# 555 Timer Circuit Short pulse Help

#### Dodgydave

Joined Jun 22, 2012
8,610
Possibly increased to 470k, assuming that all 10 switches are closed, the total resistance of all 1 meg resistors in parallel would be 100k, so the voltage at pin 2 would be 12v * 100k/570k = 2.7 V( including diode vdrop)that would be low enough to trigger the timer.

Try it with ten switches, and see, increasing the 100k to 470 to 680k should be fine.

#### Michael Knight

Joined Nov 5, 2015
90
Possibly increased to 470k, assuming that all 10 switches are closed, the total resistance of all 1 meg resistors in parallel would be 100k, so the voltage at pin 2 would be 12v * 100k/570k = 2.7 V( including diode vdrop)that would be low enough to trigger the timer.

Try it with ten switches, and see, increasing the 100k to 470 to 680k should be fine.

Awesome, I redid my diagram maybe give it going over and make sure I have not botched anything.
Also would I be able to group each rocker under a 100K resistor branched off of the 12V for each switch?
Or should I just continue the mapped out scenario on down the line with the 100K switched up to a 470K or higher?

#### Michael Knight

Joined Nov 5, 2015
90
Possibly increased to 470k, assuming that all 10 switches are closed, the total resistance of all 1 meg resistors in parallel would be 100k, so the voltage at pin 2 would be 12v * 100k/570k = 2.7 V( including diode vdrop)that would be low enough to trigger the timer.

Try it with ten switches, and see, increasing the 100k to 470 to 680k should be fine.
Wow! That is awesome, I changed the R1 to a 200K and I get a perfect pulse for a momentary button which is what the Voice Box's Random DTMF tone request needs, so that is perfect!! I find I can switch on and off the latching switches fairly quickly and still get a short pulse from the timer. Wicked!!
Now I realize this is going above and beyond a little here
But do you know if there is a way for the latching switches to also generate a pulse through the timer when they are switched off?
Or is that so complicated it would not be worth the time?

#### Michael Knight

Joined Nov 5, 2015
90
Wow! That is awesome, I changed the R1 to a 200K and I get a perfect pulse for a momentary button which is what the Voice Box's Random DTMF tone request needs, so that is perfect!! I find I can switch on and off the latching switches fairly quickly and still get a short pulse from the timer. Wicked!!
Now I realize this is going above and beyond a little here
But do you know if there is a way for the latching switches to also generate a pulse through the timer when they are switched off?
Or is that so complicated it would not be worth the time?
OK just want to share a demo video of how this is working with the lighted rockers in my Upper Console. Thanks Dodgydave for all of the awesome help.

#### marcf

Joined Dec 29, 2014
257
I am just curious, so please excuse my ignorance, but what is the DTMF tone request doing?
What is the scope of the project?
Thanks.
I do not know if this is a useful concept, but in DTMF signaling there is BU (buttons up) and /or BD (buttons down) operation.
BU is preferred as it minimizes contact bounce.
I believe this would enable a pulse to occur when the switch is released if you could incorporate this in your design.

#### Michael Knight

Joined Nov 5, 2015
90
I am just curious, so please excuse my ignorance, but what is the DTMF tone request doing?
What is the scope of the project?
Thanks.
I do not know if this is a useful concept, but in DTMF signaling there is BU (buttons up) and /or BD (buttons down) operation.
BU is preferred as it minimizes contact bounce.
I believe this would enable a pulse to occur when the switch is released if you could incorporate this in your design.
Oh hey, thanks for chiming in marcf,
Basically I'm building my own Knight Rider replica. The dash electronics are from a place called "Knight Rider World." The famous K.I.T.T. (Knight Industries 2000) voice box in their electronics has a built in random DTMF tone generator that delivers a random tone when a ground pulse is sent to it. It requires momentary pulses for the request. I have been toying with the idea for some time now of being able to achieve the same effect using my latching switches.
So far with the awesome help from Dodgydave and another fellow on FB from the Knight Rider community I have been able to get a momentary pulse from the latching switches to be sent to the DTMF request of the Voice Box. Which is pretty awesome. It would be further perfected if when the latching switch was turned off that it also was able to send a short pulse for the DTMF request.
If you like I have a blog where I have been documenting every step of my project here:
http://www.myknightrider2000.blogspot.ca

#### Michael Knight

Joined Nov 5, 2015
90
OK so this works with the rocker switches turning on, even multiple switches on at the same time, but is there a way to also have the pulse generated when each latching switch is switched off?

