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Constructing ab' with a NAND

R

Rikard Bosnjakovic

Jan 1, 1970
0
I'm designing a small alarm-circuit for a veroboard and I've run into a
problem that I hope anyone can help me with.

The truth table for triggering my alarm is this:

a b trig alarm
0 0 0
0 1 0
1 0 1 <--
1 1 0

That is, ab'.

In the circuit I'm using a 74LS03 (quad 2-NAND O/C) and up to this moment
three of the four gates are used. Because of the limitations of the
project size I cannot add another IC to the board, but there is room for a
couple of diodes and transistors.

What I need to do is manually constructing the ab' output. Since I'm
having a free NAND gate, I can invert either a or b to get 0/0 or 1/1
respectively, but I'm unable to get any further since I haven't been able
to figure out what to do next.
 
T

Terry Pinnell

Jan 1, 1970
0
Rikard Bosnjakovic said:
I'm designing a small alarm-circuit for a veroboard and I've run into a
problem that I hope anyone can help me with.

The truth table for triggering my alarm is this:

a b trig alarm
0 0 0
0 1 0
1 0 1 <--
1 1 0

That is, ab'.

In the circuit I'm using a 74LS03 (quad 2-NAND O/C) and up to this moment
three of the four gates are used. Because of the limitations of the
project size I cannot add another IC to the board, but there is room for a
couple of diodes and transistors.

What I need to do is manually constructing the ab' output. Since I'm
having a free NAND gate, I can invert either a or b to get 0/0 or 1/1
respectively, but I'm unable to get any further since I haven't been able
to figure out what to do next.

Method 1:
Invert b with a transistor stage to give b', then NAND(a,b') = 0, then
invert that with another transistor stage to get Out = 1

Method 2:
Invert b with NAND gate to give b', then AND(a,b') = 1 = Out with a
couple of diodes, like this:

5V
----------o----------
|
|
|
.-.
| |
| | 2k2
'-'
|
o-------o OUT
|
IN a --|<------o
|
|
IN b'--|<------o

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
 
T

Tim Williams

Jan 1, 1970
0
Rikard Bosnjakovic said:
The truth table for triggering my alarm is this:

a b trig alarm
0 0 0
0 1 0
1 0 1 <--
1 1 0 ...
In the circuit I'm using a 74LS03 (quad 2-NAND O/C) and up to this
moment three of the four gates are used.

Use a single transistor as inverter, or use the NAND as an inverter for DTL?

Tim
 
R

Rikard Bosnjakovic

Jan 1, 1970
0
John said:

Somehow I cannot follow these links. Doesn't work in either Mozilla's
news-reader, Outlook express, Gnus or pine.

Can you name the subject and date of the article?
 
J

John Fields

Jan 1, 1970
0
Somehow I cannot follow these links. Doesn't work in either Mozilla's
news-reader, Outlook express, Gnus or pine.

Can you name the subject and date of the article?
 
R

Rikard Bosnjakovic

Jan 1, 1970
0
Terry said:
Method 2:
Invert b with NAND gate to give b', then AND(a,b') = 1 = Out with a
couple of diodes, like this:

Thank you, that method worked fine.

I do however have some questions. Using the schematic you posted, if I
connect a LED between OUT and GND, the LED will light when a and b are
high (this is expected, since the circuit is an AND).

Since this AND-gate is controlling a buzzer (alarm), I connected it to a
transistor and the buzzer. If I dont use the transistor, it won't work
because of too low current (I think).

So, I grabbed a BC547 NPN from my box, connected OUT to the base, buzzer
red wire to +5 and buzzer black wire to the collector. The emitter I sent
to ground. However, this circuit makes the buzzer buzz all the time, just
as if there is always a current to the base of the transistor, completely
regardless of the a and b inputs.

I'm by all means an utter novice to transistors, but the fact that the LED
was off when either the a and b inputs to the AND was low made me think
that there will be absolutely no current to OUT, and therefore the
transistor should not let current flow through its collector and emitter,
and therefore the buzzer should not buzz either.

Obviously, I'm thinking wrong somewhere but I can't figure it out.
 
T

Terry Pinnell

Jan 1, 1970
0
Rikard Bosnjakovic said:
Thank you, that method worked fine.

I do however have some questions. Using the schematic you posted, if I
connect a LED between OUT and GND, the LED will light when a and b are
high (this is expected, since the circuit is an AND).

Since this AND-gate is controlling a buzzer (alarm), I connected it to a
transistor and the buzzer. If I dont use the transistor, it won't work
because of too low current (I think).

So, I grabbed a BC547 NPN from my box, connected OUT to the base, buzzer
red wire to +5 and buzzer black wire to the collector. The emitter I sent
to ground. However, this circuit makes the buzzer buzz all the time, just
as if there is always a current to the base of the transistor, completely
regardless of the a and b inputs.

I'm by all means an utter novice to transistors, but the fact that the LED
was off when either the a and b inputs to the AND was low made me think
that there will be absolutely no current to OUT, and therefore the
transistor should not let current flow through its collector and emitter,
and therefore the buzzer should not buzz either.

Obviously, I'm thinking wrong somewhere but I can't figure it out.

Having confirmed last night that my main HD is at risk of failure, my
priorities have suddenly changed! So I'll have to be brief, and anyway
I expect you just want to get it working. Wire the buzzer between the
EMITTER and ground, with collector to +5V.
 
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