative supply during the charging process only through T4 and also provides a normal negative path to to T2 and the LED array when they conduct.
The left side LED indicates, mains power ON or presence of day light.
The LED at the right side indicates, battery is charging.
Parts List
- R1 = 2M2
- R2 = 1M
- R3, R4, R5, R9, R6, R7, R8 = 4K7
- ALL LED RESISTORS = 330 OHMS
- D1, D2, D3 = 1N4007
- D4----D7 = 1N5402
- C1 = 1000uF/25V
- C2 = 1uF/25V
- T1, T3 = BC547
- T4, T2 = BD139
- Z1, Z2 = 3V/400mW
- P1 = 10K PRESET
- IC1 = IC 555
- TRANSFORMER = 12V, CURRENT = 1/10 OF BATTERY AH
- LEDS = WHITE 5mm, OR AS PER CHOICE.
- BATTERY = 12V, AH = AS PER LED POWER AND BACK-UP REQUIREMENTS.
Using a Single PNP BJT
The above circuit can be much simplified by eliminating the IC555, and by using just a single PNP transistor instead of two NPN in the battery auto-battery cut of section.
P1 is used for adjusting the ambient light threshold at which the LEDs stop illuminating.
P2 is set such that at 14.6V (across the battery terminals) the base LED becomes very dim, hardly visible, and at 12.5V it's brightly lit.
Adding a Solar Panel
The above circuit can be also coupled with a solar panel for getting an automatic charging facility from both the sources that is from the panel during day time and from mains after the sun sets.
Parts List
R1,R2,R3, R4, R5 = 1K
P1 = 470K
P2 = 1K
C1 = 1000uF/25V
D1---D5 = 1N4007
T1 = BC547
T2 = 8050
T3 = TIP127
ALL LED RESISTORS = 330 OHMS
LEDS = WHITE, 5MM
LDR = ANY STANDARD TYPE
TRANSFORMER = 0-12/1AMP
hello sir
thanks for voluntarily sharing your innovative ideas and knowledge with us on this open platform.
I am a senior student reading electrical engineering and currently working on my senior project (Automatic Emergency light) with which I’m finding it so difficult to come up with a comprehensive Circuit Diagram. Please, I am appealing that you please help me do a circuit diagram for Automatic Emergency Light with Relay (and NOT LDR) because I want the LED bulbs should glow only during power outage and using the relay as automatic switch.
Hello Prince, a relay is actually not required, you can control the LED using a transistorized circuit itself. You can refer to this post for more info:
https://www.homemade-circuits.com/how-to-make-efficient-led-emergency/
I have a 3.6V recheargeable battery powering an SMD array, is it possible to use a lower V transformer (5 v) and keep same circuit?
yes that’s OK, just remove Z1, and Z2 and replace with direct links.
is it possible to cuf off load (led panel) when battery get discharged at 10.5v
if you add 3 LEDs in series instead of two, then this can be implemented automatically
please tell me
sir which software your using for designing the circuit???
Supeeeeer!!!!!
Hi
How to adjust the over charge cut off circuit for a 4v 1ah battery?plz help..
Hello Swagatam,
I built the 2nd circuit you gave and checked a couple of times to make sure everything is connected to each other well. I got some issues though. First of all, I would like to use a led strip (rated 12V – 4,8W per meter), but I don't know how much length I need yet. I also discarded the LDR section with the LDR, P1 and R1. A couple of times I tried, it worked as expected. Later on, it didn't work. Here are the issues:
1- While it was working fine, when A/C mains was cut off, it took some 7-8 seconds for the leds to light up 🙂
2- As you mentioned in the description and in some of the answers you gave, I adjusted P2 to get 14V from battery terminals and I got a dim red light, but when it went down to 12V the light got dimmer, almost not visible.
3- With the use of strip leds, I guess I fried T2 (S8550), because after a few tries turning A/C power on and off, the leds starting lighting up with the A/C power on turning off when the power goes off 🙂 Also, the transistor was pretty hot.
Hello Alihan,
the issues which you have mentioned can never take place because there's no possibility of those happening.
there's no capacitor associated with the T1/T2 so the delay effect cannot happen.
I hope you have only removed the LDR, R1 and P1 in your circuit and kept rest as shown in the diagram with R2 connection intact with the base of T1?
T2 can only become hot if the LEDs current is much higher than its tolerance level.
The charging LED will become dim as the battery voltage reaches 14V and vice versa so that's fine.
when mains is present T1 will be triggered ON through R2, which will keep T2 switched OFF, keeping the LED bank switched OFF, so the LEDs can never light up when mains is present unless one of the transistors has gone faulty.
by the way this a very basic and crude design so you cannot expect high precision from it.
for a high precision you can probably refer to the following design.
https://www.homemade-circuits.com/2015/10/smart-emergency-lamp-circuit-with.html
Thanks for your reply, you've been pretty helpful..
The only direct connection between T1/T2 and the capacitor is the negative pole, which I guess not very important.
As I presumed, I had fried T2 and when I replaced it, it worked fine again. Then it got fried again as I forgotten to remove some part of led strip before plugging it back :))
I realize that it is a crude design, still it is important for me to start building something small and make it useful at home..I have seen the other project you have mentioned and I will surely have some questions for that one too 🙂
If I change T2 to a more powerful transistor, would I be able to use it with a load of 2-2,5A? Or I need to upgrade the whole circuit for that? For T2, I found BDX53C, which seems to be pretty powerful, still not sure if I can use it for the same purpose, or do you have another suggestion?
Thanks in advance..have a great week
yes obviously T2 must be dimensioned as per the LED current otherwise it will keep blowing of.
Only T2 and R3 will need to be upgraded as per the load current rest will as is unless any other parameter such as battery is upgraded too.
R3 = Batt supply – LED operating voltage / LED operating current
Hi Sir. We want to make a request. Is it possible to add a feature on this circuit? We would like to add a fan in this circuit so we can use it when there is power failure. Thank you.
Hi Mayu, you can connect the fan exactly where the LEDs/resistor network is connected that is across the positive of battery and the collector of T2
Sir
Please reply me
I have 12v 12AH led acid battery. I need to charge it. Please tell me the spec of 12v transformer
Varun, you can use a 0-12V 2amp transformer for it, connect a bridge-rectifier with a 6800uF/25V to getting the required 14V 1 amp charging output for the battery.