Emergency auto-on flashlight mains adapter

[Agent86]

Hey everyone,

There's a device I'd like to build, which I don't think is available for retail sale.

http://www.ple.com.au/ViewItem.aspx?InventoryItemId=601359

I have one of these UPS's and it has three circuits which are surge-protected but not on UPS, so when the power goes out, these sockets are unpowered.

I'd like to put together a 12V flashlight which charges off mains power, but when the mains power is disconnected, the light comes on. So when there's a blackout, you can find your flashlights easily, and they are always at maximum charge.

So it would have two states

- mains power on, battery charging, light off

- mains power off, battery discharging, light on

and a rocker switch which disables the light, so that if there's a blackout while you're on holiday, the lights don't come on.

Obviously, these could be plugged into any wall socket, not just a UPS, but typically, when there's a blackout, the UPS is the first place I go to safely shut down my hardware.

I've used an old (old) battery charger which I can use to charge a rechargable flashlight I have, but I don't really know enough about electronics to put together a switch that uses little to no power and is switched "ON" when the transformed 12DC power from the wall is cut off. Is there an easy circuit design I can use to acomplish this?

[steve_v]

Just use a 12v SPDT/DPDT relay, use the 12v from the mains transformer to hold the relay 'on' and wire your light across the 'normally closed' contacts.

It'll draw some current from the mains while idle, but the requirement for a relay coil is too small to worry about.

i.e.

Adjust for your existing charger circuitry ofc.

Or you could use a transistor to turn the relay on from battery power, potentially using less from the mains when idle:

In this case however, you would need to calculate the current carrying capacity of the transistor & the values for the 2 resistor voltage divider to set the trigger voltage (and keep the transistor base current to a reasonable level). This would depend on relay and transistor selected.

If the current draw of the lamp is low enough (or the transistor grunty enough), do away with the relay entirely and use the transistor for switching.

Disclaimer: I threw these together off the top of my head, while goofing off @ work. Fuses & back-emf protection diode not shown.

There are many, many ways to achieve this.

EDIT: Nope, can't resist.

Here's one with a P-channel MOSFET:

Just make sure you pick a MOSFET with Vgs max >12v.

You may need a resistor from gate to -ve, depending on your power supply.

There are some funny gaps in my wires due to png export...

Edited December 4, 2014 by steve_v

[GoatRider]

I just searched "emergency light" on Amazon. This is the first thing that came up, looks like exactly what you're looking for:

http://www.amazon.com/Amerelle-71134CC-Slimline-Power-Failure/dp/B0039UE9EG

[Tommygun]

I have had a few of these type of emergency flashlights over the years.

The power plug and recharger is built into the flash light and it comes on when the power goes off.

The problem I had with them is that the rechargeable batteries go bad pretty quickly and you have to buy a new light every 18 months or so, otherwise the light would only last 10 or 20 minutes.

http://www.homedepot.com/p/Dorcy-Failsafe-Rechargeable-LED-Emergency-Flashlight-41-1032/204305304

[Nuke]

you need a small smps power supply like whats used in phone chargers, this would output a regulated voltage, call it the power circuit. then a li-po battery and charge and safety circuit, we will just call this the battery circuit. it would take charge the battery when there is power from the supply circuit, and would have power output (possiblly regulated) from the batteries to the thing you are powering. the third thing you need is the lighting circuit, with switch and lamp. all 3 share a common ground.

to detect whether or not you have mains power, you just stick a pull down resistor on the output of the power circuit, and this would be the control line. this would control the gate of a p-channel mosfet in the lighting circuit, which would be in series with the light and switch on the lighting circuit. when the control line is high, that means there is mains power, when its low, mains power is not available. p-channel mosfets are active low, which means they come on when the gate is pulled low. so when there is no mains power, the lighting circuit is on (by default, if there is a switch it would override this behavior, unless your lighting circuit was more sophisticated).

also ninjad, with diagrams (to be fair i stopped reading your post when i saw the relay in the first diagram )

Edited December 6, 2014 by Nuke

[steve_v]

A relay may not be elegant, but it has the virtue of requiring *zero* electronics knowledge

[Nuke]

it does however give you everything you need to know to build a 1940s computer

[andrewas]

It might be worth action activating it with photo sensors rather than by detecting a mains fault. That way. It also comes on if a lightbulb fails or the lighting breaker trips.