I haven't really followed this thread.

The transistor circuit can be replaced with a simple push button switch. This circuit will restart its timing cycle (without reseting) every time it is activated, which is what I think you were after.

My blog has lots of examples how to use a 555 in different ways, including how to drive relays.

Bill's Blog

 

I haven't really followed this thread.

The transistor circuit can be replaced with a simple push button switch. This circuit will restart its timing cycle (without reseting) every time it is activated, which is what I think you were after.

My blog has lots of examples how to use a 555 in different ways, including how to drive relays.

Bill's Blog

Thanks, Bill, you have some nice writeups on there! Actually, I'm trying to send a brief pulse to a relay with a low-ohm solenoid (so higher draw than 555 can source). At the same time, I need to have a 5 second or so "lockout" where additional button presses would have no effect. One idea has been to use one 555 to put out a 5s pulse, HPF that to get the 'rapid fire' trigger I need for the relay...feed that thru a device such as power mosfet, solenoid driver or SCR to trip the relay. I'll put up a pic of what I have so far. The relay only needs to activate for < 1/4 second.

IAN: would I place the R/C in the reset line or on the output? That also came to mind...

 

Then a conventional monostable with signal conditions as shown in my basic 555 monostable article would work.

555 Monostable

Drivers are basically simple, if you are using AC then look toward a SSR, which are extra simple to use. Otherwise something like this would work.

 

Here is what I have in mind. The red is obviously the output of the 555, and the blue is the pulse to trip the relay. Anyway, it shows what I want to do. Sorry about the odd pic size!

 

It would be better to use a second 555 to generate a precision small pulse that is square. How much delay between triggers?

 

Mike33,

Here's a link to a driver chip I had mentioned earlier:
http://www.digikey.com/product-detail/en/TD62064BP1G(O,J)/TD62064BP1G(OJ)-ND/871105

4 channels at 1.5 amps per.

 

Yeah, I am leaning towards the driver, Tube. Much simpler than using a 'device' requiring outside components. I will have to figure out if the small pulse is enough to trigger it, or if like Bill says, I'd need to generate another pulse with a 2nd 555. Time to experiment! Thanks for the link.

Bill - I need my 'high time' for the 555 above to be about 4-5 seconds. It's not critical...what IS critical is to pulse the relay I'm switching for about 1/4 second to switch it, and then lock out any other pulses for about 3-4 seconds. This is for a model railroad - the relay in question changes tracks and so on. 2 555s would do this easily, I know.

The trouble this circuit had when wired up directly to a switch was that holding the toggle closed, or repeatedly triggering it (accidentally) was burning out the solenoid. Only takes 1 to 2 seconds to ruin a pricey snap switch this way. I'm just looking for a reliable and economical way to toggle the snap relay in question. There are TEN of them, so TWO circuits for each one. Can add up $$$ fast! If I can do it for $50, we'll be in business.

 

Update: Snap relay came in the mail. Each coil is 4.5 ohms.
Running at 12V, so we have 2.6A ignoring inrush current.
Ouch.

That's why I've been so clueless as to what to drive it with. Should one drive a relay with a relay? LOL. I thought it would be simpler.

 

I would drive it with a MOSFET; an IRLU8721 is what I would use (because I have some, and I like the case.)

ETA: Bill already answered that back in post #23, didn't he?

 

Yup, I just checked out using an IRF510 (I have some of those, ha ha), and it can handle it. In post #23, I wasn't sure if the circuit offered a 'lockout' where you can't retrigger it for about 5 seconds. Doing it by tapping off a longer 555 output works (at least in the computer, we'll see about in the real world).

At any rate, I think by using all the ideas presented here, I have a solution! So thanks guys!

 

Update: Snap relay came in the mail. Each coil is 4.5 ohms.
Running at 12V, so we have 2.6A ignoring inrush current.
Ouch.

That's why I've been so clueless as to what to drive it with. Should one drive a relay with a relay? LOL. I thought it would be simpler.

AFAIK you don't get inrush current with a solenoid/relay - the current rises linearly until the lines of flux saturate the core, at which point it stops behaving like an inductor and assumes the DC resistance of the winding wire.

As long as you use the solenoid in spec it shouldn't saturate, but when you reach as many lines of flux as you're going to get, the dynamic inductor behaviour ceases and you again get to the DC resistance.

With fast switched inductors such as SMPSU or horizontal flyback transformers the winding resistance can be less than 1 Ohm and B+ a few hundred volts - when the transistor stays on too long it makes quite a bang!

 

Nice! Thanks for pointing that out, you just removed that particular concern for me!