The Solar Panel Project - Update: Nightlights

This is something I've been meaning to do for a while. The kids bedroom has a nightlight that sits there burning away 11W of energy overnight every night. I don't like it - it's too bright and they never remember to turn it off. I've wanted to install some kind of LED based solution that ran off the battery of my solar system and this weekend, despite having man-flu, I finally got around to it.

Firstly I needed some suitable LED lights. I was thinking of making something in a similar vein to my porch lights but while being dragged around the new Ikea store in Coventry last weekend by the wife I came across these fellas and bought two for this project...


These are ideal - three white LEDs per unit, integral switches, self adhesive, fully recyclable packaging and less than three quid each. They also require three 1.5V batteries so they run on 4.5V total which means I could easily rejig them to run them off my 5V regulator without too much trouble...

... however trouble is my middle name and I could never spend my weekend on a project that was TOO easy. The thing I didn't like about these fellas was the current consumption - 100mA per unit at 5V. Now, in my porch light project I used a transistor based astable multivibrator to 'flash' the LED's at a fast rate in order to lower their current consumption and I decided to do the same with these lights but this time by using a 555 timer IC instead of transistors.

Here's the 555 based astable circuit I used. As usual, these component values are just what I had to hand rather than being the result of any calculation!


The 555 timer is a great IC and can source or sink up to 200mA. By sticking one of my Ikea lights in the LOAD position and using the timer to sink the load current I found consumption reduced from 100mA to just 15mA. There was a reduction in brightness (the 555 output is Vcc - 1.7V) but it was acceptable. I knocked up a little circuit for doing the job as below...


The above circuit was intended for inline use (i.e. between the regulator and the lights) however I remembered I had deliberately saved a load of space in my 'control panel' to allow for extra components to be mounted so I rebuilt the circuit here instead...


You can see the 555 chip in the centre of the above picture mounted along with other (existing) components on the control panel board. The 5V line from the regulator and the output of the chip are then wired out to the two Ikea lights mounted on the bedroom ceiling. The lights can be switched on or off at a touch and it doesn't matter if they end up being left on all day - at a total current draw of under 30mA for the pair they're not likely to cause my battery any problems for a while!

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