Timed on/off switch

AC or DC current? How much current? What voltage? What is your power supply voltage?

It will be a DC system running at 12V. It will be carrying close to 3 amps at max. It will be getting the power from a AC-DC converter plugged into the wall. The load will be a electromagnet.

What purpose is there in cycling such a magnet at 6 Hz?

Well, I guess that 6 is a little high... maybe around 2 Hz will be optimal. I am trying to make my own unique design for a tattoo gun I am making.

What you want is a solenoid

and then you could use something like a 555 timer which you could then easily control the speed.

--- View in Courier:

.+12>--+--------------------------+--------------------+ . | | | . | | +-------+ . [1000] +---------+ | |K | . | 7|_ |8 | [DIODE] [COIL] . +------------O|D Vcc|--+ | | . | 6| _|4 | +-------+ . [82K] +-----+--|TH R|O-+ IRL3303 | . | | | 2|__ |3 | \\ C . +->[500K] +-O|TR OUT|--|-------------+----B . | | | GND | | |K E . +----+ +----+----+ +------+ | | . | 1| 555 |+ | [1N4744] | . [0.68µF] | [10µF][100nF] | | . | | | | | | .GND>-------+-------------+-------+------+------+------+

The 10µF electrolytic and 100nF ceramic capacitors are to be mounted directly across pins 1 and 8 of the timer.

The 0.68µF capacitor is polyester film (Mylar).

With the 500k rheostat at minimum resistance the output frequency will be about 6Hz and at maximum resistance the output frequency will be at about 1Hz.

-- JF

gh

Wow, thanks guys!

Look at HowStuffWorks.com and search for the buzzer demo. That's basically how to do this.

Yes, it's called a transistor. But I'd put it in the negative lead (your white one), and call the negative output of the supply "ground"; it makes the control circuitry simpler to design, because then it can all be referenced to ground.

You need something to generate the pulses, like a 555 or astable multivibrator, and the output of that will switch the transistor on and off. You'll also need a "catch" diode, which is a diode in parallel with the coil, that absorbs the inductive spike when the current is turned off. Some might recommend a MOSFET - for something on this order of complexity (which isn't very much, for an old fart, albeit for a newbie it can be daunting), it's pretty much a matter of personal taste.

Do you have any electronics experience at all? If not, I'd recommend searching the web for tutorials and such so you have a handle on what you're trying to accomplish, and how to go about doing it.

Good Luck! Rich

--
Geez, Homer, do ya think he might want to be trying out something
different without asking for your permission?

This whole thread has been a great help! As for my electronics background... I am a level 2 electrician apprentice... Although this doesn't help much as far as experience on low voltage DC systems, I understand everything that has been discussed here and have a good grasp on electrical flow and I can solder well.

Now one thing I am unclear about (this may sound like a stupid question)... If I am looking at an electrical circuit diagram and it leads a few parts to the ground symbol... do I just connect it to the box I'll be putting it all in?

Only if the box is metal. :-)

And, one of the conventions in sci.electronics.basics is that there is no such thing as a stupid question, unless you ask the same one three or four or five or ten times, trying to get the answer you think you want instead of the right one (of which there are usually several). :-)

If you really want to set the world on fire, invent a tattoo eraser that's as cheap to have done as having the tattoo installed in the first place. :-)

Have Fun! Rich

You can generally find the US Army or Navy instruction books online. They are a very good starting point for learning since they have been used many many times for teaching these subjects.

You can. Or you can just connect all of the ground connections together. The notion of ground is a conceptual one - it really means common return to the power supply, sometimes with an element of shielding associated with it.

Okay... I think I found the circuit I am going to try to assemble... I am confident that I can do it:

But the load of course won't be a light, rather an electromagnet... and I will connect a MOSFET in parallel with the electromagnet to avoid the damages of any current spikes. What do you guys think?

As for a tattoo eraser... I'll worry about building one of those when I start having kids :)

Thanks!

-KV

Something like this should do it for you:

+12 ---+------------------------------------+ |a | [D1] | | | +----+--------+---+ | | | | | +----+ | [1K] --------- | | | | | 8 4 | | | | +-----|7 | [100K] | | | | | | | | [33K] | 555 | | |-+s | | | 3|---[330R]-+-,| STP12PF06 | +-----|6 | g||-+d [100uF] | | | | | +-----|2 | +----------+ | | | 5 1 | | | | |+ --------- | | | [10uF] | | [Electromagnet] [D2] | | [.1uF] | | |a | | | | | | Gnd ---+----+--------+---+------------------+----------+

D1, D2 = 1N5404

With the values shown, the 555 should give you about

2 seconds on, 2 seconds off. Use a regular NE555, not the cmos TLC555 version.

Ed

Thanks Ed, that actually looks a lot simpler than the one I planned on using (and a bit more applicable)... A few questions: What type of diodes should I use? and what components should I change if I wanted to increase the frequency slightly? Could I add a rheostat anywhere?

2s on and 2s is a bit too slow, I think.

The frequency will be inversely proportional to the value of the 10uF cap at pins 2 & 6 - this is called the "timing cap". For example, a

4.7 uF would give about 1 sec. on and 1 off, a 2.2 would give you about .5sec on and .5 sec off, and so on.

Yes, it's possible to use a rheostat, but I'm too lazy too look up the app note on the best way to do that - I don't remember exactly how the charge/discharge works, but the 1K and the 33K are used for that - I'd also cut the cap to about 1 uF and change them to 10K and 330K, and replace the 330K with a 100K pot connected as a rheostat, in series with a 220K.

But the data sheet should have lots of example circuits - just google for "555 data sheet".

I'm also thinking - I've seen an ordinary tattoo gun in operation, and it sounded just like a standard buzzer, in the tens of Hz; so you might have to keep an eye on the inductance of your solenoid and various things like that.

Good Luck! Rich

When I replied, the reply from John Fields was not yet on my server, so I didn't see it. His circuit gives you the ability to adjust the timing. On the one I posted, change the 33K resistor to a 200K pot to vary the timing. Diodes = 1N5404

John's circuit is better than mine, but I would make 2 changes to it: add a diode and change the

10 uF to 100 uF. I'll show the change to add the diode below, in a partial schematic. The remainder is the same as John posted, except the 10uF cap is changed to 100uF. The added diode is a 1N4148

.+12>--+-----------------------------------------------+ . | | . [Diode] | . |k | . +--------------------------+ | . | | +-------+ . [1000] +---------+ | |K | . | 7|_ |8 | [DIODE] [COIL]

The purpose is to stabilize the voltage to the 555 when the solenoid fires.

Ed