Alternative Technology at Home in Suffolk
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Solar Garden Lamp Repair
Our water feature uses several solar powered garden lights which were very cheap, but only lasted a year before failing to illuminate correctly. I describe below how I managed to find the faults and inexpensively repaired them.
Garden solar powered lamps use a small photo voltaic panel on the top, to generate electricity when the sun shines and this power is stored in a small battery bank sufficient to drive a couple of LED's (light emitting diodes) which provide low level illumination at night. Most of these lights are made very cheaply in China and are not designed to be repaired, but if you are handy with a soldering iron it is possible to get a faulty unit working again.
Access is often obtained to the innards by simply twisting the top of the lamp anticlockwise while holding the base. Sometimes there are a few small screws to undo. The batteries are often protected by a screwed down cover which must be removed.
When a lamp fails to light up at night, the first priority is to check the battery bank. My lamps have two AA size Ni-MH rechargeable cells fitted to give 2.8 volts to power the LED's. Check the voltage with a multimeter after they have been charged on a sunny day. If the voltage is low, it is probably the cells that are faulty and they should be replaced. It is possible that they are not receiving a charge from the solar panel, but this is usually because of dirty contacts and only rarely a faulty solar panel. Once you have cleaned the contacts, try fitting a couple of standard dry AA cells and take the lamp into a darkened room to see if it lights up.
If the rechargeable batteries prove to be OK then it is a fair bet that the cadmium sulphide light dependent resistor (LDR) or photocell, which senses darkness, has gone faulty. This LDR is mounted on a small printed circuit board and positioned so that is faces upwards in the lamp. These work by going high resistance in darkness, compared with a low resistance in daylight. The high resistance triggers the circuit to switch on the LED's when darkness falls. I have found that these light dependent resistors often degrade in sunlight over a long period and don't reach a high enough resistance to switch the lamp on. The solution is to replace it.
The circuit board can usually be accessed by springing a retaining clip up, so that the underside of the PCB is in view. Make sure you remove the batteries before commencing any work on the circuit. The printed circuit copper track feeding the LDR is often covered in lacquer, which must be scraped off before attempting to unsolder the LDR. I used a solder sucker with my soldering iron, to remove the solder holding the light dependent resistor in place. Once you have removed it you can refit the batteries and check to see if the lamp will come on. If it does, then you have found the fault, but the lamp will be on all the time, even in daylight, because removing the light dependent resistor is mimicking the high resistance of it in darkness.
Obtain a replacement LDR from a component supplier like CPC, Maplin or RS Components and carefully solder it into the PCB. Suitable types are VT935G or NSL19-MS51 or MPY54C569. The polarity doesn't matter on an LDR. Refit the batteries and test in a darkened room. If all is well, the whole lamp fitting can be re-assembled and returned to the garden.