# Switching circuit help needed

Discussion in 'The Projects Forum' started by Johnnz, Mar 22, 2010.

1. ### Johnnz Thread Starter Active Member

Dec 31, 2008
55
0
Hi all,
Have been working on a computer based mp3 jukebox lately and would like to use a novel approach to switching it on or off. Basically, just using the remote control to sit into a holder (cavity) on the top of the unit. That would be relatively simple to do since the computer is switched by a simple momentary switch, however, the remote control (receiver connected via usb) has an off button on it to, so it is possible that the unit will be switched off before the remote is placed back in the holder.

I have an idea to make use of some kind of digital circuit using two variables as input. (1) Take power status from current passing through computer LED powerlight. (2) take remote location status from a micro switch detecting the weight of the remote in the holder.

From these inputs I should imagine it is a relatively simple task to find an IC to do the job of the actual switching output to the computer.

Can anyone suggest an IC or circuit basis I could use to acheive this functionality? Attached is flow chart to clarify operation:

2. ### Johnnz Thread Starter Active Member

Dec 31, 2008
55
0
Just want to bump this question and add some more detail. Can anyone suggest an IC that can detect changes of state in two inputs and allows me to set the permutations or combinations (?) of changes possible to dictate what output is given in response?

The inputs would be power detected from (1) the computer LED power light, and (2) the on or off position of the micro switch detecting whether or not the remote is in its dock/holder.

Inputs are described as follows:

LED on = 1 on, LED off = 1 off
Micro switch on (remote in) = 2 on, Microswitch off (remote out) = 2 off

There are 8 unique possible changes in state, but only two should cause the switching circuit to produce an output pulse (that can indirectly be used to trigger the power switch on the computer)

The two changes of state that should produce an output are:

(1) on
(2) off ---> on

(1) off
(2) on --> off

The other six changes in state should not cause an output pulse. I just need help to chose an IC that could be utilized to acheive this behaviour. Obviously the switching circuit would have to be powered independently from the computer (do computer power supplies provide any low voltage stand by power output I could use?)

3. ### SgtWookie Expert

Jul 17, 2007
22,198
1,797
ATX form factor power supplies have a 5v standby output that is always on. It is on the 20 or 24pin board connector. Look up the specifications for ATX or ATX plus 12 power supply connectors. There isn't an easy way to get to it, without cutting into the harness or adding a harness extender and tapping into it there.

Using a mechanical switch introduces problems of "bouncing" contacts.
Consider using a magnet in the remote, and a Hall-effect switch in the holder.

Use the standby 5v to power the logic circuit.

The switched supply 5v is available at both the 20- or 24-pin board connector, and at any of the 4-pin molex connectors. When the computer is turned off, that power gets turned off too.

You might use 74HC series logic ICs powered by the +5v standby.

Write a truth table describing your inputs and logic states, and your desired outputs.
[eta]
If you don't know what I mean by 'truth table', have a look here:
More specifically, in this section:

Last edited: Mar 24, 2010
4. ### Johnnz Thread Starter Active Member

Dec 31, 2008
55
0
What is the best way to utilize changing input states to dictate the output rather than the current state of inputs? It seems from what I can gather that truth tables are about comparing inputs relative to one another, rather than detecting changes per se? Perhaps there is another way using analogue methods to do this project?

5. ### SgtWookie Expert

Jul 17, 2007
22,198
1,797
What's the big deal?

The holder can be the trigger mechanism.

If the remote is removed from the holder and the computer is off, the computer is turned on.

If the remote is placed in the holder and the computer is on, turn the computer off.

6. ### Johnnz Thread Starter Active Member

Dec 31, 2008
55
0
The issue is that the power switching on the computer is done with a type of standby switch, rather than just fully on or fully off (as with high voltage switching).

This method of power control allows the computer to automatically shut down when the existing soft-touch power switch (a low voltage normally open momentary switch - not a SPST switch) is pressed.

As you can see in my flow diagram, the remote holder can not be the only sensor because if power is switched off via the usb remote before placing it back in the holder I don't want the computer to switch back on again.

Perhaps, if this is really too difficult a project I will need to rewire the remote so the power button is unavailable. Then I could simply use the micro switch in the remote dock to control power to the computer. Would be a whole lot simpler then...