Current limiting resistor for led on 7408 gate output

[Ald]

I am building a 2 to 4 decoder with 7408 And gates and need to put LED's on the output as indicators, active high. 2 questions

1. Would I put a current limiting resistor in series with the LED to ground or put the resistor and LED from the gate output to Vcc?

2. The LED draws 30mA at 2V, would I use the worst case Vo of the gate to calculate the required R value?

Thanks

 

[crutschow]

If you read the 7408 data sheet (I realize that's asking a lot) it shows that the maximum low (sink) output current is 16mA at 0.4V output voltage and the maximum high (source) output current is only 0.4mA. Thus you would need to connect the diode to the +5V and the maximum diode current would be 16ma. The series resistor required is (5-0.4-2)/16mA=162 ohms (use 170-180 ohms for margin).

This connection lights the LED when the gate output is low which is opposite to what you want. To invert the signal you could use a 7400 Nand gate which has a low output when both inputs are high. If you need a high output for other purposes you could add a 7405 inverter at each output.

If you want to operate the LED with 30mA then you could use a 7406 buffer/ inverter at each AND gates output to drive the LED. The 7406 can sink 40mA maximum.

 

[Boncuk]

Use a small power transistor with a low base current to be controlled by the ICs output. Connecting an LED with or without current limiting resistor will cause problems if the output signal is used to control other IC-inputs.

A BC547 will do the job nicely.

Boncuk

 

[Ald]

To confuse matters, it turns out I need to have active high outputs.

I only have available a 7410 Nand (I need 3 inputs) I was going to place a 7404 inverter (all I have available) on the output to get active high output.

I would like the LED to light up when output is High. Circuits are a 2 to 4 and 3 to 8 decoder.

How will the inverter change anything in terms of driving the led's?

 

[crutschow]

You can drive the LEDs as well as the 7404 inverter from the Nand output. Standard TTL outputs will sink a maximum of 16mA. The 7404 input will require 1.6mA, thus leaving about 14mA to drive the LEDs. Will that be bright enough for your requirement?

 

[Ald]

So will I put the diode/resistor branch on the output of the inverter to Vcc? I want to LED to light when the output is high.

Also I only have 150 or 200 ohm resistors available, which side should I error on?

I think I was able to attach an image of the circuit?

 

[crutschow]

So will I put the diode/resistor branch on the output of the inverter to Vcc? I want to LED to light when the output is high.

No. If you connect the LEDs to the inverter outputs as you show, then they will light when the inverter's output is low. When the output is high there's no voltage across the diodes and thus no current flows.

If you want the diodes to light when the inverter output is high, then you need to connect them to the Nand output (inverter input). That way when the Nand gate is low (inverter output high) there will be voltage across the diode and it's resistor, and the diode will light.

Also I only have 150 or 200 ohm resistors available, which side should I error on?

If you have only those two resistor values available, use the 200 ohm. It will give about 13-14mA though the LED when on.

 

[Ald]

Thanks a lot for your help, you've taught me a lot.

 

[tejasvi]

i am working with 7408 ic. i read here that if u connect transistor to 7408 to power led i tried this but it is not working. i am using pnp transistor the led just glows but there is no sign that ic is working as a and gate. i also tried npn but circuit is just dead

 

[audioguru]

I think people are wrongly interpreting the spec in the datasheet of the SN7408.
Its minimum output voltage is +2.4V (the minimum recommended input high voltage of a TTL logic gate) when its load is 800uA (20 TTL inputs each with a max current of 40uA).

The output is an emitter-follower in series with a diode and it even has a series 130 ohm current-limiting resistor so it would easily provide +1.8V to a red LED with a current of a few mA.
But I haven't seen an old TTL IC for many years for me to try it.

 

[nike6]

use the modern HC variant. they can power LEDs directly.
if you are scared about burning out or IC heating up, work at 3v or 4v.

also I have another suggestion.

get low-current/high efficiency LEDs, and then use 2.2k to 4.7k resistors.
it is enough for indication.
I can get 3mm King's Red (Dark Red) low-current LEDs at $7/100pc.

you can also use the 5mm LEDs from Sure Electronics with a 4.7k resistor.
however, they did not answer my request for 3mm LEDs, and also otherwise are slow to respond to custom requests.
They just supply what's on their webshop.

