4017 Wired to NPNs to make LEDS brighter

Thread Starter


Joined Oct 27, 2008
Hey everyone......so I'm in the final stages of building a LED spectrum analyzer and its looking great so far....I just had a hunch of trying to make my LEDs brighter from the outsputs of a 4017 decade counter.....I read somewhere maye on the data sheet that the current would make the LEDs seem dim (which they are) and I found a similar schematic in which I'm trying to manipulate into my circuit but I hooked it up like the schematic below and not only did the LEDs not get brighter...they didn't work at all.....I don't understand the theory behind a transistor I guess...I get the whole base collector emitter and NPN PNP thing....but why isn't it incresing the intensity???



Joined Sep 26, 2009
Since with the 4017, only 1 output is always HIGH, you will need a NPN (2N3904) transistor to switch the LED's on...... The way the circuit is that you show, all the LED's are tied to ground with 1 Resistor, I would use a resistor for each LED....... Here is a sample circuit showing 1 of 10 outputs of the 4017 connected to an NPN and LED...... connect each LED the way I have shown and see if this works for you....

You might also want to try connecting a resistor in between the output of the 4017 to the NPN, use a 100 ohm, or 220 ohm resistor.....

4017 to LED.png

My .02


Joined Jan 10, 2010
Since your only working with 5-6 V you must put the led into the col. return, if you do your circuit as illustrated will work fine .
Change the common emitter return resistor to adjust the brightness of all the leds.
If the variation of intensity is objectionable, then change the led or use current limiting res. for each LED.
One other thing, If you have a cmos version of the 555 then you can go up to 18v supply and no changes need be made to your original circuit exept for the emitter current limit Res.


Joined Dec 20, 2007
If the power supply voltage is only 6V, the output high of your CD4017 is about 5.5V when loaded and red LEDs have a forward voltage of about 2.0V then the 120 ohm current-limiting resistor has a current of 29.2mA.

Like I said in your other topic, "The transistor switches are also selected by the CD4017 and pass the outputs of the filters to the input of the LM3915 one-at-a-time. This is called "multiplexing".
Since only one filter lights its LEDs at a time then its LED indication is turned on for only 1/10th of the total time and will appear at 1/10th the normal brightness. It results in a very dim display (2.9mA for each LED).

The display can be made brighter if the outputs of the LM3915 are boosted 10 times with transistors and if the transistors at the outputs of the CD4017 have their currents increased 10 times."