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Old 6th Jul 2008, 05:04   #1
eric_f
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Simple flasher circuit

Hey everyone
I am working on a project at work and one of the things I need is a simple flasher for a 120v light bulb. I was thinking of using a 555 timer circuit.
I have 12vac coming for power (easily rectified and made DC). I would like the 555 timer to control a relay that I happen to have. I have been google-ing around and found a whole bunch of examples, but they all seem to be for LEDs. The one I did find said I need to use diodes on the relay, but I'm not sure where.
I would like the relay to flash on and off about once a second, but preferably that could be adjusted.

Any help/ideas would be great.
Thanks
-Eric
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Old 6th Jul 2008, 05:16   #2
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As for fitting a diode to the relay, http://www.kpsec.freeuk.com/components/relay.htm ~about halfway down.

But i'd rather use a transistor/mosfet to do the switching, as the relay will make a noise, and the relay wouldn't last that long.

And it's easy enough to have a fully adjustable 555 timer circuit, so you can set how long the off & on time is with two pots/variable resistors.
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Old 6th Jul 2008, 05:35   #3
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Thanks for the quick reply
As for the relay, first the noise doesn't matter one bit. Second, at an absolute maximum it would be on 3 hours a day, which would be VERY unlikely, so I think the lasting should be ok.. for now anyways.

Do you have, or can you find a schematic for me? I've been looking for a while now, and I just don't know enough to make a decision on what to go with.

Thanks again
-Eric
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Old 6th Jul 2008, 11:00   #4
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hello eric

dont use the relays as mvagusta said use a high power rating BJT or mosfet this would last much much longer. i once used a relay to switch a motor on-off so as to control its speed, the relays didnt last 15 min of testing and it took the driver board with it.

so never never use relays for fast switching
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Old 6th Jul 2008, 11:05   #5
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if you want to switch mains ac you really want to use either a solid state relay SSR , or build one with a Triac and an optotriac , that way you have complete electrical isoslation between the low voltage side and the mains - plus you can switch the mains on and off at any speed without burning out and components!
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Old 6th Jul 2008, 16:33   #6
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OK. I guess I will have to order some stuff then, seeing as I don't have any SSR's or Triacs. That being said, I still don't know how to construct this.
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Old 7th Jul 2008, 03:00   #7
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opto isolator into a FET
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Old 7th Jul 2008, 03:38   #8
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If you use an opto isolator, the isolated transistor would be connected to the same dc as the rest of the electronics! Right?

Nothing wrong with this, but the only protection offered is to the output of the 555, but the power supply of the 555 is still connected to the transistor output of the opto isolator, so if the fet was to fail and allow 120vac to go through it's gate, the 555 & optoisolator & dc supply are all still gonna be blown. So i don't see the point of using an opto isolator, unless i'm missing something?
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Old 7th Jul 2008, 10:21   #9
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If you're sure you want to use a 555 and a relay, here's the circuit you'll need:

The diode on pin 3 is there to protect against the back EMF generated by the relay switching, connect the input of the relay to this diode. The values for R1, R2 and C are 20k, 62k and 10 micro farads respectivly. That should give you about a 1 second on-off action with a respectable 56% duty cycle.
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Old 7th Jul 2008, 10:41   #10
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I'd whack a transistor in between the diode & 555 to drive the relay, as the 555 would only be able to power a small relay by itself.

I think we had actually convinced him not to use a relay actually...
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Old 7th Jul 2008, 11:57   #11
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If you're switching a mains filament bulb every second you need all sorts of tricks to keep it from blowing in the first half-hour - zero-crossing switching, hi-lo powering, some under-volting, maybe.

For the first, use one of these on the 555.

datasheet
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Old 7th Jul 2008, 20:10   #12
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the point of the opto triac is that you switch the mains which fires the triac , the mains ac side and the dc side have no common connections - complete electrical isolation , you can get opto triacs that have the zero crossing detectors built in reduce the rf harmonics and stress on the light bulb

I have built this before and it works well

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Old 7th Jul 2008, 23:40   #13
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Quote:
Originally Posted by mvagusta View Post
If you use an opto isolator, the isolated transistor would be connected to the same dc as the rest of the electronics! Right?

Nothing wrong with this, but the only protection offered is to the output of the 555, but the power supply of the 555 is still connected to the transistor output of the opto isolator, so if the fet was to fail and allow 120vac to go through it's gate, the 555 & optoisolator & dc supply are all still gonna be blown. So i don't see the point of using an opto isolator, unless i'm missing something?
well, i meant to tie the 120V to the collector of the opto and use the 120V to drive the FET, but use the FET to handle the current.

But now i realize it is 120VAC. so nevermind, unless you design a 3rd power supply to drive the fet's gate...

And on top of that, 120V at the gate would probably smoke the FET. That was just an all around bad idea. Sorry...
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Old 8th Jul 2008, 03:18   #14
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I was thinking that it would be good if you make a small rectified dc circuit to drive the isolated side from the 120vac, but then cpemma posted the ssd relay!

Very nice, i didn't know those existed! Well i've learnt something new today, can i go home now?
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Old 8th Jul 2008, 03:43   #15
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oh my, seems like we've had a nice discussion since i've been gone.

i will look into all these ideas more when i get home, but for now, thanks.
the relay i have, if used, would be on for maybe 5 minutes at a time, and no more than 20 times a day. but i still am up for suggestions
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Old 8th Jul 2008, 12:25   #16
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cpemma's suggestion is the way to go
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