Emergency Light

After a recent power outage, I thought I would order some inexpensive battery backup LEDs from ebay. It takes very little light to overcome total darkness in an emergency.

I was shocked to see there weren't any little inexpensive units, everything was industrial size and not easily hidden. So I designed a circuit to charge an 18650 Li battery and, when power goes out, light a few LED's. The original circuit worked rather well. It is designed to always be active (no off switch) as a switch would ALWAYS be in to the OFF position when you needed it!!!

A later enhancement, was the addition of a simple PIR sensor. It enables the LED's when motion is detected. This works even when the battery is charging and the LED's are normally off. The result is a compact unit which lights EITHER

  • when motion is detected OR
  • when power is removed.

To simplify life, the unit charges from the ubiquitous 5v wall wart with a micro USB connector. There is a single LED to indicate that external power is available and there are 4 BRIGHT WHITE LEDs that illuminate.

The unit is supplied as a partial kit: the SMD components are all installed. Through-hole parts are NOT installed as there are a couple different ways to build the board. Depending on the orientation of the unit, the LEDs can be all pointed in the same direction or pointed left, right, down to fit the users particular application. A plastic case is available which can easily be drilled with a 1/4" bit to allow the LEDs to point in the desired directions.

The battery is an 18650 Li cell. These are available in a variety of power capacities up to nearly 10AmpHours. The circuit on the PCB protects the cell from overcharge and over discharge, which is essential for Li batteries. The is also a LED driver that maintains the LED brightness over the range of the battery voltage. Near the end of the battery capacity, the LEDs may blink on/off. When the battery reaches its lower limit and the LEDs go off.

PIR

A PIR sensor is used to detect motion. The design of the sensor is such that a person walking across the pattern is easily detected while there is lower sensitivity to a person walking directly towards the sensor. This is an effect of the optics. There is no ambient light detection. Therefore the LEDs will illuminate regardless of ambient light. The PIR triggers the LEDs for about 10 seconds.

External 5v

External may be applied from a wall wart or any source of +5v with a USB Micro connector. Once the +5v is applied, the circuit goes active and any removal of power will result in the LED's illuminating until power is return or the battery falls to its lower limit. The LED's will operate for hours from a single 18650.

Kit Assembly

The only trick to assembling the kit is to install the LED's correctly. The LONGER LED lead goes into the hole with the ROUND PAD. This is true for both the charging indicator and the bright LED's. Install the LED's with about 1/2" of lead length from the bottom of the LED to the PCB. This will provide some flexibility in turning the LED's to the side or down, depending on the final orientation of the box. There are a total of 5 LED's to install and solder.

Install the battery holder in the case and solder the 2 leads from the battery holder to the PCB as shown in the photo. Make sure the unit is working before installing the PIR sensor.

Install and solder a 3 pin female header connector as shown. This is the receptacle for the PIR sensor. Do not mount the connector close to the PCB. It should be raised up 3-4mm (1/8") when the body is horizontal. The photo indicates the correct polarity for the PIR sensor. Check the small PCB for the + and - symbols and install as shown. The (-) lead is towards the battery.

Mounting the Unit

It is best if the unit is mounted under a cabinet shelf below eye level. This prevents the LEDs from shining in your eyes. It is like a sharp cutoff projector. Velcro with strong adhesive can be used to mount the unit to the underside of a shelf or kitchen cabinet. Place the Velcro off center, above the battery.

The light can illuminate a large area; orient the unit to project across a room or down a hallway to maximize the range. It is easily good for 20 x 20 ft.

Ordering

There are several options depending on your interests.

1. SMD Assembled PCB Only - This is a PCB with all SMD devices installed. User installs 5x LED's and 1x USB connector (supplied). Battery holder and battery included. $8

2. Adds the case to #1. This is a simple black plastic case. The PCB is designed to match the case. You need to make holes for the LED's and the USB conn. $10

3. Adds PIR detector to #2. You need to solder the connector (3 pins) and make a hole in the case. $12

4. Options

4.1 Case $2.50

4.2 PIR kit (PIR assembled PCB + connector) $3.50

4.3 5VDC wall wart $4

4.4 High Capacity Battery 9000 Amp Hour (in place of standard battery) $3