Need help in using SN74LS47..

sn74ls47 circuit

Hi all,
I hv problem using this IC. For some reason, it just doesnt drive out enough output voltage or current to light up the 7 segments LED.
I supplied +5V an +7V but both dont work. With +5V, I'm getting Output voltage: 0.11V (for HIGH), 0.03V (for LOW), and Output current: 1.42mA (for HIGH), 0mA (for LOW).

Can anyone help me? I try to search from the web but some of them just simply connect to the 7 segments LED and it seems to be working but not for me! I hv no idea with the Open collector output too. I did try to use 2k ohm as a pull-up resistor but it fails too!

I really need help in urgent! So pls do help me if u know how to use this IC!

Thanks a lot!

Regards,
Eric

ic sn74ls47 used in seven segment displaay

mmm, did you read the datasheet???
you need to add pull up resistors to the output.
Or use a common anode display without the pull up resitors

what is sn74ls47

Hi,

Common anode of your seven segment display must be connected to the +5V power supply. Each cathode of the seven segment display must be connected in serial with a resistance of arround 270 ohms to the dedicated open collector output of the 74ls47.

sn74ls47 connect to 7-seg

An open collector output is one where the last transistor output stage has a collector NOT connected to the supply rail. As a result, it can only sink current, but not source current. This implies that the output of the IC can only be pu lled to ground on low, but floats on high. Floating means that the actual voltage is determined by the external circuit. The common method is to use a pull-up resistor to pull the voltage high when it floats. Or like what seinfeld has suggested, use a common-anode display.

open collector not connected

Thanks everybody! I use 2k ohm resistor as pull-up resistor now to pull the voltage but I dont know why it somehow like working on the other way. When the output is supposed to be high, I'm getting 0V but when it's supposed to be low, I got a voltage higher than 0V!

+display +sn74ls47

Hi Ericmar,

First, here's a link to a datasheet:

**broken link removed**

Now, do you realise that the segment outputs of the '47 are active low? The chip is intended to drive a common anode display - that is, one with all the positive sides of the LED segments joined together and the connecting pin joined directly to +V. The segment outputs of the chip then go to the negative sides of the LED segments, each though a resistor (of about 220 ohms) to limit the current from your 5V supply.

When the segment is activated to light, the chip's output for that segment goes to 0V, letting current flow through the segment from +V, through the segment, through the resistor and finally though the chip to ground.

If the segment is not activated, the output 'floats' as previously mentioned. That is, it is effectively unconnected. So, no current can flow.

See this link for a simple example:

http://www.electronics-lab.com/projects/misc/012/

Note that you can't easily use a common-cathode display. If you use a pull-up resistor, the output will be forced high (+V, through the resistor) when the segment output is inactive, and pulled low when active. With a common-cathode display connected, this is the opposite of how the device expects to decode the data to 7-segment - the segments will be inverted - segments will light when they should be off and be off when they should light.

Cheers,
FoxyRick.

google.74ls47pdf

hi,
u can get useful material from
www.completedigitaldesign.com/articles/web_counter.pdf

how to use sn74ls47

can you send the circuit to me and i will tell you where is the error.