555 Timer Circuit Short pulse Help

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
View attachment 94100

I am working on a way to make latching DPDT rocker switches and other latching switches be able to generate a 1 second negative pulse needed for a DTMF tone request. So far I have this 555 timer circuit which works well for one rocker switch. What I'm wondering is if there is a way to make the circuit reset after the ground pulse from the DPDT rocker switch has been sent?
Otherwise I would need a circuit like this for each DPDT rocker switch, I'm working with 5 rocker switches but would also like to implement SPST latching switches into the scenario as well so that they too could be made to generate a 1 second negative pulse for the DTMF request.
If anyone knows a good way to do this that would be awesome :)

DTMF_TimerCircuit_993x768.jpg
 

Dodgydave

Joined Jun 22, 2012
8,466
The 555 is wired as an auto triggered monostable, so it needs a negative pulse applied to pin 2, to make it work which it gets at power up,
 

Alec_t

Joined Sep 17, 2013
10,449
If you want any one rocker switch closure to be able to trigger the pulse even if another switch is already latched closed then you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded.
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
If you want any one rocker switch closure to be able to trigger the pulse even if another switch is already latched closed then you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded.
Thanks Alec_t, that sounds like a possibility, is it possible that you could digram that out based on what I have here? That would be a great help.
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
If you want any one rocker switch closure to be able to trigger the pulse even if another switch is already latched closed then you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded.
Hi Alec_t,
I'm not totally sure I follow, in my diagram I already have the DPDT rocker switch connected to the #2 pin via a capacitor. I did try connecting the # 1 pin directly to ground so it has a connection to that all the time and I did not get the results I was hoping for so I'm sure I don't follow what you mean by:
"you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded."
 

Dodgydave

Joined Jun 22, 2012
8,466
Use a multple capacitor switch pulse on each switch like this,

DSC_0042.JPG

When each switch is pressed a pulse is sent to pin 2,try 100k resistor and 100nf capacitor, for starters..
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
Ok increase the capacitor to 1uF, how fast do you want your 555 pulse output.
I need a pulse to just simulate the pressing of a momentary button, a second or less should be more that sufficient to request the DTMF tone, but you didn't answer my question about my Rocker switches sending a negative pulse and NOT a positive pulse. You'll have to forgive me but I'm still pretty new to a lot of electronics ;)
 

Dodgydave

Joined Jun 22, 2012
8,466
No the rocker switches are connected to negative supply, so they pull the capacitor down when pressed a give a negative pulse.
 

ScottWang

Joined Aug 23, 2012
6,831
The files size that you attached was a little big, I was compressed them to the normal size, they are from about 760Kb compressed to about 69KB, next time before you upload the files, please compress the photos to 800x600 or 640x480, if necessary then you can compress to 1024x768, almost the photos using 800x600 image is enough.

Too much file size will affecting the connecting speed of members get to the forums, thanks for your understanding and cooperation.
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
NO, its drawn wrong remove C2 and R2

and connect the switches like this

View attachment 94140

your making this trigger circuit , but adding extra switches and capacitors
http://www.instructables.com/files/deriv/FD8/HJ51/FC2BTKS2/FD8HJ51FC2BTKS2.LARGE.gif
OK I thought it was getting a little confusing with the two different drawings so I tried it Exactly like you have here, I hooked the pin 3 output to an LED, and the out to device to another LED, the Device LED stays on as it should but so too does the LED from the number 3 output pin??
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
NO, its drawn wrong remove C2 and R2

and connect the switches like this

View attachment 94140

your making this trigger circuit , but adding extra switches and capacitors
http://www.instructables.com/files/deriv/FD8/HJ51/FC2BTKS2/FD8HJ51FC2BTKS2.LARGE.gif
I also tried that other circuit you posted a link too. It works and gives a 1 second pulse but I am still unable to get it to work with multiple switches, I mean each switch will activate the pulse but if a switch is latched then the second switch when pressed is still not able to get a pulse from the timer IC.... I feel we are missing something here, perhaps the reset pin needs to be triggered somehow when each switch is turned off??
 

Thread Starter

Michael Knight

Joined Nov 5, 2015
90
Ct and Rt are the timing cap,resistor for your output pulse, R1,C1 in your circuit.
"Well Fog a Monkey!"
This one actually works!!
I added and extra momentary button into the scenario, and I held the buttons down to simulate my latching switches and I'll be buggered it works.
Now my guess is that for 10 switches I would need to change the value of that 100K resistor right?
 
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