problem with activating relay

[Lasermad]

mmm wonder if any of you guys can offer ideas

Im running 4 x 5v lasers on a timer

First im dropping the 240v to 12v with one of these

http://www.ebay.co.uk/itm/Atrix-18-O...item3a98f95d80


this supplies 12v into these timers with a relay
http://www.ebay.co.uk/itm/LED-Home-A...item566c759e91

kind of irrelvent with the problem im having but the output from above is then
push through one of these to drop to 5v
http://www.ebay.co.uk/itm/3A-15W-DC-...item5b06c75f62

Now heres the problem im having , two are working perfect on a 5m cable run , the other two are at the end
of a 100m cable run , and although when i meter test it shows just over 12v arriving at the end of the cable run , it seems theres not enough
voltage to activate the relay in the timer unit ? in theory they should work , but they work perfect on 50m , but at the
100m end , just not enough juice

So throw me some bones guys , any quick easy fixes ? bear in mind im not an electronic
engineer

[VDX]

... 100 meters is a pretty big resistor which drops the current, so it won't be enough for triggering your parts.

This is even more a problem for DC than for AC.

So either you'll drive with higher voltages or with AC.

It shouldn't be a problem to feed the 240VAC (or 24V DC) through the 100m and convert it at the end to 12V or 5V ... or you can use much thicker wires to reduce the resistance of the cable.

For another similar problem with >200m cable lengths I've used glass fibres and LED's instead of voltage to switch/activate the target electronics ...

Viktor

[Lasermad]

Hi Viktor ,
many thanks for reply , for safety reasons and positions of cables i simply wont
ever run 240v down the 100m portion of cable.

Glass fibre / led , interesting , im going to guess thats a pretty complicated solution , but sounds interesting
anyways so i would be keen for some links if nothing else for filling the knowledge base :-)

the two problem units are on their own 12v line coming out of power distribution box .. so i did a search
for 12v to 15v dc step up transformer .... and came across this .... Any thoughts on if this would work in theory
bear in mind its a relay that needs the juice , the relay control unit can handle upto 16v according to specs , but
then its also supposed to handle 10v !
http://www.ebay.co.uk/itm/Solar-DC-t...item2c841b9ae4

[Draco]

That should work.
I have a 24 volt dc buss in my house that is derived from 4 solar panels on the roof. I went with 24 volt because of the loss over the cable, because it's dc and needs a return path your 100m long cable is 200m long to the circuit. Using heavy gauge wire has lower loss per foot or even using 12-24volts AC would work better over that distance