Need time delay circuit

[goofeedad]

Hey All, I was wondering does anyone have a simple time delay circuit, maybe using an RC circuit that would give me an on time of about 20 - 30 seconds. I have a relay driven by a 5VDC transformer so connecting it in line shouldn't be a problem.

I have a float with an on / off switch hooked up to it that turns on a water valve. My problem is that when the float switch turns the water on, it doesn't stay on long enough and the valve is constantly turning on and then off, almost vibrating.

 

[birdman0_o]

Look into using a 555 timer, they can do a 30 second delay pretty easily, there will be many hits on google.

 

[MikeMl]

Hey All, I was wondering does anyone have a simple time delay circuit, maybe using an RC circuit that would give me an on time of about 20 - 30 seconds. I have a relay driven by a 5VDC transformer so connecting it in line shouldn't be a problem.

I have a float with an on / off switch hooked up to it that turns on a water valve. My problem is that when the float switch turns the water on, it doesn't stay on long enough and the valve is constantly turning on and then off, almost vibrating.

Post a circuit of how your existing relay/float switch/5VDC supply is wired.
Do you want the delay when the float switch opens, or when it closes?

 

[goofeedad]

Post a circuit of how your existing relay/float switch/5VDC supply is wired.
Do you want the delay when the float switch opens, or when it closes?

Don't have a written drawing of the circuit and I'm not sure how I can post one.

I have a 5VDC cell phone power converter spiced in series with the float switch and a solid state 5V relay. When the float switch lowers in the water, it closes the circuit causing the relay to close activating the water valve.

To answer your question, I believe I want the delay when the float opens. Causing the water valve to stay open for 20 seconds or so.

 

[MikeMl]

... I believe I want the delay when the float opens. Causing the water valve to stay open for 20 seconds or so.

Can you work with this:

 

[goofeedad]

Can you work with this:

Mike, thanks so much. It's been a long time since I worked with an actual schematic. I'm having a bit of trouble with the symbols and abbreviations but it looks like what I need. How did you write this up, some SPICE equivalent? BTW, is there still a free basic SPICE program available that you know of?

 

[ericgibbs]

Mike, thanks so much. It's been a long time since I worked with an actual schematic. I'm having a bit of trouble with the symbols and abbreviations but it looks like what I need. How did you write this up, some SPICE equivalent? BTW, is there still a free basic SPICE program available that you know of?

hi,
We use LTspice, its lite free version of Spice.

A quick web search will give the address.

https://www.linear.com/designtools/software/ltspice.jsp

 

[john1]

Sounds to me like this might be a simple lack of hysteresis.
(unless i misunderstood something)

 

[MikeMl]

Sounds to me like this might be a simple lack of hysteresis.
(unless i misunderstood something)

How do you add hysteresis to a switch contact closure?

 

[john1]

Hi Mike,

Generally a float operating a switch will naturally have a reasonable amount of hysteresis. However the description of this particular set-up reads like the float is on an arm straight out into the container which it is topping up.
"almost vibrating" was mentioned, so maybe the surface is being somewhat disturbed by the inflow of water.

There are a couple of things i would try. Firstly turn down the water delivery to the tank so that its duty cycle is longer when the relay kicks in. This should disturb the water less, and this may be enough to sort it out.

If that has little effect, then watch it go through a cycle or two, and consider altering the positioning of the switch operating arm.

In some cases the switch is not on an arm, but the float has a tube for it to rise and fall inside a tube within the tank. This gives a much more steady rise and fall to the water level for the float. The natural hysteresis of the switch action then generally reflects in the action of the float.

A delay on the relay would of course prevent any 'chatter' or 'bouncing' from the float mechanism being repeated by the valve, although i would rather try to get it working as it should before making a 'workaround'.

Cheers, John

 

[goofeedad]

Pretty much summed it up. The float is on a tube as you mentioned, the water level does have "waves" due to the high water flow going in as-well-as the mechanics also going on in the water. It just occurred that a simple delay circuit would alleviate any water baffle needs. I intend on using baffles to stop the water surface from having waves.

Thanks all, for the responses. I am currently checking into the answers supplied, I'm sure that I can now go on.