Advice on a buffer/inverter chip

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cepheid
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Main Question or Discussion Point

I'm working on a circuit for a hobby project (not homework). In this circuit is an IC that carries out the functions of being a countdown timer and driving a seven segment LED display. I want to be able to do either of the two following things:

1. Have the logic level of one of the pins on the aforementioned IC determine whether a load is driven or not driven.

2. Drive one of the pins on the IC high or low depending on the the logic level of some signal.

For number 1, as an example, there is a pin on the timing chip that is always at logic low whenever the value of the counter is 00:00 (and logic high otherwise). I want to use this "zero" pin to cause a load to be driven whenever it is low. I don't want to use a BJT, because I don't think that the pins on this IC can source or sink very much current at all, and I don't want to have to worry about having sufficient base current to make the transistor work. A MOSFET would be perfect, but I don't think there are discrete, low-level MOSFETs, and a power MOSFET would be overkill (when I say a "load", I'm talking about an LED powered by 5 V. I'm not talking about switching huge currents here). I thought maybe I could use a logic buffer/driver chips like a 7406 or 7407 (depending on the specific case). However, it doesn't seem to be quite what I need. Instead of a truth table like:

input --> output
------------------
high --> high

low --> low

What I really want is:

input --> output
------------------
high --> high impedance

low --> conducting

For number 2, it's the same idea, except that I would connect the pin on the timing IC to the *output* of my driver/buffer/whatever, instead of having it be the input.

Can you recommend something that will do the job?
 
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Answers and Replies

  • #2
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Why can't you use the 7406? Have you checked the cmos inverter CD4049? What is this IC you're trying to interface with? You don't really need an inverter though. Just use a buffer (because they have higher sink current) then connect a transistor switch if needed. But you can probably drive the led directly.
 
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cepheid
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You don't really need an inverter though. Just use a buffer (because they have higher sink current) then connect a transistor switch if needed. But you can probably drive the led directly.
Good point. I guess that's the whole point of a buffer. I'll look into your other suggestions as well. I just thought that the 7406 wouldn't act like a switch, but maybe I'm wrong.

This chip I'm using is an ICM7217 from Maxim. Thanks for your reply!
 
  • #4
887
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I just checked the data sheet on the ICM7217 and it shows that the ZERO output low (IBOL) can sink 2ma. That should be enough to drive a transistor switch. But it wouldn't hurt to have a buffer in there.
 

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