Where can I buy an active-low tri-state buffer?

Question

I am building a circuit and I believe I need an active-low tri-state buffer to accomplish the task at hand. Let me know what you think.

I have two signals say A and B, and output C. If only A is on, I want C = A. If only B is on, I want C = B. But if both A and B are on, I want C = A.

This means I would take the said buffer and connect A to the inverted switch, correct?

I have drawn a simple circuit, you can see it here http://s555.photobucket.com/albums/jj453/Bostonaholic/?action=view¤t=circuit_paint-1.jpg

Answer 1

The circuit won't work, for two reasons.



First, most gates have limited drive capability. Typical LED requires 20mA, and most gates cannot drive.



Second, the circuits will interfere. Remember that when A or B is low on your circuit, they draw current through their resistors from the other circuit. Consider when your AND gate is on, and A is off. The LED will only see about 1V, which is not enough to drive most LED's.



To solve the first problem, use a CMOS quad NOR with high current capability, like an ACT or AC (both should sink/source 20mA). This gives you rail-to-rail voltage. Buy online from someone like Digikey, Mouser, or Newark in the US, similar suppliers in UK and Australia.



Here's the circuit that fixes the second problem by using the diode to isolate the circuit functions:



A---.|….…….|---.|

B---.|.NOR.--|---.|.NOR.---------|

..…………...…….……....….…|

..…………...…….……....…..LED

..…………...…….……....….…|

….|--.|….…..A---.|…………..…|

B--|--.|.NOR.-----.|.NOR.-- R1--|

..…………...…….……...…..…|

.……...….……………….…...R2

..…………...…….…….........…|

…………..……....................GND



Circuit Operation: There are two parts isolated by the diode. One is above the LED, and one below. The top part powers the LED if A or B, while the bottom part has the current limiting resistor and a dimmer. NAND and NOR gates can be used to make anything. Typically, 4 gates on one chip, so best to use 4 identical gates. I used NOR gates, but good chance 4 NAND gates would work also.



Top Circuit Description:

A and B go into a NOR gate. The output is backwards, so put it into both inputs of a second NOR gate to invert. This goes to the top of the LED.



Bottom of the LED:

B goes into both inputs of third NOR gate. This inverts B. B' and A go into fourth NOR gate. This dimmer signal is on to dim, and goes through R1 to the bottom of the LED. R2 goes from the bottom of the LED to ground.



Sizing the Resistors:

Two constraints are used to size R1 and R2, depending on current you want through LED for bright and dim modes.



Dimmer is zero for bright. Select R1 and R2 so that R1 and R2 together sink the desired current for the LED.



Other constraint is when Dimmer is 1, fourth NOR gate sources some current. Size R1 and R2 for ratio that gives correct current through LED. R1 should probably be larger than R2.



This is way cheaper and easier than gates + relay.



Good luck!

Answer 2

It's not entirely clear if you want A, B, and C to be digital or analog signals. If by 'on' you mean a digital 1, then you've described a truth table something like:



A - B | C

0 - 0 | X

0 - 1 | 1

1 - 0 | 1

1 - 1 | 1



for which a standard OR gate would work. You can order these at any of a large number of online parts suppliers, e.g. www.jameco.com



but given that you specify that you want C = A (and not just 'ON') when A & B are both on, I'm guessing that what you might really want is a multiplexor. A 2-to-1 multiplexor would do the job, but you'd still need to decide at what analog level A & B qualify as 'ON'. Once you've decided that, you can use a comparator to convert your analog levels to digital signals for controlling the multiplexor.



Having said all that, your circuit looks like you're wanting an LED to turn on in either a dim or bright mode depending on these A & B signals. If these A & B signals are really digital, and both at the same voltage level, then it would appear you want the LED to be off when both are off, dim when only B is on, and bright when A is on, regardless of what B is doing. If that is the case, then your circuit will work as designed provided that your buffer is capable of supplying the current needed for your load. Again, refer to the online suppliers or stop by a local Radio Shack (they might have the parts on hand). Alternatively, you can build what you want with a relay where A drives the relay and B is connected through it on the normally closed (NC) connection.

Answer 3

A tri-state buffer is a circuit that can either copy an input to produce an output (produce the two states of the input) or have the output turned off, so that it is effectively disconnected (a high impedance third state). The choice of following the input or going into the high impedance state is controlled by a second input.



But what you describe sounds more like an OR gate. If either input is a high (on) you want the output to be high (on). But if both are on, you also want the output to be on, which happens to be the same as A or B at hat moment.



Since you are driving an LED with the output (no need to pull the output down for the off state, just to block current) you could either connect both signals to the anode of diodes and connect the cathodes together and to the single current limiting resistor of the LED. Of you could connect A and B to an OR gate and connect its output to the current limiting resistor of the LED.



--

Regards,



John Popelish

Answer 4

Looks like a logic circuit of some sort.



An OR gate will give you:

A on, C on.

B on, C on.

A and B on, C on



It's just a simple OR gate.



Now I have to change your wording:

"A is on, I want C = A"

but since A is on, this is equivalent to"

"A is on, I want C on"



Perhaps you are not phrasing your requirement correctly?

for example, what happens when an input if off?



Actually 2 diodes will work also.



EDIT, new data.



Sorry, don't want to speculate on hidden meanings any more. "the path of A", etc. is actually meaningless. State exactly what your requirements are, and you will get a good answer. I suspect you have a complicated signal in A and B that you want to gate, but please say so and describe it, instead of meaningless phrases like "the path of A".



You statement still appears to require a simple OR gate, as does the truth table.





.

Answer 5

It sounds like you need a more complicated circuit. It looks to me like you are expecting A and B to be a different voltage. Neither an OR gate nor a buffer would work like you expect if this were the case, they are logic elements and work at a set voltage. You could look at using a selector circuit, this would require additional external triggers to make it work, or you could look into a summing amplifier. of some sorts, we really need more information about what the rest of your circuit is.



I see you added stuff about taking paths, in that case TTL or Digital logic will not work. You will want to use a selector.

Answer 6

When two open-collector outputs are wired together, if either one is low, it pulls the other one down.