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Author Topic: Which IC for 1/2 second closed-1/2 second open circuit control  (Read 1737 times)
K1PEK
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Posts: 51




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« on: June 27, 2010, 08:19:36 PM »

Hi, need an IC or trans to control a 12 VDC load of 270 ma .  Timing needs to be about .5 second ON / .5 OFF  ANybody have a recommendation?  Tnx,  Steve  K1PEK 
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KZ1X
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« Reply #1 on: June 27, 2010, 08:25:53 PM »

555, plus a transistor to switch the load
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AC5UP
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« Reply #2 on: June 27, 2010, 09:48:39 PM »

Ditto.

http://www.national.com/ds/LM/LM555.pdf
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WB6BYU
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Posts: 13009




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« Reply #3 on: June 28, 2010, 12:52:18 PM »

Depends how accurately you need the timing.  The standard 555 configuration timing
isn't symmetric, though there are some variants shown in the data sheet that allow you
to adjust the duty cycle to exactly 50%.  The CMOS version (TLC555, etc.) can be
configured so the output drives the timing capacitor directly through a resistor, which
gives close to 50% duty cycle.  (The data sheet shows a variant for the standard
LM555, too, but it isn't as simple.)

Most varieties can source 20 to 200mA (depending on the specific part number) so you'll
still want a transistor or two to switch the actual current to the load.

You can download a datasheet for the LM555 or TLC555 from National Semiconductor
(www.National.com) with all the required info.
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K1PEK
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« Reply #4 on: June 28, 2010, 05:04:37 PM »

OM's. tnx for help !! 
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KE3WD
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« Reply #5 on: June 28, 2010, 06:01:28 PM »

Yep, the CMOS Triple Nickle would be my first thought also, barring the need for precise and accurate timing, that is. 
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K4TL
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Posts: 23




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« Reply #6 on: June 29, 2010, 05:36:25 AM »

This is probably overkill, but if you need a 50% duty cycle, you can set the speed of the 555 timer to 1/4 of a second and use it's output to trigger a flip-flop to divide by two.
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K1PEK
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Posts: 51




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« Reply #7 on: June 29, 2010, 06:07:32 AM »

K4TL ,  Tnx.  I'm in the dark ref. the "flip - flop".   Is that a transistor, if so, do you happen to know which transistor to use in conjunction with the IC to achieve your noted result??   Thanks,   Steve   
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WB6BYU
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« Reply #8 on: June 29, 2010, 09:21:23 AM »

Any transistor that is rated at sufficient current - perhaps 500ma would be sufficient.

There are a couple likely circuits for the transistors - I'd probably use a PNP as the series
DC switch (input power to emitter, resistor from emitter to base, load on collector), in
which case grounding the base through an appropriate resistor will turn on the power.
Something in a TO-39 case may work, depending on the saturation voltage.  Practically
any of the PNP transistors in a TO-220 case will probably work, as they are rarely put
in that package if they aren't rated at least for 1A @ 30V.  Some variant of TIP32 or
TIP42 should be fine, they are rated for 5A at 40 to 100V (depending on the last letter).
You can bolt a small heatsink to the tab of the transistor if it tends to get warm, but
it MUST be insulated from ground.
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K1PEK
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« Reply #9 on: June 29, 2010, 09:35:43 AM »

WB6BYU, and the others, tnx so much,  Steve
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AA4PB
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« Reply #10 on: June 29, 2010, 09:56:24 AM »

I'd suggest an NPN transistor in an emitter follower configuration. Relay coil between emitter and ground. Base directly to output of the NE555 and collector directly to the same 12V source that supplies the NE555. No other components needed.

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WB6BYU
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« Reply #11 on: June 29, 2010, 11:26:56 AM »

If you are going to use a relay anyway, why not get one with a 50mA coil and drive it
directly from the 555?

What I like about the PNP circuit is that the output voltage is not limited by the maximum
drive level of the 555 timer.  With the NPN, the maximum available output is about 0.6V
below the timer peak output (the difference between the base and emitter voltages.)
With the PNP the resistor between emitter and base keeps it off even if the 555 output
drops a volt or two below your supply voltage (depending on the relative resistor values)
but the output voltage when on is limited only by the saturation voltage of the transistor.

In fact, in some cases I'd use an NPN between the base of the PNP and ground and let
the 555 drive that base through a resistor - then the only purpose of the 555 is to drive
the lower transistor, and the output voltage of the entire circuit is practically independent
of the output drive level of the 555 timer.  (Which could be running on a +5V regulator,
for example, without any adverse effect.)

If the switched supply is, for example, a battery charger, where the exact voltage drop
across the load is important for proper operation, then a relay may be your best option
as it won't add to the voltage drop, though it may make an annoying clicking if it is near
your desk, and relays have a finite number of operations.  If it is flashing a lamp then it
really doesn't make a lot of difference whether it sees 12.3V or 12.1V, though the circuit
needs to be able to handle the peak current when the lamp turns on after being cold.

So there are a number of other factors that might influence your choice of a circuit.
In any case, I've found it poor economy to simply minimize the parts count:  a couple
resistors (and sometimes an extra transistor or two) can make the circuit much more
robust and able to operate over a wider range of input voltages, current demands, etc.
We're talking about $0.60 each for the IC and the power transistor from DigiKey, and
you can get a handful of resistors and transistors to improve the circuit for the
change from a $2 bill.

You do have to do a bit of planning - the base drive current to the transistor, for
example, needs to be sufficient to pass the required current, while switching it
reliably off at other times.  You need to know how much voltage drop your load can
tolerate, and potential switching transients (especially with relay coils.)  But it is
quite easy to design the switching circuit to handle 60V @ 5A and it should run
nearly forever in your application.
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AA4PB
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Posts: 12667




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« Reply #12 on: June 29, 2010, 11:48:05 AM »

The thing about the NE555 is that it has a totem pole output. It switches hard to ground and VCC and handles 200mA in either direction. I've even used them for their output circuit when I don't need a timer (disable the timer function and use it for a switch). One 8-pin DIP at 0.45 and available locally is easier that building a similar output using multiple descrete components. In my opinion, in most cases using anything more than an emitter follower is unnecessary and usually just duplicating circuitry that is already contained inside the NE555.

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KC5CQW
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Posts: 98




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« Reply #13 on: June 30, 2010, 06:53:52 PM »

The "Triple Nickel" with Ra at 4.7M and Rb at 680k  with a 0.47uF cap yields 0.506636Hz
I like the Arduino (sparkfun.com) development platform. It uses an Amtel AT Mega PIC.
It also uses a 16kHz or 20kHz ref XTAL.
This will also build on your C programing skills.

73, Damon
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