Switch de-bounce circuit questions.

Hello,

I have put together a switch circuit using two tactile switches with
555 de-bouncers to a positive edge triggered flip-flop controling
transistors/mosfets.

The on-board tactile switches work fine but I have also added pins for
off-board switches.

With off-board switches what do I need to do in addition to using
shielded wires to prevent false triggering? Twisted wires caused the
switch circuit to flip-flop when a fluorescent light was turned on.

In addition I have LEDs indicating the flip-flops output ON state but
how do I indicate the OFF state with LEDs? Formerly I used a double
pole switch.

Cheers,

Andrew.

 

[Bill Bowden]

(Rubicon) wrote in message news:<418dc8b9.720652@news.netaccess.co.nz>...

Hello,

I have put together a switch circuit using two tactile switches with
555 de-bouncers to a positive edge triggered flip-flop controling
transistors/mosfets.

The on-board tactile switches work fine but I have also added pins for
off-board switches.

With off-board switches what do I need to do in addition to using
shielded wires to prevent false triggering? Twisted wires caused the
switch circuit to flip-flop when a fluorescent light was turned on.

Did you use a termination resistor at the flip-flop input?
If the trigger signal is from a 555 timer, you can use a fairly
low value resistor at the flip-flop input, maybe 1K will help.
The resistor will short out the noise on the long wires.

In addition I have LEDs indicating the flip-flops output ON state but
how do I indicate the OFF state with LEDs? Formerly I used a double
pole switch.

If you have access to both sides of the flip-flop, you can use the
other side to indicate the off condition.

-Bill

 

On 6 Nov 2004 22:01:45 -0800, wrongaddress@att.net (Bill Bowden)
wrote:

(Rubicon) wrote in message news:<418dc8b9.720652@news.netaccess.co.nz>...
Hello,

I have put together a switch circuit using two tactile switches with
555 de-bouncers to a positive edge triggered flip-flop controling
transistors/mosfets.

The on-board tactile switches work fine but I have also added pins for
off-board switches.

With off-board switches what do I need to do in addition to using
shielded wires to prevent false triggering? Twisted wires caused the
switch circuit to flip-flop when a fluorescent light was turned on.


Did you use a termination resistor at the flip-flop input?
If the trigger signal is from a 555 timer, you can use a fairly
low value resistor at the flip-flop input, maybe 1K will help.
The resistor will short out the noise on the long wires.

No Bill I did not. 1K approx to V- at F/F input.

In addition I have LEDs indicating the flip-flops output ON state but
how do I indicate the OFF state with LEDs? Formerly I used a double
pole switch.


If you have access to both sides of the flip-flop, you can use the
other side to indicate the off condition.

Unfortunately the other side if the F/F has an identical 555 trigger
circuit.

I can't help but think that it's a waste of a 555 timer here - perhaps
I should have gone for a transistor/resistor/capacitor circuit
instead?

Cheers,

Andrew.




>-Bill

 

[Terry Pinnell]

(Rubicon) wrote:

Chris,

Thanks for the reply. The circuit I used was a general 5V-15VDC 555
de-bouncer circuit and has a 100K resistor from my 5VDC+ to Pin#2 of
the 555 then via the on-board push-button (tactile) switch to V-. A
100K resistor goes from V+ to 555 Pin#7/6 and on to a 1uF ceramic cap
to V-. I chose the cap value via experimentation and I would estimate
that the circuit is about 95% effective like this as sometimes it
"misses a beat".

I will make the alterations you suggested though I've already gone and
chipped away the stripboard! Blast!

1. Your one-shot is currently about 110 ms; I'd make it a bit longer,
say 300 ms, by increasing either the Rt or Ct.

2. You don't show a schematic, but have you included a small cap, say
10 nF ceramic, at pin 5?

3. Similarly, have you added a smoothing cap, say 22 - 100 uF
electrolytic or tantalum on 5V rail, and also for good measure a 0.1uF
ceramic, both close to 555?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[CFoley1064]

Subject: Switch de-bounce circuit questions.
From: (Rubicon)
Date: 11/7/2004 1:19 AM Central Standard Time
Message-id: <418dc8b9.720652@news.netaccess.co.nz


Hello,

I have put together a switch circuit using two tactile switches with
555 de-bouncers to a positive edge triggered flip-flop controling
transistors/mosfets.

The on-board tactile switches work fine but I have also added pins for
off-board switches.

With off-board switches what do I need to do in addition to using
shielded wires to prevent false triggering? Twisted wires caused the
switch circuit to flip-flop when a fluorescent light was turned on.

In addition I have LEDs indicating the flip-flops output ON state but
how do I indicate the OFF state with LEDs? Formerly I used a double
pole switch.

Cheers,

Andrew.

Hi, Andrew. The 555 trigger input is level-triggered rather than
edge-triggered. That gives you some options, depending on how your circuit is
built.

