Timer - Relay

[ncoad]

It seems that my previous post has been deleted from the message board; so here's my repost with a few more details.

What I'm trying to do it trigger a relay (from closed to open) for a variable amount of time.

The start trigger is going to be a momentary switch (mercury switch). When depressed, I need a timer (555?) to open the relay for a specific amount of time, at which point it will revert to being closed again. The times will be from 10ms to as much as 1s.

If the switch is still depressed at the end of the timer sequence, I'd like to leave the relay open and reset the timer and start again.

The whole thing needs to run off a DC power supply between 11-15 volts (maybe a little more...) and hopefully the timer should not be affect by the voltage too significantly.

As I mentioned before, I not very much of a hardware kind of guy; I'm more the programmer type. However I think this circut might be very useful for many people.

Any help would be appreciated. I'd like to get something basic going at first and then build on it as I learn more.

Thanks very much!

Nicholas
erkware@yahoo.com

 

[BrianG]

From your description I take it that by the relay being "open" you mean the coil is energised and also that you really only want the relay to hang in there after the switch is released ? If so, it seems to me like you are looking at the problem from the wrong way around - you only want the delay for the relay to start when the mercury switch opens after being pressed for a random amount of time.

To do this you need to trigger the timer with the opening of the switch contact, not the closing switch contact. You will need some additional logic to provide an OR function so that either the closed switch or the timer, while it is running, keep the relay energised. This energises the relay for an indefinite time, while the switch remains pressed, and then maintains it for your pre-set timer period after it is released. The "logic" could just be two diodes in the simplest case, or for the supply voltage you specified you could use CMOS gates and timer from the 4000 series.

 

[dpc]

I'm not sure of your application, but you can probably just use a time delay relay, which is a relay with the timer built in.

 

[ncoad]

Do they make time delay relays that are adjustable between 10ms and 1s?

Any recommendations on a specific product number?

Nicholas

 

[dpc]

http://www.natcon.com/timer1-9.htm

This is claimed to go down to 50ms. 10 ms is pretty short, but do a Google search on time delay relays and maybe you can turn one up.

dpc

 

[dydt]

check an omron timer...

http://www.omron.com

i know they have a timer with reset and start configuration..you can use use it as an off-delay, on delay, etc....there is a corresponding chart for its application which will help you select what you need..
runs at 24VDC...other voltages also available but i am not sure if they have 12V ac or dc supply..you can set the dial for sec or hr....if i'm not wrong the part number is H3CR...i use this timer in lots of applications and i am quite happy with it because of its "flexibility"..

this timer should do the job.

good luck
dydt

 

[buzzp]

10mS is pretty small when your talking about an electromechanical relay. The typical response times will certainly cause some errors in the times. Typical response times are 5-20 mS. If this is critical you may consider using a solid state switching device.

 

[melone]

Does it have to be a relay? I am just concerned about welding your contacts closed due to high switching frequencies.

 

[ncoad]

If a relay is not recommended, what are my other options?

Nicholas

 

[melone]

A transistor might work, depending on your requirements. By simply "turning ON or OFF" the transistor (base or gate), you can in essence, connect or disconnect a load. If you want to put this item in series with mulitple components, I would suggest trying to use a PNP transistor. The devices (BJT Transistors) operate primarily on current, and can give you very little voltage drop. If you try to use a FET, the you must be very carefull to supply sufficient gate voltage to guarentee turning on the FET.

Hope this helps!

 

[buzzp]

One note: Look for TO-220 package or larger as these can handle more current with an appropriate heat sink.