Hi Guys,

I have a piece of equipment that requires a 1sec pulse ( 5volt IO pulled to Ground ) to Start and possibly hours later another separate 1 sec pulse
to stop.
The problem is there is only a on/off SP remote Switch to control this function - on to to Start and off to stop.
I have made up a (1sec) 1 shot timer with one half of a 556 timer which will work fine for the start sequence but i think i need to use some logic to
control a 2nd 556 timer for the off command ! The switch grounds the one control wire when ON and open when OFF

Any ideas would be appreciated

Thanks

 

One problem you are going to have to deal with is how to tell whether or not the system is on or off to begin with.

With the remote (which is presumably operated by a human?) the button to stop the equipment isn't going to be pressed if the equipment is already stopped because the human doesn't just push the button, they evaluate whether or not the button needs to be pushed.

Leaving that aside, you can use the same timer to generate both pulses, since both pulses are simply a one second pulse on a particular wire. What you need to add is something to control the amount of time between those two activations of those pulses.

The whole thing pretty well screams out for using a cheap microcontroller (or something like an Arduino or similar developer-friendly platform).

But in either case, you need to consider how you are going to evaluate whether or not the button needs to be pushed in the first place.

 

Thanks for the reply WBahn,

The remote Switch is a standard push ON / OFF latching switch with the control wire grounded when pushed on and open
circuit when the switch is released.
i agree i can use the same one shot timer in the 2nd half of the NE556 IC but need some logic to interface with it
when the same control wire from the switch is a open circuit to ground not a short ( ie switch on ) maybe a NAND gate ?

Regards,

Ross

 

I must not be following correctly.
You say the equipment needs a 1 second pulse to activate on or off but the remote switch grounds a control wire to activate on and open to turn off. How does that work? Sounds like a contradictory statement.

 

sorry i thought i explained that at the start. The equipment has 2x control inputs, one to turn it ON and another to turn it OFF
both requiring a momentary pulse ( 1 sec pulse ) the OFF state puts the equipment into a standbye state waiting for the next time its used or given a on command by the SP switch.

I hope that explain my dilemma.

Cheers,
Ross

 

I must not be following correctly.
You say the equipment needs a 1 second pulse to activate on or off but the remote switch grounds a control wire to activate on and open to turn off. How does that work? Sounds like a contradictory statement.

My impression is that this is possibly like the two-wire interface on some garage door opener wall switches. For example, I have one that is lit continuously (so that you can find it in the dark) and it has a motion detector that turns on the light, as well as a button that turns on the light, plus a large button that operates the door itself. There is no battery. But there are only two wires going between the opener and the switch.

I haven't explored it yet, but if you short the two wires together, the door opens/closes (or stops, as the case may be). I'm guessing that the opener provides power and ground over these two wires with a high-impedance source and that this is used to keep a capacitor charged to supply power to the circuitry in the switch (there's a small PCB in there, but I haven't taken it out). I'm further guessing that pushing the light button (or the motion sensor detecting motion) shorts the two wires together briefly, while pushing the open/close button shorts them for a longer period of time. The circuitry in the opener is monitoring the power wire looking for it to be pulled low.

 

Ross_Rocks
Are you saying the equipment can be turned On and Off by using pulse or the SP switch?

 

equipment that requires a 1sec pulse ( 5volt IO pulled to Ground ) to Start and possibly hours later another separate 1 sec pulse
to stop.

The switch grounds the one control wire when ON and open when OFF

If I understand correctly, the sim of the circuit below using the two 555's (or one 556) does that.
It generates a 1s ON pulse (red trace) when the switch closes and the control signal (green trace) goes low, and a 1s OFF pulse (yellow trace) when the switch opens and the control signal goes high.

Edit: R6 and C4 may not be needed. You can try the circuit without them (TRIG and RST connected together) to see if it works okay that way if you like.)

 

That will work but I read in post 1 it requires a negative pulse.

 

That will work but I read in post 1 it requires a negative pulse.

If you are referring to his statement "1sec pulse ( 5volt IO pulled to Ground )", I think he's referring to the input, not the output, since he said he already has one 555 circuit that does what he wants.

 

Possibly. I didn't interpret that way but will see.

 

If I understand correctly, the sim of the circuit below using the two 555's (or one 556) does that.
It generates a 1s ON pulse (red trace) when the switch closes and the control signal (green trace) goes low, and a 1s OFF pulse (yellow trace) when the switch opens and the control signal goes high.

Edit: R6 and C4 may not be needed. You can try the circuit without them (TRIG and RST connected together) to see if it works okay that way if you like.)

View attachment 296829

Thanks for the Circuit, i made it up on a bread board but it didn't work correctly i had a LED with a 470 R in series on each output and with the switch ON the 555 On output didnt come on just a very quick flash but with the switch off the OFF 555 timed out correctly and turned on the LED for the 1 sec

 

The requirements may be met using three electromagnetic relays.

The pulses are generated as the capacitors 'C' charge through relay coils 'K2' & 'K3'.

Relay contacts 'K2' & 'K3' then discharge capacitors 'C' through resistors 'R'.

Here's the schematic.



Another set of relay contacts 'K2' & 'K3' trigger the equipment to start & stop when the SPST switch is actuated.



Nandu.

 

Thanks for that Nandu but the remote switch is wired with 1 control wire which is grounded. i cant change that unfortunatly

its a good use of relay logic though.

Thanks again

Ross

 

Anytime, Ross !

Many thanks.

Nandu.

 

i made it up on a bread board but it didn't work correctly

Did you include R6 and C4?
If so, try increasing the value of C4 to about 1µF or higher.

Also double-check all the wiring.

 

Hi There Zapper, Thanks again for the circuit it worked a treat after i sorted out a wiring problem, i didn't include R6 & C4 as it worked better without them.

Cheers,
Ross

 

Thanks again for the circuit it worked a treat after i sorted out a wiring problem,

Glad it worked for you.
Always nice to hear a suggested design worked.

i didn't include R6 & C4 as it worked better without them.

What's "better"?