#### marcf

Joined Dec 29, 2014
257
You might try something like this. BU would be when switch opens, BD would be when switch closes.

#### Attachments

• 29.4 KB Views: 4

#### Michael Knight

Joined Nov 5, 2015
90
You might try something like this. BU would be when switch opens, BD would be when switch closes.
Thanks marcf,
It looks interesting but being new to electronics I have very little clue as to what this is or how I would use it with what I have?
More detailed information would be helpful here

#### Michael Knight

Joined Nov 5, 2015
90
I wonder if an answer for being able to generate a short pulse for the rockers in the off position might be in the use of 2 555 timers?
I found this page that has a whole bunch of circuit scenarios but not sure if anything there would fit my particular situation not being too familiar with a lot of this stuff?
http://home.cogeco.ca/~rpaisley4/LM555.html#6

#### Michael Knight

Joined Nov 5, 2015
90
You already have a 555 giving you a pulse, do you need to make a pulse with a positive signal input or negative, if its negative input, use another 555, or use a 556 dual timer.

Well the pulses from the Rocker switches are negative pulses, I usually try to use (-) pulses from all of my switches to trigger relays for device switching and activating.

So far we have the rockers able to send out short pulses even if more than one rocker is switched on. (Thanks for that very much Dodgydave)
But if I can somehow get the same effect when each rocker is switched off to send out a short pulse then I'm golden.
My thought was maybe it could be done with an additional 555 timer triggered by a normally closed state?

#### Dodgydave

Joined Jun 22, 2012
8,610
Best thing is to draw a diagram of what you want to do, then we can see what pulses or signals, switch, relays etc to give, put the timer in a block diagram, dtmf also.

#### Michael Knight

Joined Nov 5, 2015
90
Best thing is to draw a diagram of what you want to do, then we can see what pulses or signals, switch, relays etc to give, put the timer in a block diagram, dtmf also.
Not sure I follow you dave, Basically we are almost there, so far we have it working when the switches are turned on, I just need it to work when the switches are turned off, the DTMF is simple and only requires a momentary pulse, we already have that so we are good there.

#### Dodgydave

Joined Jun 22, 2012
8,610
So what you need is when the rocker switches from negative to positive off, the timer gives out a pulse?

#### Michael Knight

Joined Nov 5, 2015
90
Best thing is to draw a diagram of what you want to do, then we can see what pulses or signals, switch, relays etc to give, put the timer in a block diagram, dtmf also.
Hey Dodgydave,
I found this circuit that produces a short pulse with a normally closed switch here:

Is it possible to somehow use this in conjunction with what I have here to produce a short pulse for when the switches are turned off?

#### Dodgydave

Joined Jun 22, 2012
8,610
its the same thing, that gives a negative pulse when the switch is pressed,

#### Michael Knight

Joined Nov 5, 2015
90
No, its the same thing, that gives a negative pulse when the switch is pressed,
The normally closed switch diagram you mean?

#### Dodgydave

Joined Jun 22, 2012
8,610
Yes, i think your getting us confused here,

Do you want to trigger the timer when the rocker is pressed, and again when the rocker is released?

#### Michael Knight

Joined Nov 5, 2015
90
Yes, i think your getting us confused here,

Do you want to trigger the timer when the rocker is pressed, and again when the rocker is released?
Yeah it's not hard in these forums, sorry, I'll try and take this a little slower

Ok my rocker switches are DPDT Rocker switches OK So the middle position of the switch is OFF, flipping the switch to the Right will activate a device, flipping it to the left (Past it's middle OFF position) will turn off the first device and turn on the new device. See my video for the demo

OK so far we have that when you flip the switches on we get our short pulse, which is perfect for devices turned on.
Now I basically want to do this in reverse where when the rocker switch is turned off (Middle position) we get another short pulse sent out from the timer.

I'll do another quick video to try and explain better and post it here. I added a little piezo buzzer just for demonstrative purposes so you can hear what is going on too.... BRB with a video buddy