Dusk to dawn switch using low voltage ac with ac output

Thread Starter

punkieys17

Joined Oct 4, 2014
4
The garden pump had a LED ring light around it with a separate transformer supplying 14V AC which passed through a LDR switch then onto the LED ring. While I can find very simple 230V triac based circuits I'automatic-night-lamp-circuit-230v.gifm struggling to find one for such low AC voltages. I have attached an example of such a 230V AC. Can I modify this to work...my calculations suggested that a 1n4001 diode and a 270K pot would be OK. Any thoughts greatly appreciated. Julian
 

LesJones

Joined Jan 8, 2017
2,343
LEDs run on DC so there is probably a rectifier between the AC input and the actual LEDs. It is just possible that there pairs of LEDs are connected with reversed polarity so one lights on the positive half of the cycle and the other one lights on the negative part of the cycle. I suggest investigating exactly how the LEDs are fed from the 14 volt AC supply. As the circuit in your post uses an SCR it will only conduct on half of the cycle. If you replaced the SCR with a triac and removed the diode it should work. The value of the resistor will also depend on the type of LDR that you use.'

Les.
 

Thread Starter

punkieys17

Joined Oct 4, 2014
4
Hi guys, many thanks for the reply. The light ring (just like the sensor) is encapsulated so impossible to see anything. I checked the Ac transformer output and that links to the sensor. Both transformer and sensor have a connector with no polarity so was happy to just connect the transformer to the LED ring - which lit up correctly. So just doing some investigation on what triacs I have in the bottom of the tool box....an somewhere I have a ORP12 from memory - though I see 5mm LDRs are really cheap on ebay.
 

Thread Starter

punkieys17

Joined Oct 4, 2014
4
So found a BT136 triac and a LDR with 100mW max power and resistance 20k to 1M. Think a 1M resistor would work fine as the gate voltage is 1.5V. Can anyone please explain the use of the diac and if it will still work on the 14AC power. streetlight.jpg
 
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