invert signal from momentary switch

[Russ Caslis]

I was/am building myself a custom mouse for my computer. I took the
guts from an existing mouse and moved it over to my custolm enclosure,
and got a couple switches to mount on the outside that fit the theme
well.

Well, it turns out I didn't check first and the switches an normally
closed instead of normally open. Building a new enclosure will take
weeks, so I was hoping to use something like an inverter to change the
signal for the left and right buttons.

I've tried everything I can think of using a 4069 inverter, but no
luck. I'm not too good with electronics, so could someone smart out
there tell me what I need to do to get this to work?

Thanks.

 

[John Fields]

On 9 Jul 2004 23:27:28 -0700, rcaslis@xkill.net (Russ Caslis) wrote:

I was/am building myself a custom mouse for my computer. I took the
guts from an existing mouse and moved it over to my custolm enclosure,
and got a couple switches to mount on the outside that fit the theme
well.

Well, it turns out I didn't check first and the switches an normally
closed instead of normally open. Building a new enclosure will take
weeks, so I was hoping to use something like an inverter to change the
signal for the left and right buttons.

I've tried everything I can think of using a 4069 inverter, but no
luck. I'm not too good with electronics, so could someone smart out
there tell me what I need to do to get this to work?

---
View with a fixed-pitch font:

Vcc
|
[10k]
| |\
|<--+----O| >-->OUT
| |/
O
|
GND


--
John Fields

 

[Russ Caslis]

John Fields <jfields@austininstruments.com> wrote in message news:<il00f0p5mcnidtf5t28269vkc0s4p1h9oi@4ax.com>...

View with a fixed-pitch font:

Vcc
|
[10k]
| |\
|<--+----O| >-->OUT
| |/
O
|
GND

Thanks so much for the response, but I'm not sure I understand.

The switch goes to ground on one wire, and the output of the circuit
on the other wire. What's happening with Vcc and the input/output of
the 4069 isn't clear to me.

Also, there are two connections for the switch on the circuit board.
One seems to be ground, and the other seems to have 5v on it and
registers a button press when it's tied to ground.

Can you provide a little more info? Thanks again.

 

[Peter Bennett]

On 10 Jul 2004 15:59:35 -0700, rcaslis@xkill.net (Russ Caslis) wrote:

John Fields <jfields@austininstruments.com> wrote in message news:<il00f0p5mcnidtf5t28269vkc0s4p1h9oi@4ax.com>...
View with a fixed-pitch font:

Vcc
|
[10k]
| |\
|<--+----O| >-->OUT
| |/
O
|
GND

Thanks so much for the response, but I'm not sure I understand.

The switch goes to ground on one wire, and the output of the circuit
on the other wire. What's happening with Vcc and the input/output of
the 4069 isn't clear to me.

The 4069 inverts - when its input is high, the output is low, and vice
versa.

CMOS logic parts like the 4069 have a very high input impedance - if
they are not connected to anything (as when the switch is open, and a
resistor is not used), the input will wander aimlessly between high
and low - as a result, the output of the inverter is unpredictable.
To ensure that the input of the inverter goes high when the switch is
open, you need a "pull-up" resistor (the 10K resistor shown in the
drawing) to hold the input high.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[John Fields]

On 10 Jul 2004 15:59:35 -0700, rcaslis@xkill.net (Russ Caslis) wrote:

John Fields <jfields@austininstruments.com> wrote in message news:<il00f0p5mcnidtf5t28269vkc0s4p1h9oi@4ax.com>...
View with a fixed-pitch font:

Vcc
|
[10k]
| |\
|<--+----O| >-->OUT
| |/
O
|
GND

Thanks so much for the response, but I'm not sure I understand.

The switch goes to ground on one wire, and the output of the circuit
on the other wire. What's happening with Vcc and the input/output of
the 4069 isn't clear to me.

Also, there are two connections for the switch on the circuit board.
One seems to be ground, and the other seems to have 5v on it and
registers a button press when it's tied to ground.

Can you provide a little more info? Thanks again.

---
Actually, the simplest thing to do would probably be to replace the
switches you've got with normally-open switches, but...

On your ciruit board, there are basically two ways it could be wired
for the switch:


+5V
|
[R]
|
| <--O--A1
|
O-------A2
|
GND



+5V
|
|
| <--O--B1
|
O-------B2
|
[R]
|
GND

In either case, if you measure the voltages at the connections (pads)
for the switch, you'll read 5V (Vcc) on one pad and 0V (ground) on the
other, as you found. However, if you measure the voltage on either
pad when they're tied together (as would be the case when a
normally-open switch was made) they will read either +5V or 0V. If
they read 0V, then the circuit will be wired as in A1A2, above. If
they read +5V, then it'll be wired as in B1B2. The difference is that
in A1A2 the pullup resistor is wired to +5V, and in B1B2 it's a
pull-down resistor and it's wired to GND.

What you'll need to do is to find out which pad the resistor is
connected to, and to do that you'll either need to inspect the board
visually and track down what the pads are connected to, or measure the
resistance from the pads to ground and Vcc. You'll almost certainly
find that one pad is wired directly to Vcc and the other pad is wired
to ground through a resistor, or that one pad is wired directly to
ground and the other pad is wired to Vcc through a resistor.

You could also measure the voltage on one of the pads and then short
the pads together. If the voltage changes, then the pad the voltmeter
is connected to is the pad the resistor is connected to. If the
voltage doesn't change, then the resistor is connected to the other
pad.

In any case, the pad connected to the resistor is the one you'll be
interested in, and you'll need to connect one of the 4069's inverters
to it like this:

NEW SWITCH MODS | | EXISTING MOUSE CKTY
| |
+-----+---->| |>-----Vcc
| | | |
| | | | |
[10k] | | | [R??]
| | \ | | |
|<--+---O| >-->| |>--+--
| | / | |
O | | |
| | | |
+---------+---->| |>-----GND
| |

The switch shown is your normally-closed switch, and it shorts the 10k
pullup resistor to ground, forcing the input of the 4069 low, which
causes its output to go high, fooling the mouse into thinking that the
switch is [normally] open. When the switch is pressed, it opens,
pulling the input of the 4069 high, forcing its output low, fooling
the mouse into thinking that the switch is closed. You'll need to add
the 10k resistor to the circuit as shown, and you may need to remove
the resistor (R??) on the PCB if it loads down the output of the 4069.

Try it; if the circuit doesn't work when it's connected to the mouse
guts, measure the voltage on the output of the 4069 with the switch
un-pressed and pressed. Un-pressed should read pretty close to +5V
and pressed should read pretty close to 0V. If it doesn't, unsolder
one end of R?? and try again. If it still doesn't, well... we'll
cross that bridge when we get to it.

Finally, you'll need to connect the unused inputs of the 4069 to
ground or Vcc. It doesn't matter which, they just shouldn't be left
floating.


--
John Fields