Addendum to this project based on one of the comments received: It was suggested that the PIR could be used directly as a switch, thereby eliminating the transistor. While I couldn't figure a way around the transistor, I did hit on a way to further simplify the already simple circuit used for this project. I have included it as Configuration 2. A valid point was made that the light produced is not very bright, but it does produce enough illumination so that you can safely navigate the stairway.

Night lights are a favorite project for DIYers and there are many, many projects both on Instructables and the internet in general. However, I was in need of a light that was fairly bright, compact, and self powered to place on a stairwell with no electrical outlets. My (admittedly brief) internet searches did not turn up what I needed, so this seemed like a perfect project for someone relatively new to electronics.

Design Features

1. The light had to be bright enough to illuminate 2 sets of stairs so someone could safely navigate them at night.

I settled on a 12 V LED panel light consisting of a 3 x 3 array of piranha LEDs that I found on eBay.

2. The light had to contain its own power.

I decided on an A23 12 V battery. Solar powered rechargeables were ruled out since the unit had to be compact.

3. The night light should be designed for maximum battery life.

The 12 V A23 battery only has a 55mAh capacity so the design of the light had to optimize battery life, or I'd be changing batteries every few days. The obvious fix was to have the light turn on only at night and only when someone was using the stairs. Both these features were accomplished by including a PIR motion sensor with built-in light level activation.

OK, enough intro - let's get on with the project.

Step 1: List of Materials

List of Materials for Original Configuration

12 V 3x3 piranha LED array panel light (found this on eBay)

Parallax wide angle PIR sensor (part # 28032)

LM 7805 voltage regulator

BC547 NPN transistor or equivalent

1k ohm resistor

2k ohm resistor

A23 battery and holder

breadboard, hookup wire, PCB

project box ( I used a plastic 100x60x25mm box)

Skills needed for this project: Ability to solder and read a simple schematic.

List of Materials for Second Configuration

12 V 3x3 piranha LED array panel light

HC-SR501 PIR Sensor

1k ohm resistor

BC547 NPN transistor or equivalent

A23 battery and holder

breadboard, hookup wire, PCB, project box

Step 2: Schematic

Original Configuration - works good

The top schematic is fairly simple. This design draws about 6mA of current from the battery, with about 3mA across R1 and the LED display. Voltage to the PIR motion sensor (3 to 6 VDC operating range) is about 3.5 V, and voltage and current delivered to the transistor base are well below max values. The PIR sensor has built-in continuous and nighttime modes that are set with a jumper. This eliminates the need to include a light sensing circuit in the project design. The sensor also has a sensitivity adjustment.

Configuration 2 - simpler, cheaper, and more reliable - this is the circuit of choice

The bottom schematic is the simplified design using a HC-SR501 PIR sensor. It is considerably cheaper than the Parallax sensor and has an operating voltage range of 5 to 20V, so the voltage regulator and 2K resistor can be eliminated. This sensor also has sensitivity (3 to 7 meters) and time delay (5 sec to 5 min) controls.

Step 3: Hacking the HC-SR501 PIR Sensor

The only problem with the sensor used for Configuration 2 is that continuous or nighttime capability is not factory installed. You can convert this sensor to operate only when it's dark by adding a photoresistor. The photo on the left shows the sensor with the lens removed. The two solder pads enclosed by the red circle are where the LDR needs to be attached. Right hand photo shows the photoresistor in place.

Step 4: Breadboard Configuration

I seemed to have gotten everything right on the breadboard layout, because every time I moved, the light came on.The picture shows the light panel turned on with the PIR sensor activated. Notice the red LED that lights in the sensor when it's activated.

Step 5: Fritzing Diagram

This is the breadboard layout for configuration 2, but you'll have to use your imagination for a few of the components (I couldn't find the right ones in Fritzing). First, pretend the battery is an A23 rather than AAA. Second, imagine the single LED shown is actually the 3x3 panel display we are using. Third, the little box labeled PIR is really the PIR sensor. Configuration 2 is the best circuit to use for this project.

Step 6: The Power Supply Circuit

Here, I've installed the components for the power supply in my project box. I've done a couple of things here that I haven't mentioned previously. First, I glued in 2 battery holders and wired them in parallel to increase the total battery capacity to 110 mAh. The battery holders are connected to one throw of a DPDT switch, and the other throw connected to a 5.5mm x 2.1mm power jack. So now the light can be powered either by the internal batteries or from a plug-in AC/DC converter (remember, 12 V). A nice plus is that the switch can be used to turn the night light off. Of course these are totally optional add-ons.

Step 7: Installation of Sensor and LED Panel

These pictures show the front and back of the project box cover with PIR sensor and LED panel installed. The LED panel mounts with 4 screws and spacers. The sensor was a little more work. It required cutting a 1&3/8" hole in the cover (I used a hole drill bit and drill press). The easiest thing would have been to glue the components on, but I had do it the hard way and attach everything with screws. No, seriously, I used screws so that I could salvage the components at a later date if needed.

I was able to get the rest of the circuit on a 1x1" piece to PCB with lots of room to spare. The leads from the switch and LED panel were soldered directly to the circuit board, while the PIR sensor was connected using 3 lead Dupont cable. I apologize for not including a picture of this last bit, but it was the simplest part of the project.

Step 8: Finished

So there you have it. A compact, completely self contained night light that can be used almost anywhere.

It's relatively inexpensive (the most expensive component is the PIR sensor ~ $12) and easy to build - took me about 8 hours.

Including the additions in Step 5, and assuming the light will be on about 20 minutes per day, I calculate the light should use on average about 2mAh per day (6mA x 0.33h). The batteries then should last 50 to 60 days. This calculation is still being tested.

I hope you've enjoyed the presentation and have fun building the project. Feedback would be appreciated as this is my first Instructable.