Light Sensor switch help

[Streak]

I have an LED emergency light.
It consists of a series (24) of 5mm LED's in a series parallel configuration directly driven off a 6v SLA battery.
The battery charges via a transformer and some diodes while there is mains. When the mains fail the LED's come on having been switched on by a relay. The circuit is very basic.

I want to make use of an LDR or suchlike to ensure that if there is a daytime power failure that the light will not switch on.

As there is already a relay in the circuit I guess the best would be to control the relay some how.

Total current draw from the battery is about 250mA.

Please can you give me some really simple (cheap) ideas to achieve the above?

Thanks.

 

[SafetyBob]

I have been thinking about this all day after I read it earlier. Obviously, simplicity is the key here. If you have access to one of those plain old photo sensors they use on the cheap outdoor lights at Lowes or Home Depot, depending on the current it can carry you could either use it in series with the power going from the relay to your lights or if it can't take much power you will have to add another relay...have it be DC powered from the battery. You might be able to adapt an already made plug in type unit also.

I have essentially the same setup for emergency hallway lighting at my house. My 12VDC led's are X-10 controlled to come on around dark to light the hallway (for those trips to the bathroom at night....or so the wife can see to get the coffee pot going in the morning without getting everyone up with a big, blinding hallway light). In a power outage, they come on automatically day or night because usually the hallway is kind of dark.

Bottom line, just think extra relay.....

You might post the same question in the fixed lighting section for more inventors to see. It really belongs there.

Bob E.

 

[Mr Happy]

This seems like a really simple job for a little electronic circuit. I think you want the light sensing circuit downstream of the relay so it doesn't do anything unless the relay switches first. 250 mA is low enough that no big devices or heat sinking should be required.

As an aside, LDR's seem to be going the way of the dodo these days, due to the ban on heavy metals like cadmium. (So how do NiCd's survive?) The replacement for the defunct LDR is the phototransistor.

Since phototransistors turn on when light shines on them I think you need an additional ordinary transistor to invert the signal. The circuit would probably have a phototransistor, a switching transistor and a couple of resistors for biasing and current limiting. Maybe one of the electrical engineers around here will come up with a quick circuit diagram for you? I'm no expert in electronics so I won't venture a circuit without testing it first.

 

[lctorana]

As an aside, LDR's seem to be going the way of the dodo these days... ...replacement for the defunct LDR is the phototransistor.

Funny the things you learn on CPF. I never would have guessed that.

Australia must be years behinf the times - LDRs are sold in all our retail electronics shops, are installed in all solar garden lights sold here, and I have never ever laid eyes on a phototransistor. Ever.

Time to stock up.

 

[Mr Happy]

I think the disappearance must be mainly in Europe due to RoHS. There do seem to be plenty of CdS sensors available elsewhere.