Looking for a counter chip

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husslemk4

New Member
Hello all,

I'm looking for a ripple counter chip like a 7490 decade counter; however my problem is that I need them to go LOW when triggered. I want to control the common cathode pins of a 4 digit 7-segment display so I can multiplex the digits.

I know I could use the HIGH output of a counter and then just switch a transistor to ground the common cathode pins, but is there any other way to do this?

Thanks,
Huss

------------Backround details----------------
I have 16 7-segment displays (i.e. 16 digits). All of their letter segments (a to g) will be connected to a 3-8 line decoder chip. I then want to control the ground of each digit, and switch the digits on sequentially with their value from the output of the decoder chip. Of course I will do this fast enough so that all the displays appear on.
 
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BeanieBots

Moderator
Even if you did find a suitable chip, I expect it would not be able to supply enough current on it's output when you wanted to display an "8".
Your best bet would be to use a trnasistor to do both the inversion and provide sufficient current.
 
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Minifig666

Senior Member
Why not just use a microcontroller say a PICAXE for a change. (I joke) No what you said with the transistors sounds good but where you said 'common cathode pins' do you mean there is just one common cathode and the rest are anodes or a common anode?
 
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husslemk4

New Member
Hi BeanieBots and Minifig66, thanks for the replies

I'm slightly confused over the whole current issue, because on the datasheet it quotes 20mA typical, but I don't know if that is for the whole digit or each LED in the digit.

When I said common cathode, I meant that the grounds of the LEDs of each digit are tied together. It's shown better than I can explain it on the datasheet. Not to worry, a PICAXE will be used! :)

Here is the datasheet link:
http://www.rapidonline.com/netalogue/specs/57-0252.pdf
So I will have 4 of these units making a total of 16 digits
 
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Minifig666

Senior Member
Sorry I misread that there. It relates to 20mA per LED not per unit unless they are some very efficient LEDs. I see know, thanks for the datasheet. As long as your transistors can handle about 120mA (SMD opportunity to shrink it?) they should work very well.
If they where to release the 18M2 sooner then you could probably have it and its magic powers do it all for you.
What will you be using this project for? A scrolling message display, or something more sinister...?
 
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BeanieBots

Moderator
That's 20mA each segment.
If you are multiplexing, then you can run at higher (peak) current but the average should remain at 20mA and the peak should not exceed 160mA.

For all 8 segments (including decimal point) at max peak current that would require a transistor that can handle 1.3A and also a power supply that won't collapse with that sort of current. Of course, you could run at much less than that at the expense of brightness.
 
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westaust55

Moderator
Many LED's have an absolute max rating of 20mA. If you look at most circuits, they include say a 330 Ohm or 470 Ohm resitor in series to limit the current to something like 11mA or 8mA.
In the past forum members here have reported that LED's can still be quite visible down to around 3 or 4mA.
So in that regard it will depend upon how far away and thus how bright you want your display to be.

With respect to IC's you need to learn to read the datasheets.
Datatsheets are your friend.

Even if there was a version of the 7490 with low active outputs consider that:
a 7490 can handle a max of +16mA per output (that is it can only source current to each output) and a typical total of just 29 mA for the entire chip.

the ubiquitous 74HC595 shift register which is often pressed into service to drive 7-segment displays
can handle up to +/- 35mA per output (that is sink or source) but has a total chip Icc current limitation of +/-70mA so if driving 7 segments and a dp must limit the individual segemnt currents to around 8mA each.

When mutliplexing you can strech the boundaries by considering averaging of the current (but personnal I prefer a robust circuit).

Transistors for higher current drivers are therefore essential if you are wishing to handle more than something like 10mA per chip output.
 
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