Monday, 6 July 2015

Hacking a Solar Security Light

Introduction

I've got a few LED lights around my home with infrared motion detectors. They provide soft light at night in the hall, stairs and inside cupboards, and another provides a surprisingly bright light over the back door. So no more fumbling in the dark for light switches or keys on cold dark nights. But the one thing that's been bugging me is the light in the garage.

I replaced the single light fitting with a 6 foot fluorescent tube last year. It does a good job lighting the space, but the kids sometimes leave it on, and the second or so delay is annoying if you're repeatedly going in an out of the side door.

Having been impressed by the light over the back door I decided to get another for inside the garage. Not only will this provide some convenience, but solar = free power.

Problems With the New Light

Solar LED Light
I picked up this 36 LED light from Maplin which provides 150 lumens and has a sensor range of 8 metres. Not bright enough to do any work by, but enough to stop miscellaneous clutter from being a tripping hazard.

It was different from the previous one I'd bought, being a bit cheaper, and the on-line reviews showed that some had problems with water getting in. But as I'd planned to install it inside my garage, I disregarded that as a problem, and purchased the light.

It wasn't until testing that I realised the light wouldn't come on in daylight. (this is determined by measuring the voltage coming from the solar cell). I found I had to cover the solar panel with my hand, or disconnect it to get the light to come on.

This was NOT good!

Hacking the Light

I mused that perhaps the circuit could be altered so that the solar panel only charges the battery and doesn't factor into the sensor circuit. I grabbed my screw driver and started to investigate.

After removing the screws from the main case I found a 3 cell battery pack and a small circuit board.

The light's casing open showing circuit board.

You don't get wiring diagrams these days so I had to trace the circuit and sketch it out on scrap paper.

1. The voltage from the solar cell passes through the blocker diode D1, which then feeds onto the battery connected to the (slightly blurred) white connector.

2. The white return wire from the IR sensor connects to the OUT pad on the circuit board, which feeds the base of larger transistor on the far left. This turns the light on when the sensor is triggered.

3. The smaller transistor Q2 shorts the base to 0v if the voltage through R4 (from solar cell) is high. This stops the light coming on in the day.

Remove R4 from the circuit.

So,.. I grabbed my soldiering iron and desoldered this resistor.

The removed resistor on my finger

Bingo, the light comes on, and this time I didn't have to cover the solar panel.

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