 

[tejasvi]

hey audio guru when are u online

 

[tejasvi]

i hate all 74xx sereis of ic (Texas instrumentation). man what is the point of making such kind of ic which has reverse current flow in logic high . i can not directly cascade 7408 ic to 7400,7475,7404 etc. i require transistors for interfacing.now if thats the case i shell use simple ttl logic to make and gate. it will not be costly also.when ever i use them they easily get ground through my body.
same is the case with 7475 d latch ic. what a stupid i was to purchase it just look at its data sheet . it requires 20uA an 80uA as its input How the hell would that be achieved very high value of resistence would be required if i use 3V of supply.it gives 400uA at its output i dont think any thing could be connected to it.
is there no company which makes logic ic .can any body here just please tell me any other sereis of ic that can power normal led directly

 

[audioguru]

i hate all 74xx sereis of ic (Texas instrumentation). man what is the point of making such kind of ic which has reverse current flow in logic high . i can not directly cascade 7408 ic to 7400,7475,7404 etc. i require transistors for interfacing.now if thats the case i shell use simple ttl logic to make and gate. it will not be costly also.when ever i use them they easily get ground through my body.
same is the case with 7475 d latch ic. what a stupid i was to purchase it just look at its data sheet . it requires 20uA an 80uA as its input How the hell would that be achieved very high value of resistence would be required if i use 3V of supply. it gives 400uA at its output i dont think any thing could be connected to it.
is there no company which makes logic ic .can any body here just please tell me any other sereis of ic that can power normal led directly

The 74xx series of TTL logic is extremely old. It needs a regulated 5V supply. It uses a lot of supply current.
Its max output low current is 16mA max at 0.4V which is fine to drive up to 10 TTL inputs that need a max input low current of 1.6mA at 0.8V or less.

Its max output high current is 400uA at 2.4V which is fine to drive up to 10 TTL inputs that need 40uA at 2.0V or more.

I threw out my "TTL cookbook" over 30 years ago when I started using CD4xxx and 74HCxxxx Cmos logic ICs. They have NO input current and use a very low supply current. The CD4xxx use an unregulated supply from 3.0V to 18.0V and the 74HCxxxx use an unregulated supply from 2.0V to 6.0V. Their output current is high enough to drive LEDs when high or when low.

 

[tejasvi]

cd4xxx and 74hcxx sereis are cmos technology
dont they easily get damaged by static electricity for example by
merely touching it.
and what about the 74ls sereis i heard they are new

 

[Hero999]

The LS series is hardly new.

All ICs can be damaged by ESD but CMOS are more sensitive. I don't know about he CD4xxx series but the 74HC series have built-in protection diodes to minimise ESD.

 

[audioguru]

cd4xxx and 74hcxx series are cmos technology
dont they easily get damaged by static electricity for example by
merely touching it.

Yes, they have input protection diodes. A pretty big static spark can still destroy them.

what about the 74ls series i heard they are new

LSTTL is also old but is a little newer and better than ordinary older TTL.
LSTTL uses Schottky diodes to prevent its transistors from saturating therefore has high speed and has lower supply current than old TTL.

 

[tejasvi]

**broken link removed**
it doesnt say that 4011 can power led directly

 

[audioguru]

**broken link removed**
it doesnt say that 4011 can power led directly

All CD4xxx ICs are made the same by many manufacturers.
Look at the datasheet from Texas Instruments for their CD4011 or any other CD4xxx logic IC. The datasheet has graphs of typical and minimum output current.

With a 5V supply the output current into a 3V LED is typically 3mA.

With a 10V supply the output current into a 3V LED is typically 14mA low or 18mA high. Without a current-limiting resistor the output transistor will dissipate 100mW at 14mA which is its maximum allowed.

 

[tejasvi]

i have to make 10x10 led display can any body tell me any micro controller or ic that can be employed in this task.
the display is to have custom display. meaning it can be programmed to display anything.
i was thinking that memory chips can employed in this task like one memory cell for each led. or is it just a foolish idea??
& it has to be cheap.