I have a need where I want to control 24 individual LED array connected through TLC5940 & CAT4101 LED driver.

I have 144 LED's in 24X6 LED array. These are high power 3W LED's. After searching I found out that TLC5940 can be used to increase the PWM pin's on Arduino.

I'm not sure how to connect TLC's output to CAT's input. Since the TLC is current sinking driver how can I use it to control LED brightness by sending signal to the PWM Pin of the CAT? If I connect TLC's output to CAT's PWM input will it work?


1 Answer 1


You connect the base of any small-signal PNP transistor (for example: BC327, BC807, BC556/557/558, 2N3906) to the output of the TLC via a series resistor (500-10k ohms). The emitter of the transistor goes to Vdd, the collector goes to the input of the CAT. Another parallel resistor (1k-50k ohms) should be connected between the input of the CAT and GND.

You should configure the TLC to generate only a small (<1mA) current, by selecting the maximum reference current resistor value supported (see page 14 of the TLC data sheet), and programming the dot correction to lower the current.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks for the reply. I will have a look into this setup later on. If I go by this setup than this will be a big mess and will become more complicated when I want to control 24 pins from TLC. As per this setup I will need 24 PNP transistors & 48 Resistors. Is there a way I can use DARLINGTON TRANSISTOR ARRAY instead on standard transistors. if yes than which one and how. regards BW \$\endgroup\$ Oct 7, 2013 at 15:49
  • \$\begingroup\$ @BhaskarWankhede no, darlington transistors are for high current usage. They would be useful if you want to replace the cat4101 and just use the tlc5940 as the led driver. You could use transistor arrays and resistor packs though, to save space/part count. \$\endgroup\$
    – Passerby
    Oct 7, 2013 at 17:57
  • \$\begingroup\$ @laszlo won't this have a problem with leaving the PNP's base floating during the off period of the TLC's output? Shouldn't there be a weak pull-up from vdd? \$\endgroup\$
    – Passerby
    Oct 7, 2013 at 17:58
  • \$\begingroup\$ @Passerby: You can put a resistor there, if you feel so inclined, but considering the currents involved in this circuit, I don't think it is neccessary. \$\endgroup\$ Oct 7, 2013 at 18:20
  • \$\begingroup\$ @Bhaskar Wankhede: if space is your problem, you could use some other component. My first recommendation would be some kind of 74LS TTL inverter, for example: ti.com/product/sn74ls04 as these can be connected to the TLC output without any resistor, and their output can also be connected to the CAT input with any resistor. However, they might not be that easy to buy these days... \$\endgroup\$ Oct 7, 2013 at 18:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.