# Transistor switch for LED array

Discussion in 'General Electronics Chat' started by Phatness, Jun 9, 2013.

1. ### Phatness Thread Starter New Member

Jun 9, 2013
2
0
Hello everyone, this is my first time building a circuit and I'm having trouble understanding transistors. I don't know which values in the datasheet tell me how many volts and amps I need to turn on the transistor.

I am trying to create a 16x16 RGB Led matrix controlled with 8-bit shift registers (74HC595N). 1 output from the shift registers switches on a row of LEDs, if all columns are enabled that would be 48 LEDs.

With a 5V power supply I think this gives a total of almost 1A over the row transistor. Could I use a TIP120 with the base voltage coming from the shift registers, or would I have to amplify the logic current first?

I was also thinking about using 2 of the PN2222A per row so they each only had 24 LEDs which should require less volts and amps on the base pin...

The columns are controlled by a 2N3904, only 1 LED should be on at a time in a column so I'm less worried about this transistor. I want to control all of the transistors with signals directly from the shift registers (with 1 resistor), is this reasonable?

I haven't built any circuits before so I expect to be totally off, I've tried doing the math but I'm not really sure what values to use from the datasheets.

My schematic for the 2 transistors per row (24 LEDs) idea.

I'm basing this off of Kevin Darrah's 8x8x8 cube http://www.kevindarrah.com/?p=1424 which has 192 LEDs per "row" since my power needs are smaller I'm trying to reduce the complexity of the circuit.

2. ### Evil Lurker Member

Aug 25, 2011
117
23
Thats a pretty neat looking toy.

Personally if you are looking to control a bunch of transistors would suggest using something like ULN2004 (I can't remember might be a ULN2002) transistor array which can directly interface with 5v signals as the base resistors are built directly into the IC.

Again, I can't remember exactly how many transistors are in the array, I'm thinking its either 8 or 10.

Also each LED should have its own current limiting resistor if operating them in parallel because it can cause a cascaded failure if one or more goes out. Try and control them in series if possible using a higher voltage.

3. ### LDC3 Active Member

Apr 27, 2013
920
160
Why is there a 100R resistor on the emitter of the PN2222A? It seems to me it would just drain the current away from the diode array. A capacitor would make more sense.

4. ### Phatness Thread Starter New Member

Jun 9, 2013
2
0
Each LED has 100R resistors, the bottom part of the schematic is repeated once for all 48 LEDs, so I can individually control them with the 2N3904.

Your right a capacitor would make more sense... I'm not sure I need anything between the emitter and LEDs, just a cap at the collector. I want to do PWM, the cap should help with that.

What I am still unsure about is how much voltage/amps I need to supply to the base of the PN2222A (or transistor IC which I like) to supply the full 5V and 0.48A to the LEDs (I would like to supply 0.96A, I don't think the PN2222A can handle it, could/should I use a Darlington?)

5. ### LDC3 Active Member

Apr 27, 2013
920
160
According to the datasheet, PN2222A is capable of a 1 A current. I suspect that it would get pretty hot.