Help please - electronics techheads


New member
Username: Madboy

Post Number: 1
Registered: Jul-09

Appreciate this forum might not be the best one to ask this question in (if it's not, can anyone point me in the right direction?)

I'm trying to marry a cheap PIR motion sensor security light to a wireless doorbell. As in, motion sensor is tripped and doorbell rings. I was intending to wire the output from the motion sensor (normally to light bulbs) to the switch on the doorbell.

However, it occurred to me that it's 240v AC going into the light bulb and a 12V DC battery running the doorbell. I assume I'd fry the poor little circuit board if I sent that voltage into it.

Without spending any money, how can I get around this problem?

Can I hack up a phone charger and use it to step the mains power down to 12v? Or will that still be too much for the doorbell circuitboard?

Reason I'm doing this is that I'm too cheap to spend $278 buying the proper off the shelf unit.

Any suggestions or pointers would be appreciated. Thanks in advance.

Platinum Member
Username: Nd4spd18

Philadelphia, PA

Post Number: 10561
Registered: Jul-06
So you need a 240vac to 12vdc transformer.

Now where are you getting 240v from.... not in north america perhaps?

Gold Member
Username: The_image_dynamic

San Diego, California

Post Number: 5257
Registered: Dec-06
With the 240VAC and the use of the word "proper" I would say he is in the UK somewhere.

New member
Username: Madboy

Post Number: 2
Registered: Jul-09
Hi. New Zealand. We use the Queen's English here ;)

So would a mobile phone charger (I have heaps of them lying around from various phones I've had over the years) not be suitable? Is it because they would have a fixed output not a variable one?

I'm just trying to think of what I can get fast and cheap! Thanks.

Gold Member
Username: The_image_dynamic

San Diego, California

Post Number: 5259
Registered: Dec-06
Ahhhh NZ, love that place. We trout fish at North Island as often as we get out there. And all the hot blonde girls my goodness....

Yes, depending on the milliamps that the light can handle, a phone charger will work. Find the lowest mA@ 12V output one you have and try that. Depending on how far it is from the 240V, you can always sacrifice another PS's wire and patch that length on to whichever one works.

Gold Member
Username: Van_man

Boston South, MA

Post Number: 4909
Registered: Mar-06
I think a simple relay wired in would accomplish your objective.
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