Hi,

i want to build a circuit that will detect the current drop and sound an alarm if a bulb is removed from a set of 24VAC 20VA low voltage 80 mini outdoor xmas lights. The 80 bulbs are divided into 4 sections wired in parallel. Each section has 20 bulbs wired in series.
That means if 1 bulb blows/is removed only that section goes out.
I've looked at the high sensitivity current sensing thread here:
http://forum.allaboutcircuits.com/showthread.php?t=11782 but that is for mains voltage and detects an increase in current.

For the past few years i've put them in the hedge at the front of the house with no problems but last year someone deliberately ripped out some of the bulbs. The hedge separates the front garden from the public pavement/sidewalk. This year I want to catch anyone vandalising them.

I'm also going to setup a CCTV camera that i've had spare for ages (the cheap B/W type that just plugs into the AV input of a TV) but i want to interface it to a motion detection circuit if possible that will detect whenever anyone walks past to light an LED and sound an alarm. i'm not going to actually record anyone, i just want to detect movement.

 

i've also had a look at this circuit: http://www.4qdtec.com/csrd.html but you have to pay for more info. in that schematic it doesn't say what part numbers the transistors should be. Would any general purpose low power one be ok such as BC548?

 

Actually I think you don't need anything as fancy as that. You just need a continuity alarm in series with each string. I'll bet somebody here knows a good way to trigger when a circuit opens. It might involve a 555 circuit that will sound an alarm if it isn't continually reset by the power in the string.

 

If one bulb is removed, they all go out?

If so, detecting a 25% current drop should be fairly straightforward.

Do the 4 different strings blink relative to each other? Or are all 4 lit up all the time?

What is the total wattage of the string with all 4 lit?

Do you have a photo of the transformer with the wires? Something can be worked out I think.

 

No they don't blink, it all just simply stays on the time.
i think the transformer output is 24VAC 20VA but i'll be taking the boxes down from the attic this weekend so i'll take some pictures for you.
2 output wires plug in to the transformer via a "barrel" plug and it splits every 1/4 of the way along. i'll draw a schematic of the layout and upload a pic.

 

Schematic pic.

 

This could be done via a sense resistor in series with each of the strings, and the equivalent of a 4 input NAND gate triggered off the voltage of the resistors.

Need a way to make the NAND with AC voltages, or convert to DC.

How versed are you in reading electronic schematics, soldering, etc?

 

yeah i'm experienced in soldering and have built complex circuits from schematics before and often repair things for friends/family.
i have an electronics work area with a temp controlled iron, tools, components, etc.

i realise that the lights should work fine on DC and that it'll need a bridge rectifier and smoothing capacitor from the transformer output. i was thinking of running 4 feedback wires to a box inside containing the circuit. these 4 wires would be ANDed together using 4 diodes.

 

I believe you could use a simple diode+cap peak detector to eliminate the AC aspect? I was thinking you could then use a quad comparator with the outputs ORed together, normally high. A failure of any one of the strings would trigger the comparator's output to change state to low if any one of its 4 outputs go low.

 

Assuming each branch of the lamp string takes approximately the same current, a four winding common mode choke with an output winding can be arranged to give near zero volt/current output if all four strings are lit.

A common mode choke is just the fancy name for a transformer with four identical windings plus a fifth output winding. It can be made easily with an old transformer.

Should any bulb in any string goes open or been removed, then the balanced condition is destroyed and the large AC voltage produced across the fifth winding can be detected easily to operate an alarm or relay.

 

eblc1388: i like your idea of using an old transformer (and i actually have some metal plates from one that burnt out without the glue you get with newer ones) but i've never seen one with 4 identical windings.

Also i've never rewound a transformer before. What gage enameld copper wire should i use and how many turns should each winding have?

i understand that the 5th output winding would be different to the 4 others but how would i arrange it to produce zero output? i assume the 4 identical input windings would be wired in series at the beggining of each lamp string?
Could you draw a schematic?

 

wayneh: i just had a read of this page: http://home.cogeco.ca/~rpaisley4/Comparators.html
Would the 4 Level - voltage Detector near the bottom of the page be suitable using an LM339. In that diagram i noticed each output is sinking current for the LEDs.

How would i reconfigure it to source current so i can OR the outputs together using 4 diodes, which would then connect to the base of a common emiter NPN transistor to drive a relay? Also, what do you mean by a diode+cap peak detector?

 

Also i've never rewound a transformer before. What gage enameld copper wire should i use and how many turns should each winding have?

It does not have to be exact in number of turns or gauge as long as the current rating of the enamel wire is sufficient. One only needs about 20 turns of wire for each windings, perhaps more for the output detection windings.

i understand that the 5th output winding would be different to the 4 others but how would i arrange it to produce zero output? i assume the 4 identical input windings would be wired in series at the beggining of each lamp string?

In fact the more I think about this setup I realize that one don't even need four identical windings but two will suffice.

This enable us to use a ready made mains transformer with two identical secondary windings like 110V/6V+6V or a center-tapped transformer. No additional modification of the transformer will be needed too.

You can then wire up the secondary of the transformer in configuration shown and you then use the 110V winding as detection winding. For center tapped transformer, the mains 24V will going into the center connection(i.e. 0V terminal) and the two remaining terminals connect to common 24V supply of string1&2 and string3&4 respectively.

You have to use a transformer with 4 secondary connections as a three wire 6V-0-6V transformer *WILL* not work.

Edited: The above underlined statement is not true. One can also use a center-tapped transformer in this case.

 

Here's what I was thinking of, assuming your lights are using AC: To each of the four non-inverting inputs of the comparator I would place a capacitor to ground, and a diode from one of the 4 light strings. (A resistor voltage divider might also be needed if the voltage is too high for the comparator.) The diode would allow the cap to be charged only by positive pulses of the AC, essentially converting it to DC. The cap will hold the charge at the peak since there is very little drain on it, only the comparator and self-leakage. In fact, you might want to add a resistor in parallel with the cap, so that it discharges more quickly.

If voltage is lost on one string, the cap will discharge and the voltage at the comparator's pin will fall below a reference. I would use the same reference for all 4 comparator inverting inputs. Just take the DC source you're powering the IC with, divide it in half (or more) with 2 resistors, and feed it to all 4 inverting inputs.

The comparator outputs can all be connected and pulled up with a single ~3K resistor. The output will stay high unless/until one comparator goes low, pulling them all low. You can then use the output to trigger a MOSFET, a transistor+relay, whatever.

I really liked the transformer idea because it's an AC solution to an AC problem. Dead simple, passive, low component count. But like you I'm not sure I could find the right transformer and get it rewound properly.

 

thanks, i think i will go for the comparator circuit as i already have all the components except the LM339. Even though it uses more components, it's more likely to work correctly at the end. It's pretty straight forward and logical.
i have little experience rewinding transformers, so there are too many variables.

 

Even the Shack has the LM339. Not that I'm recommending them or anything, just sayin...

Now that I think of it, that's where I got my last one.

 

i'm in the UK remember, we don't have radioshack. we used to have tandy years ago.
if i want something big or a lot of stuff at once i usually mailorder from rapid electronics (www.rapidonline.com), cricklewood or ebay.
if it's just 1 or 2 things or i want to look at it (e.g. the inside of a plastic box) i go to my local maplin store. it works out cheaper cos i'm not paying postage.