LED Sound to light unit - UK Vintage Radio Repair and Restoration Discussion Forum

davew · 11th Mar 2016, 11:38 pm

I've just bought a simple Velleman Sound to light unit kit (MK103)
It works pretty well - A BC557B output transistor drives 4 X LEDs in series with a 47R resistor.
My question is: can I add further LEDs (maybe around 6) to this array?
I'm not sure how to work out the series resistance needed or whether 2 paralleled arrays would be better.
Also, I assume it depends how much current the BC557 can pass.
The LEDs I have are a general purpose pack from Maplin
Grateful for any advice (or how I can work out any resistance values required)
Thanks
Dave

Diabolical Artificer · 12th Mar 2016, 7:42 am

A typical LED consumes around 20mA. One way to find an unknown LED's safe operating current is to wire one up with a pot in series to a suitable PSU.

That said you may not need one at all. not sure though if a BC557B could drive 10 LED's, though you could use another in a Darlington pair to provide the extra current.

Looking at the BC557 datasheet collector current is given as 100mA, so you'd be struggling to provide I for 10 LED's. If it's configured as a CCS you might need to change an emitter resistor so you could use one module for 5 LED's

To work out resistor value's, ohms law is your friend here if you know your voltage and current. Let's say your PSU voltage is 10v and you want to power 10 LED's, 10 x 20mA = 200mA, so 10 divided by 0.200 = 50 or nearest value 47r.

Not familiar with the module in question, but if each LED has a transistor "driver" your ok, if not you'll struggle as I said. Best way is to try it, if it goes bang you can replace a component and you'll have learnt something, if it works, even better.

Andy.

dseymo1 · 12th Mar 2016, 7:53 am

Why not add one LED at a time?
When the transistor starts to get warm, you'll know you're approaching the limit.

Terry_VK5TM · 12th Mar 2016, 11:46 am

I've had a look at the circuit and all the LED's are in a series string.

Without doing the actual maths, looking at the circuit, I reckon they are drawing ~15mA. So you can add another parallel string of 4 LED's and 47R resistor to increase the number of LED's.

Total draw through the BC557 would be ~30mA, well within it's ratings.

Terry VK5TM

davew · 14th Mar 2016, 8:04 pm

Thanks for the replies - I'll do some experimenting!

trickie_dickie · 14th Mar 2016, 9:07 pm

Why not put the extra LEDs in series with the existing four. That way, the current remains the same. You may need a higher voltage to power the LEDs. Can you post a circuit of the output?

TonyDuell · 14th Mar 2016, 10:06 pm

You can download the instructions (including the circuit diagram) from one of the dealers on this page :

https://www.quasarelectronics.co.uk/...d-to-light-kit

I wonder if you could make a second copy of the last stage (R5, R9, R12, Q4 and the LEDs) and wire it to the battery +ve and -ve terminals and to the collector of Q3 in the obvious way.

11th Mar 2016, 11:38 pm	#1
davew Tetrode Join Date: Feb 2004 Location: Deal, Kent, UK. Posts: 74	LED Sound to light unit I've just bought a simple Velleman Sound to light unit kit (MK103) It works pretty well - A BC557B output transistor drives 4 X LEDs in series with a 47R resistor. My question is: can I add further LEDs (maybe around 6) to this array? I'm not sure how to work out the series resistance needed or whether 2 paralleled arrays would be better. Also, I assume it depends how much current the BC557 can pass. The LEDs I have are a general purpose pack from Maplin Grateful for any advice (or how I can work out any resistance values required) Thanks Dave

12th Mar 2016, 7:42 am	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: LED Sound to light unit A typical LED consumes around 20mA. One way to find an unknown LED's safe operating current is to wire one up with a pot in series to a suitable PSU. That said you may not need one at all. not sure though if a BC557B could drive 10 LED's, though you could use another in a Darlington pair to provide the extra current. Looking at the BC557 datasheet collector current is given as 100mA, so you'd be struggling to provide I for 10 LED's. If it's configured as a CCS you might need to change an emitter resistor so you could use one module for 5 LED's To work out resistor value's, ohms law is your friend here if you know your voltage and current. Let's say your PSU voltage is 10v and you want to power 10 LED's, 10 x 20mA = 200mA, so 10 divided by 0.200 = 50 or nearest value 47r. Not familiar with the module in question, but if each LED has a transistor "driver" your ok, if not you'll struggle as I said. Best way is to try it, if it goes bang you can replace a component and you'll have learnt something, if it works, even better. Andy. __________________ Curiosity hasn't killed this cat...so far.

12th Mar 2016, 7:53 am	#3
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: LED Sound to light unit Why not add one LED at a time? When the transistor starts to get warm, you'll know you're approaching the limit.

12th Mar 2016, 11:46 am	#4
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: LED Sound to light unit I've had a look at the circuit and all the LED's are in a series string. Without doing the actual maths, looking at the circuit, I reckon they are drawing ~15mA. So you can add another parallel string of 4 LED's and 47R resistor to increase the number of LED's. Total draw through the BC557 would be ~30mA, well within it's ratings. Terry VK5TM

14th Mar 2016, 8:04 pm	#5
davew Tetrode Join Date: Feb 2004 Location: Deal, Kent, UK. Posts: 74	Re: LED Sound to light unit Thanks for the replies - I'll do some experimenting!

14th Mar 2016, 9:07 pm	#6
trickie_dickie Hexode Join Date: Jan 2011 Location: Aberdare, South Wales, UK Posts: 403	Re: LED Sound to light unit Why not put the extra LEDs in series with the existing four. That way, the current remains the same. You may need a higher voltage to power the LEDs. Can you post a circuit of the output? __________________ Richard

14th Mar 2016, 10:06 pm	#7
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: LED Sound to light unit You can download the instructions (including the circuit diagram) from one of the dealers on this page : https://www.quasarelectronics.co.uk/...d-to-light-kit I wonder if you could make a second copy of the last stage (R5, R9, R12, Q4 and the LEDs) and wire it to the battery +ve and -ve terminals and to the collector of Q3 in the obvious way.