Help needed to boost output current in a 7 LED chaser

Thread Starter

Tony Spence

Joined Dec 20, 2013
64
I have a 7 LED chaser circuit (BELOW) that i moded to get the results that i need.
Firstly I increased the voltage in the whole circuit from 9V to 18V then i added 2 BC108C's in the darlington pair formation after each output leg of the 4017b chip that i used, before each LED output.

I think i have blown the first transistors i used I have got new transistors which i believe turn on at 25V they are also BC108C's is there a way i can boost the voltage after the output pins of the 4017B chip to turn on my new transistors to give me the current i need, or would i be able to use another component to replace the Darlington pair with and get the same current gain.

This is the circuit i started with:7-Led chaser cct.jpg
 

bertus

Joined Apr 5, 2008
20,888
Hello,

How did you connect the transistors to the 4017?
You will need a resistor between the 4017 and the base of the transistor.

Bertus
 

eetech00

Joined Jun 8, 2013
2,056
Hi

I don't know how much current you want to drive the LED's but, for 18v, you could drive each LED at about 10ma like this. Adjust the collector resistor for higher current.

eT
 

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Thread Starter

Tony Spence

Joined Dec 20, 2013
64
I have a 7 LED chaser circuit (BELOW) that i moded to get the results that i need.
Firstly I increased the voltage in the whole circuit from 9V to 18V then i added 2 BC108C's in the darlington pair formation after each output leg of the 4017b chip that i used, before each LED output.

I think i have blown the first transistors i used I have got new transistors which i believe turn on at 25V they are also BC108C's is there a way i can boost the voltage after the output pins of the 4017B chip to turn on my new transistors to give me the current i need, or would i be able to use another component to replace the Darlington pair with and get the same current gain.

This is the circuit i started with:
7-Led-light-running-adjust-speed-by-4017-and-5551-600x530.jpg
 

Thread Starter

Tony Spence

Joined Dec 20, 2013
64
I have a 7 LED chaser circuit (BELOW) that i moded to get the results that i need.
Firstly I increased the voltage in the whole circuit from 9V to 18V then i added 2 BC108C's in the darlington pair formation after each output leg of the 4017b chip that i used, before each LED output.

I think i have blown the first transistors i used I have got new transistors which i believe turn on at 25V they are also BC108C's is there a way i can boost the voltage after the output pins of the 4017B chip to turn on my new transistors to give me the current i need, or would i be able to use another component to replace the Darlington pair with and get the same current gain.

This is the circuit i started with:
View attachment 76895
View attachment 76896
Above is how I connected the two transistors to make the darlington pair (two BC108C's). I added them before each output LED and connected the LED to the emitter. I dont know how much current is needed to my output this is also why i need help, but what i do know is that the modifications i made to the below circuit work and that i need help perfecting it.
View attachment 76895
 

Thread Starter

Tony Spence

Joined Dec 20, 2013
64
I have a 7 LED chaser circuit (BELOW) that i moded to get the results that i need.
Firstly I increased the voltage in the whole circuit from 9V to 18V then i added 2 BC108C's in the darlington pair formation after each output leg of the 4017b chip that i used, before each LED output.

I think i have blown the first transistors i used I have got new transistors which i believe turn on at 25V they are also BC108C's is there a way i can boost the voltage after the output pins of the 4017B chip to turn on my new transistors to give me the current i need, or would i be able to use another component to replace the Darlington pair with and get the same current gain.
The attachment 76896 has something wrong, I can't see it.
This is the circuit i started with:View attachment 76749
hi current gain darlington pair transistors.png
View attachment 76896
Above is how I connected the two transistors to make the darlington pair (two BC108C's). I added them before each output LED and connected the LED to the emitter. I dont know how much current is needed to my output this is also why i need help, but what i do know is that the modifications i made to the below circuit work and that i need help perfecting it.
View attachment 76895
The attachment 76896 has something wrong, I can't see it.
 

ScottWang

Joined Aug 23, 2012
6,963
I'm not sure why you wan to using 18V, that is too high for a led driver, and the darlington pair need a Rb resistor to limiting the current.

For many applications that you just need to using one bjt to be the led driver, it is unnecessary to using two bjts.
 

Alec_t

Joined Sep 17, 2013
11,749
If your LEDs are the usual indicator type with a max current of ~30mA you don't need Darlington drivers.
Here's how you could do it (I've shown only 3 LED sub-circuits, but you get the drift).
LED-chaser.gif
 

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Alec_t

Joined Sep 17, 2013
11,749
The CD4017 model is in CD4000.lib, included in the zipped Part1 of the LTspice library files freely downloadable from the Yahoo LTspice User Group. But any CMOS gate model should be usable just to demonstrate LED driving via an NPN transistor.
 
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