Emergency lights are very useful electrical gadgets and are quite popular, too. During a power failure these portable lights are able to instantly illuminate through battery back up and never allow us to stumble in the darkness. Although you will find a variety of them in different shapes and sizes in the market, but building your own emergency light at home can be a totally different experience. It will not only help you to create a quality design but also will acquaint you to the technical aspects of the unit. Besides, you also get an opportunity to customize the circuit as per your own needs.
My previous design of an LED emergency rechargeable light, though very consistent, unfortunately, is not a plug-in type and also the charging facility is a bit primitive.
Due to the repeated requests from the folks I designed the present circuit, which hopefully should fulfill most of the required demands.
The idea of the present emergency light circuit is truly compact, highly advanced, and may be kept plugged into the mains socket for a permanent automatic operation. The use of a Li-Ion battery makes it infinitely long lasting.
Remember, since the proposed circuit is not isolated from AC mains, is extremely dangerous when connected to the supply line in an uncovered position (box unscrewed), maintain extreme caution while testing or handling.
Let’s try to understand its circuit description.
The circuit operation can be understood through the following points:
Referring to the figure (click to enlarge), we see that a single window discriminator IC TCA 965 forms the heart of the circuit and is entirely responsible for providing a safe charging voltage to the Li-Ion battery (5 - 6 V/800mA).
These types of batteries specifically are quite vulnerable to over charging, and “dislike” being over charged. The situation may cause drastic results like shortening its life (reduce no. of charge/discharge cycles), swelling it up, or even an explosion.
The IC TCA 965 plays a major role in avoiding the above conditions and switches OFF the charging process when the battery reaches about full charge. To get complete information regarding the functioning of this wonderful IC please see How to Build an Automatic Battery Charger.
As long as AC supply input is present, the circuit appropriately charges the battery and switches OFF as explained above.
The rectified and stabilized positive supply through D1, Z1, C3 and D2 prevents transistor T2 from switching ON the LEDs and thus they are kept switched OFF.
The moment mains power is disrupted, T2 becomes forward biased through resistor R7 and instantly connects the battery positive to the LED bank and illuminates them.
The battery now starts discharging through the LEDs and after a period of time (when the battery voltage falls below 5.5 V) the LEDs hardly produce any light and shuts OFF automatically. This happens because the minimum forward voltage required to light up white LEDs is around 3.5 volts (plus series voltage drop).
Once the power is restored back to this emergency rechargeable light circuit, T2 is switched OFF, the battery starts charging and the cycle repeats.
The construction procedure of this plug in emergency light circuit is simple and is carried out through the following points:
Over a correctly sized general purpose PCB, begin the construction by inserting the IC first and soldering its leads.
Now go on stuffing the other associated parts over the PCB, properly optimizing their positions around the IC so as to facilitate easy interconnections and make the circuit as compact as possible.
You will need another piece of general purpose PCB, for fixing the LEDs. Do it as per the shown circuit schematic. Be careful not to overheat the LEDs while soldering as that may reduce its efficiency. Also, maintain correct polarity while fixing them.
After completing the assemblies of both the PCBs connect their relevant points together using small lengths of flexible wires.
All resistors are ¼ watt CFR 5% unless otherwise stated.
R1 = 1M,
R2 = 1K,
R3, 4, 5, 6 = 470Ω,
R7 = 2K7,
R8 = 56Ω,
C1 = 2µ/400V PPC,
C2 = 1µ/25V,
C3 = 100µ/25V,
P1, P2 = 10K LINEAR PRESET,
P3 = 100Ω PRESET LINEAR,
Z1 = 9V/1W ZENER,
DI, D2 = 1N4007,
T1 = 8550,
T2 = BC557,
IC = TCA 965,
LEDs = WHITE, 5mm, HI-EFFICIENCY,
BATTERY = 6V/800mAH, Li-Ion
GENERAL PURPOSE PCB AS PER SIZE,
PLASTIC ENCLOSURE, AC PINS, WIRES, HARDWARE ETC.
Testing procedure is simple and is done as follows:
Do not connect the circuit to the AC mains for the testing purpose; rather use an external variable 0 – 6V AC/DC power supply across capacitor C2. Also, do not connect the battery to it as yet.
Initially keep the presets P1 and P2 at their midways (don’t bother about the LEDs lighting up randomly).
Reduce the supply voltage to about 5.5 volts and adjust P1 so that LED connected at pin #2 just lights.
Increase the supply voltage to 6.5 volts, adjust P2 to make LED connected at pin #14 just light up, shutting OFF the previous LED.
The above procedures will set up the lower and the higher charging levels respectively.
Now connect the battery to the circuit, the LEDs should remain switched OFF, indicating that T2 is operating properly.
Removing the externally connected power supply should instantly illuminate the LEDs, again indicating the proper functioning of T2.
Now, maintaining UTMOST CAUTION, connect the circuit to the AC mains - you will find LED connected to pin # 13 lights up, displaying a normal charging process. If the LED at pin #14 lights up, would mean the battery is fully charged and the process inhibited.
Enclose the whole assembly inside a suitable good looking PP box. Fix the LEDs appropriately at the front panel and also the AC mains pin outlet.
This concludes the setting procedure and now you may just plug-in the unit permanently into the mains socket, so that it operates and illuminates your house the moment power fails. Occasionally, manually switch OFF the AC mains to check the proper functioning and the consistency of this plug in emergency light.