# Very simple question about switching

Discussion in 'The Projects Forum' started by Just_starting, Sep 25, 2012.

1. ### Just_starting Thread Starter New Member

Sep 25, 2012
5
0
I am just starting out and need a little help..

I have 3 points in my circuit A, B and C.

I need pulses to travel unhindered from A to B & C

I need pulses to travel unhindered from B to A but in this direction I don't want them going to C

I know it's simple but we all have to start somewhere.

I have made a start using a pair of signal diodes and was hoping to use "D" to break the circuit between A and C.

Is this the way to do it or am I barking up the wrong tree?

The circuit is 5V DC and the pulse can be anything from 3mS onward and are positive square waves, this part of the circuit is pulled low via 11K of resistors when no signal is coming from either A or B.

Thanks.

Last edited: Sep 25, 2012
2. ### wayneh Expert

Sep 9, 2010
12,362
3,220
Are you aware that diodes introduce a voltage drop of about 0.7V? Perhaps this is fine in your application, but it's not exactly "unhindered".

3. ### mlog Member

Feb 11, 2012
276
36
What exactly does 'D' do? How does it break anything?

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,130
766
Woof ! comes to mind....

5. ### crutschow Expert

Mar 14, 2008
13,462
3,353
Below is a simple circuit to do what you want. The top circuit uses an OR gate so the signal is not attenuated. The bottom circuit uses diodes if you don't mind the 0.7V diode drop.

In order to simplify the circuit you need to isolate the signal source from the signal destination. If you don't do that the circuit becomes significantly more complicated since the two sources will interfere with each other.

Thus [A] source is directly connected to C destination since they are the same signal.

[A] and sources are simply OR'd together to get the A/B signal which goes to both the A destination and the B destination.

Last edited: Sep 25, 2012
6. ### Just_starting Thread Starter New Member

Sep 25, 2012
5
0
C goes to a transistor which switches a FET so the 0.7v drop does not matter but looking at the circuit it does not quite do what was described unless I'm mistaken....

What I need to happen:

A high = C high and B irrelevant

B high = A high and C low

On yours....

A high = C high - correct

B high = A ???? - C low (pulled low by res)

I'm not sure about the A/B bit ???? I'd rather sort it with descrete components diodes/simple transistor rather than a logic chip. A 74 series chip would do it but I was hoping not to go down that route.

Last edited: Sep 25, 2012
7. ### crutschow Expert

Mar 14, 2008
13,462
3,353
On my circuit...

A high = C high
A low = C low
B has no effect on C
A high or B high = A/B high.

A/B is the combined signal to both the A destination and the B destination. You can separate A and B destination into individual wires but they both come from the A/B output. As I previously, stated you need to separate the source signals from the destination signals for this simple circuit to work.

8. ### Just_starting Thread Starter New Member

Sep 25, 2012
5
0
A & B are I/O lines connected to devices I cannot do anything about so I cannot split them up they are just as they are. Thanks for the suggestion but not sure how it would fit with my circuit....

9. ### crutschow Expert

Mar 14, 2008
13,462
3,353
So A and B are bidirectional?

Edit: Post a schematic of the circuit.

Last edited: Sep 26, 2012
10. ### Just_starting Thread Starter New Member

Sep 25, 2012
5
0
Thanks, can't post schematic a moment as off to work but what I've posted is the relevant section of it. A and B are bi-directional IO lines and C is the input to a transistor switch which in turn controls a MOSFET to switch a larger load. The input from B is passing through the circuit and going out of A but cannot affect C whilst input A controls C whilst also passing though to B in order to control something else that might be attached.

11. ### crutschow Expert

Mar 14, 2008
13,462
3,353
OK, after some brain storming and realizing that I needed to sense the current direction not voltage, I came up with the circuit below. It uses one diode and one transistor.

When A is high the transistor turns on and applies the signal to C as well as conducting the signal to B through the base-emitter junction.

When B is high, the transistor base-emitter junction is reverse biased and it is off, so C is low and the diode conducts the signal to A.

Can't get much simpler than that.

Last edited: Sep 26, 2012
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