Low power LED night-light
Every time I see household lighting left on overnight for "security", I think of the coal-smoke belching into the atmosphere to power it. This kind of silliness has got to stop, besides the environmental cost it will soon become prohibitively expensive if carbon-taxing ever gets off the ground. My kids, however, (aged variously 1-7) have a habit of getting up several times in the night to bump their way to the bathroom, and they always leave every light switch they encounter on the way ON (not actually being awake enough to deal with turning them off).
We've been leaving a (low wattage) light on to give enough light to get around. Now, you can go out and buy motion-sensitive night-light devices for basically peanuts (eg, dealextreme.com), but the motion sensors are often fake (just a visible light sensor, which does not work in darknesS). Besides, I never did like to do anything the easy way when I could make a production of it, so I designed my own. It makes a neat trainer project for the Atmel ATtiny microcontrollers. The primary design elements are:
- Pre-built Passive-Infrared motion detector module
- Atmel ATTiny45 Microcontroller. Also works on Arduino & variants.
- 0.5 watt high-brightness LED (or an SMD LED-array)
- Ability to override sensor with an on/off switch
- Adjustable brightness via holding down on/off switch
- Timed fade-out to allow enough time to leave the room
- Also suitable (without PIR sensor) for fishtank lighting
I orginally chose a 10mm 0.5W LED (from Oatley Electronics), because it is bright enough to light a bathroom or hallway at night, yet does not require any heatsink. A resistor is used to limit the maximum current to around 150mA at 5v.
Arrays of 10-100 SMD leds, either in grids or strings are available pre-assembled with integrated current-limiting resistors suitable for 12v power. Typically 4 or 5 LEDs are chained with 1 resistor, and 1-20 chains are connected in parallel. The entire array consumes only tens or hundreds of milliamps at 12v, and produces a quite impressive amount of light. The version of the project presented here is capable of driving LEDs from either 5v (via regulator) or from the external voltage input (eg 12v).
Pre-assembled Passive Infra-red sensor modules are available that combine a PIR sensor, control circuirty and lens. You simply supply power and monitor a TTL output line. One inexpensive source of these modules is futurlec.com
Circuit and software detailsSource code and hardware files are available at GitHub.
You can power the circuit from battery, from a regulated power source (say an old phone charger), or use a regulator such as a 7805 or MCP1700 to get 5v from an unregulated supply. If you are powering your LED from the regulator (rather from the main supply), choose a regulator that can supply enough current.
An 8-pin ATTiny45 is used here, althoug it is hardly worked hard. The code can be made to run on an Arduino (or other ATmega device) easily enough. Only about 400 bytes of flash is used, and there are several pins spare. There's enough capacity left over to implement a communications interface, so your household LED lighting could be remote-controlled. I plan to work on this enhancement soon.
A transistor is used to control power to the LED. A general purpose medium power TO92 switching transistor capable of handling around 500ma (eg 2N2222, or 2n3907) is suitable.
The pads for the transistor are arranged to allow use of a TO92 or TO220 package.
A jumper selects whether the LED and transistor are powered from regulated 5v or external (12v) power.
A current limiting resitor may be fitted to the board, or replaced by a link when using a LED array with integral resistors.
The transistor itself may be used to limit LED current if operated in analog mode. I'm a digital guy, so I didn't try that. A good guide to selecting a base-current resistor value is at The Electronics Club
The microntroller drives the transistor with a Pulse-Width Modulated (PWM) signal, allowing the LED (or LED array) brightness may be varied over 256 steps from 0 to 255. This allows the main LED to be left in a low-brightness state as a night light, and also allows the light to be faded in and out gradually, which is kinder on sleep-deprived eyes.
PIR sensor interface
Aa 3 pin header provides power and ground to the PIR module, and reads the sensor's the TTL output signal. The PIR signal is connected to an input pin and monitored via a pin change interrupt.
The PIR module itself inhibits re-triggering for an interval after each trigger which can be adjusted via a trimmer pot.
If the LED array is off, and the PIR triggers, the LED array is faded on quickly. A 5 minute timer is started after which the LED array is faded off again. If a PIR trigger occurs while the array is lit, the 5 minute timer is reset. The effect of this is for the light to remain on while you are moving about, and to slowly fade off off five minutes after you leave the room. The on/off switch can override the timer for fast-off or remain-on.
Internal pullup is used to hold the button input pin high unless the push-button connects the pin directly to ground. De-bouncing is performed in software.
If the LED is off, the push button causes it to fade on quickly (over about 1 second). A release or second push while fading on freezes the light at that brightness.
If the LED is on, the push button causes it to begin fading off (over about 4 sec). A second push during fade-off freezes the light at that brighness (a dimmer). A third push resumes the fade-to-black (off).
Diagnostic LED and spare pins
There are at least 2 pins spare (more if you disable the RESET pin, or use fewer inputs). One of them has a normal LED connected to act as a power-on status light, handy for finding the on-switch in the dark if you do not employ a PIR sensor.
Headers are included for other spare pins for future use.
Some possible expansion ideas
a photosensor to turn LEDs off if room lighting or daylight is present
a communication interface (I2C, DOW or RS422) to allow remote control of lighting modules by home-automation systems, and use of the PIR sensors in as an alarm system
a Sonar or IR object sensor as a no-touch switch
a potentiometer connected to an analog input pin for setting minimum brightness level for 'night light' mode.
The circuit was prototyped and tested on a solderless breadboard, where the microcontroller was programmed in-circuit.
A version was then constructed on proto-board, of the type whose tracks are laid out like a solderless breadboard. This is a very handy way of migrating a breadboarded circuit to a soldered equivalent.
Once the design was proven to be workable, a PCB version was etched. A few shortcomings were made evident by the first PCB version - some of the tracks could have been made wider to give resistance to lifting due to component torquing. It would have been handy to run 5v to all the input jumpers, so that active sensors could be connected to any of the spare pins.
More pictures of construction and deployment in my pir-lite photoset.