I have a glue head that is purged by a simple momentry switch. If the switch is not pressed every 8 mins for 5 seconds the glue dries and the head needs replacing. So I would like to wire in a system that would purge it automatically if nothing happens for 8 mins. However if the system is used i would need that timer to be reset to start counting again. I cant seem to find a timing relay that would do this....

Any adivce or tips would be greatly welcomed,
cheers

 

Well, you win today's oxymoron award for "continuous pulse"!

You need to provide more information. A pulse generator with a 7 minute delay is not too tough, but anyone trying to help needs to know the power supply available for the circuit, as well as the details about what has to be switched. Is it AC? How much current, or wattage?

 

Sorry,
the power supply is 24 Vdc. What I cant find on most timing devices is after the initial pluse to start the count down, do furthur pulses then reset the timer which is what id like. Also after the timer has counted down and sent a signal for the glue head to purge would it automatically reset itself and countdown again.

 

And the current being switched? Probably an amp or two?

I'm no expert but I believe what you want is called a "missing pulse detector" circuit. It will fire off a pulse at set intervals unless a reset signal is seen, in which case it resets to start looking again for a pulse.

You can use the 555 timer for this, and I'm sure you can find plenty of examples at this site and many others.

The biggest issue you have is the relatively long interval time you need. Most example circuits for the 555 are in the seconds range or less. An expert needs to comment for us if 7 minutes is practical with a 555 circuit. You might need a hack to extend the normal range.

 

8 minutes is a problem for a standard 555 timer.

Here's a watchdog timer using a CD4060 that can be readily adjusted to get 8 minutes or more.

It stops after 8 minutes and restarts after receiving a RESET pulse. (Note: The reset pulse must go to V+ and back to ground. If using a SPST switch to V+ to generate the pulse, then add a 10k-100k ohm resistor from the RESET input to ground).

Also note that the absolute maximum operating voltage of a CD4060 is 20V so you will need a simple regulator to reduce the 24V. Anywhere between 15V and 5V is suggested. A 78xx series regulator is an easy way to do that.

If you need a 24V output signal from the circuit then you will need to add a transistor driver (and a relay if needed).

 

This sound like it may be an industrial application, so I'm thinking you might have some money to spend. Here's my suggestion...

Use off-delay timing relays. These require constant power, plus a trigger signal usually, unless they're "true off delay" relays, in which case they continue to time even after power is removed.

Here's a basic description (circled in red)

Now, this should look familiar, no?

Now, you need to add 2 off delay timers; one to handle the 8 minute part, and one to handle the 5 second part. It should look like this:

The way this works:

• You turn the machine on, current flows through the N.C. contact of TD1 and triggers TD2.
• TD2 closes it's N.O. contact, which bypasses the Purge PB and it will it send a signal for 5 seconds to purge, and this also triggers TD1.
• Once TD1 triggers, it will open it's N.C. contacts that feed TD2. It will maintain this open condition for 8 minutes; after which TD1 will deenergize and close the contact which feeds TD2, starting the cycle over again at step #1.
• When you press purge for the first time (and every time after that), you send a new signal to trigger TD1, and TD1 will start it's 8 minute countdown again.

The 2 relay coil representations I drew at the bottom are assuming you are using relays which need constant power and a trigger signal. If you use a "true off delay" relay like the on that I linked to, this will not be necessary.

Sorry, I couldn't find a cheaper off-delay timer that's capable of 8 minutes; maybe you can, but I don't have time to look.