Led chaser for cinema awnings etc

[tramcar trev]

I have a KJ8066 LED scanner/chaser kit that I am going to use to provide chasing warm white leds under my cinema awning... I cant get the schematic without buying the entire project book... Ok its simple enough to put together but it only drives 20 leds. There is a 330 Ohm resistor that is the resistor connected in common with all the anodes of all the leds. This gives me 3v @30mA for each LED from a 12V supply so if I connect up 2 leds ( in parallel) to each connection that would give me 3V at 15Ma ( or 3 leds at 3V X 10mA) for each led, the LEDS are switched by a bank of BC548 transistors each of those can handle 500mA according to the data sheet...
I am wanting to use 1.8mm warm white leds....
I guess the best way in absence of a schematic is to assemble it and try it out, worst case scenario is that ith leds burn out; best case is that Leds don't illuminate I have to tweak the 330 Ohm resistor value

 

[DaveB2]

I'm not familiar with the circuit you mention but would you be better with 2 in series which would increase the total volt drop and reducing the resistor to about 200 ohms or whatever gives you a current less than 30mA and the right level of illumination?

I'm thinking that way to keep the same current and everything happy plus I'm not keen on parallel LEDs and single resistors.

Dave

 

[sparky230]

The normal way to wire up multiple LEDs is Series, Most commerical divers for LEDS are Series Wired and normaly constant current

 

[tramcar trev]

Yes I agree but I don't wish to have to redesign the circuit board... I'll assemble it and fiddle with it. If it doesn't work then I may have to use as is and get 2 more, my Cinema will be a 1930's Art Deco masterpiece and if you can recall this was the heyday of new found electricity and I wonder how come the patrons did not all have epileptic seizures with all the chasing flashing lights.....

 

[gregh]

First up - BC548 transistors are only rated at 100 mA not 500 mA.

Usually light chasers just have 3 leds in turn alight. But as you have 20 leds and that's not divisible by 3, I don't know what you've got.

How many BC548 are there? - this will tell me how many lights in a sequence. maybe 4 or 5?

 

[tramcar trev]

Yes sorry my mistake BC548 is 100mA and there are 10 of them on the board.
There is a switch on the board that looks like it either has the leds chasing along the string or chasing from the centre to the outside depending on where its put... The instruction sheet KJ8067 is $2.50, next time I'm in Jaycar I'll get it...