Here's the worst choice (view in fixed font or M$ Notepad):


T .---------.
--- | |
..--o o-----o-----|2 |
| | |
| | |
'----------o--. | 555 |
| | |
| | |
=== | |
GND | |
'---------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

A standard 555 has an input that may permit this, but it will false trigger all
over the place.

This setup is somewhat better:

VCC
+
|
1K.-.
| |
| |
T '-' .---------.
--- || | | |
..--o o---o--||-o--o2 |
| || | |
| .01uF | |
'--------o--. | 555 |
| | |
| | |
=== | |
GND | |
'---------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The lower value you can make the pullup resistor, the better. The cap keeps
the input signal from exceeding the length of the one shot.

A 555 is very sensitive to false triggering. This will help some. If this
doesn't do it, and you are still getting false triggering when you bring the 1K
resistor down to 150 ohms or so, you might have to add further input signal
conditioning.

Good luck
Chris

 

Chris,

Thanks for the reply. The circuit I used was a general 5V-15VDC 555
de-bouncer circuit and has a 100K resistor from my 5VDC+ to Pin#2 of
the 555 then via the on-board push-button (tactile) switch to V-. A
100K resistor goes from V+ to 555 Pin#7/6 and on to a 1uF ceramic cap
to V-. I chose the cap value via experimentation and I would estimate
that the circuit is about 95% effective like this as sometimes it
"misses a beat".

I will make the alterations you suggested though I've already gone and
chipped away the stripboard! Blast!

Cheers,

Andrew.



On 06 Nov 2004 19:35:20 GMT, cfoley1064@aol.com (CFoley1064) wrote:

Subject: Switch de-bounce circuit questions.
From: (Rubicon)
Date: 11/7/2004 1:19 AM Central Standard Time
Message-id: <418dc8b9.720652@news.netaccess.co.nz


Hello,

I have put together a switch circuit using two tactile switches with
555 de-bouncers to a positive edge triggered flip-flop controling
transistors/mosfets.

The on-board tactile switches work fine but I have also added pins for
off-board switches.

With off-board switches what do I need to do in addition to using
shielded wires to prevent false triggering? Twisted wires caused the
switch circuit to flip-flop when a fluorescent light was turned on.

In addition I have LEDs indicating the flip-flops output ON state but
how do I indicate the OFF state with LEDs? Formerly I used a double
pole switch.

Cheers,

Andrew.

Hi, Andrew. The 555 trigger input is level-triggered rather than
edge-triggered. That gives you some options, depending on how your circuit is
built.

Here's the worst choice (view in fixed font or M$ Notepad):


T .---------.
--- | |
.--o o-----o-----|2 |
| | |
| | |
'----------o--. | 555 |
| | |
| | |
=== | |
GND | |
'---------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

A standard 555 has an input that may permit this, but it will false trigger all
over the place.

This setup is somewhat better:

VCC
+
|
1K.-.
| |
| |
T '-' .---------.
--- || | | |
.--o o---o--||-o--o2 |
| || | |
| .01uF | |
'--------o--. | 555 |
| | |
| | |
=== | |
GND | |
'---------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The lower value you can make the pullup resistor, the better. The cap keeps
the input signal from exceeding the length of the one shot.

A 555 is very sensitive to false triggering. This will help some. If this
doesn't do it, and you are still getting false triggering when you bring the 1K
resistor down to 150 ohms or so, you might have to add further input signal
conditioning.

Good luck
Chris

 

On Sun, 07 Nov 2004 17:02:44 +0000, Terry Pinnell
<terrypinDELETE@THESEdial.pipex.com> wrote:

(Rubicon) wrote:


Chris,

Thanks for the reply. The circuit I used was a general 5V-15VDC 555
de-bouncer circuit and has a 100K resistor from my 5VDC+ to Pin#2 of
the 555 then via the on-board push-button (tactile) switch to V-. A
100K resistor goes from V+ to 555 Pin#7/6 and on to a 1uF ceramic cap
to V-. I chose the cap value via experimentation and I would estimate
that the circuit is about 95% effective like this as sometimes it
"misses a beat".

I will make the alterations you suggested though I've already gone and
chipped away the stripboard! Blast!

1. Your one-shot is currently about 110 ms; I'd make it a bit longer,
say 300 ms, by increasing either the Rt or Ct.

O.K. I'll do that.

2. You don't show a schematic, but have you included a small cap, say
10 nF ceramic, at pin 5?

Yes I have the 10nF ceramic cap on Pin#5.

3. Similarly, have you added a smoothing cap, say 22 - 100 uF
electrolytic or tantalum on 5V rail, and also for good measure a 0.1uF
ceramic, both close to 555?

I have 0.1uF ceramic caps on each 555 de-bounce circuits V+, 0.1uF

ceramic bypass caps next to the 555s and a single 10uF electrolytic
cap on the V+ for both de-bounce circuits.

Thanks for your advice. I'll alter the 10uF cap value and add the
other electrolytic caps as you suggested.

Cheers,

Andrew.

--
Terry Pinnell
Hobbyist, West Sussex, UK