Problem with RGB LED color combinations.

Thread Starter

lilrips1

Joined May 7, 2010
44
Well, the low level output voltage is only .2 V for the flip flops.

Yes, that was an old thread just about the schematic. This one's purpose was to get the LED to light up.

I see how common anode would be more convenient.
 

Thread Starter

lilrips1

Joined May 7, 2010
44
That works in Multisim. I haven't wired up the whole circuit yet in real life. The first thing I did as far as putting the circuit together was test the RGB LED, and that is where I ran into a problem, and is what this thread is about. Then I asked for clarification about the need for transistors (post #26) if I am using three JK FF outputs. I didn't fully understand what the transistors did, but it worked in Multisim, so I simply went with it.
 

Audioguru

Joined Dec 20, 2007
11,248
That works in Multisim. I haven't wired up the whole circuit yet in real life.
Multisim usually gets things wrong because it is old and doesn't read the datasheets.
The output high voltage and output current of the very old TTL IC is too low to light an LED.
 

Thread Starter

lilrips1

Joined May 7, 2010
44
Well the datasheet says that each output has somewhere between 2.4 V and 3.4 V. The current won't be enough even with Bernard's transistor recommendation? Thatoneguy indicated that Bernard's sketch is my solution.
 

Audioguru

Joined Dec 20, 2007
11,248
If the output of the very old TTL gate is only 2.4V at a low current then adding the transistor emitter-follower will result in a voltage of only 1.7V which is too low to light most LEDs.
The circuit with the 2N3906 PNP current-boosting transistor will work but it inverts the logic.
 

Thread Starter

lilrips1

Joined May 7, 2010
44
Well in that case I could take the !Q outputs instead of the Q outputs.

However, in that other thread I ended up going with the 2N3904 in place of the MPSA13. The 2N3904 is an NPN transistor, which doesn't invert the logic, but will that amplify just as much as the 2N3906?
 

Audioguru

Joined Dec 20, 2007
11,248
Why are there two theads about the same thing?

The datasheets for old TTL logic ICs show that they have plenty of output current when they go low but very little output current and a fairly low voltage when they go high.

An MPSA13 is a darlington (two transistors) that has a current gain of at least 1000 but it has a voltage loss that might be too high for your circuit.

The problems are that the TTL IC is the wrong type and is much too old, its supply voltage is too low and your displays are common-cathode instead of common-anode.
 

Thread Starter

lilrips1

Joined May 7, 2010
44
Well I'll pick up a common anode RGB LED at RadioShack so if I need to I can just use two different battery sources: one to power the LED, and one to power the logic gates, and then I can switch over to using the !Q outputs. That would all work then, wouldn't it? It seems like an easy enough solution to me, but I'm probably incorrect.

Plus, I just realized jameco.com messed up my order. Instead of giving me two JK flipflops, they gave me a flip flop and a ripple-carry binary counter in the same tube. :mad:
 

SgtWookie

Joined Jul 17, 2007
22,230
You probably don't have a high enough Vcc to make the LED work on blue for the left schematic posted above, and the old TTL won't have an output level high enough to turn off the transistor on the right. You'd need more like the attached schematic.

Since both Q1 and Q2 are used as saturated switches, the double inversion results in the LED going on when the TTL output is high, and off when the TTL output is low.

R1 limits base current for Q1. R2 ensures Q1 turns off when the TTL out goes low, as it's only guaranteed to go below 0.8v, which is not enough to turn off a transistor.

R3 limits the base current for Q2. R4 ensures Q2 turns off when Q1 turns off.

R5 limits current through the red LED.

You can substitute 2N2222 or 2N4401 transistors for the 2N3904.
You can substitute 2N2907 or 2N4403 transistors for the 2N3906.

All of these are available at your local Radio Shack in the 15-pack NPN and 15-pack PNP assortments.

It would be easier to use 74HC or 74AC components instead of the old 74LS or 74S or plain 74 series.
 

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Thread Starter

lilrips1

Joined May 7, 2010
44
I plugged that into my Multisim circuit with each LED's respective resistor value for 4.5 V (however, should I use the output voltage of the IC instead?), and again it worked nicely. I already have some 2N3904's, so ill pick up the PNP package and a variety pack of resistors.
 

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Audioguru

Joined Dec 20, 2007
11,248
You have the 2N3906 transistors connected upside-down. The emitter should be connected to the +5V supply, not the collector.
But since the very old TTL 74LS76 IC outputs do not go high enough then the transistors and the LEDs they drive will never turn off.
 

SgtWookie

Joined Jul 17, 2007
22,230
Like AudoGuru says, you have Q1, Q3, and Q5 installed with the collector and emitter connections reversed. You'll need to flip them around.

In the Radio Shack PNP assortment you'll be getting, the 2N3906 transistors are good for up to around 30mA, the 2N4403's good for up to around 100mA, and the 2N2907's up to around 500mA when your base current is 1/10 of the desired collector current. If you attempt to exceed those suggestions, you'll find that your Vce (collector-emitter voltage) becomes excessive and the transistors will heat up.

AudioGuru,
Have a look at the simulation that I posted on the bottom of page 5 of this thread; I used a square wave input that had a logic low of 0.8v and a logic high of 2.8v. I selected R1 and R2 so that Q1 would be in cutoff at 0.8v, and be saturated at 2.8v.
 

CDRIVE

Joined Jul 1, 2008
2,219
I really must be missing something here. Three more pages were added to this thread since AAC went down for maintenance, but I re-read them all from the beginning, just to be sure I didn't miss anything. My question is.. Why are we discussing TTL, Flip Flops, and LED driver Transistors, when the OP hasn't even resolved his RGB LED problem that (he clearly stated) was being tested directly from his 12V battery with limiting resistors for each led? He said he purchased a DMM but he hasn't posted any measurements across the Bat or any other nodes.

Did I miss something? :confused:

EDIT: Never mind. I found it the second time I read through all posts.
http://forum.allaboutcircuits.com/showpost.php?p=320758&postcount=25
 
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