Firstly..     What is a MAL ?

It stands for “Motion Activated Light” of course !
These are quite popular as outside lights and can be found in various forms.
These units sense Infra Red radiation at a certain wavelength which matches the heat radiated by the human body.
These particular units are designed to be installed over the existing lamp battern, and held in place by the battern and supplementary screws.
As no electrician is needed to install them,  they prove to be quite popular.
Some are described as “Oyster Lamps”,  as the over all appearance of them suggests Oysters ?

A good friend approached me with this problem :
His sensor light had stopped working !
Inside the bag he had presented to me,  was a familiar looking lamp minus its cover.
(He had given me one to fix a long time ago but at that time I couldn’t fix it).
He was desperate to get it going as he lives in a semi rural area with no street lighting.
Having the light come on at night was very convenient for him.
I told him I would see what I could do …
(Privately though, I didn’t hold much hope of repairing it.)

Whilst the overall design was quite clever in so far as ease of installation was concerned,  that was where the cleverness stopped !
The electronics within was an appalling design !
The circuitry was designed to operate directly from the mains.
Now,  there is nothing wrong with this providing it is done correctly,  but unfortunately,  the whole lot was assembled on cheap phenolic circuit board.
Phenolic circuit board burns and will catch fire quite easily !

However. 
To work on the beasts safely,  an isolation transformer is required.
I decided to use a transformer with a 36V output.
It was just a matter of calculating the new value of capacitor required as a voltage dropping element.
The reactance of the capacitor acts like a resistor in this circuit.
A suitable value was to hand and temporarily installed.
A suitable lamp was made from three 12Volt 5Watt automotive lamps in series.

As I had no circuit available,  some tracing out of the circuitry was necessary.
It seemed that there was no output from the PIR sensor.
There should have been some output as I was sitting right in front of it !
There was a 0.22uF tantalum capacitor stuffing it up !
After replacing that item,  the unit began working again.
Finding that 1N914/1N4148 were being used as mains rectifier diodes was ridiculous,  so I replaced them with 1N4007's !
The original small signal diodes only have a reverse voltage rating of about 75V.
The nominally 240V RMS mains however peaks at around 340V (excluding common spikes).
The 1000V volt rating of the replacement diodes meant peace of mind for me!

Feeling confident,  I then dug out the other unit that I had given up on a year or so ago.
Now that I had one unit working I could make measurements and compare the voltages at various points.
I replaced the tantalum capacitor and the rectifier diodes.
I have found that tantalum capacitors should only be used as decoupling capacitors in higher powered supplies …
They go very leaky in signal circuits or as timing capacitors.

The other unit still wouldn’t work.
I determined that the LDR or something around it was to blame.
These units sense the amount of ambient light and will only allow activation at night time.

I replaced the LDR and that seemed to be the culprit.
The second unit was now working,  or so I thought !
When the lamps were returned to their owner,  one of them was installed.
It just happened to be the one that I had replaced the LDR in.
The owner then reported the fact that it would still turn on during the day !
That would mean that some juggling of resistor values was going to be necessary to solve that problem. I told him to use the other one in the mean time.

I haven’t seen my friend or his MAL for a while now.
I’ll tackle the faulty lamp again one day.
I just keep thinking of that appalling design and the many others like it there must be out there.
I keep asking myself …   HOW DID THEY EVER GET IT APPROVED ?    Or didn’t they bother!

Until next time ….Bye!