Re: International standards

___ If you are trying to do what I think you are trying to do, you'd better reread Newton's third law. Will the fans be attached to the boat? ___

GRAVITY:

It's not just a good idea-IT'S THE LAW!

assuming that you don't have a +/- supply i guess you could construct a mini op AMP osc with POS feed back then Rectify that via CAP and Diode. the output can be directly.. you could use an isolation diode to switch the osc on when the out goes low. etc..

Kev wrote:

You didn't give the current requirements for the negative supply. Most charge pump inverters have a max current of 100mA or less.

Here's how I'd do it to keep it simple. To generate the negative voltage use a standard charge pump, like the cat660, here:

The part standard is called the 7660. Other parts in this class of chips includes the OnSemi 7662 and the Linear Tech LTC1044. Each takes a positive voltage (+9V in your case) and inverts it.

Then you can use an open ended opamp as your switch. Connect the +9V and -9V supplies to V+ and V- of the opamp. In its most basic form an opamp will drive the output so that the two input voltages match. Typically feedback is used to accomplish this where the output is fed back into one of the inputs. However in an open ended setup, the output will try to drive the voltage towards the rails. So the basic rule is that the output will drive positive as long as the + input is greater than the - input, and vice versa. So here's how you can set up it. Use a voltage divider to get a 2.5V to the

- input of the opamp. This can be as simple as two equal 100k resistors connected between +5 and GND. The connect the microcontroller output to the

• input of the omp amp. When the uC output is +5 the + input is above the

- input so the ompamp output will swing to +9V. OTOH when the microcontroller output is 0V the + input is below the - input and the opamp output will drive to -9V. Problem solved.

Be sure to get a rail to rail opamp for the job.

So two ICs, two resistors, and two caps. Not two ;-) much.

BAJ

Nope, wheels aren't going to help. He's going to need to put a propeller on it.

"Does anal-retentive have a hyphen?" 8*)

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LED current is not proportional to the base current of the phototransistor.

That is what I was thinking of, but I have no idea if you need any regulation of the +-9 or if crude copies of the 5 volt regulator input voltage is okay. What current will load these 9 volt outputs?

These two requirements need opposite extremes. You need to define how much collector current the output must pass, and what voltage drop is acceptable. Then using the data sheet, see if you can extrapolate what LED current that will require and if it is low enough for acceptable LED stability. This last part is hardest to guess from most data sheets. But it is safe to assume that operating at the absolute maximum LED current is not a good solution and that staying at or below the current where most of the specifications are done will give a reasonable life.

The terms 'just saturate' and 'fully saturate' are not very well defined. What voltage drop can you tolerate?

The forward current transfer ration for the ISP847 is more than 50% at

At saturation (defined as no more than .2 volts collector to emitter, and a 1 mA collector current, 20 mA LED current may be needed. This represents a forward current transfer ratio of only 5%, a 10 times reduction of the linear case. But it is one point only. If you could tolerate .3 volts drop, a much smaller drive may do. You can get some idea how the drive varies with saturation voltage from the graphical data.

Based on what I can only assume is typical characteristics (since it is so much better than the one data point guaranteed), the guaranteed value is very conservative compared to the graph of saturation voltage versus input current and for several collector currents.

You have to start with what you need. How much collector current must the device pass, and what voltage drop is acceptable? Then you can start the difficult process of extrapolating what LED current may be needed based on the fragmentary info on the data sheet.

actually not - the standard rating for solar panels is based on an illumination of 1kW/m^2, but actual sunlight will be less than this unless you live on the equator. the sheet i have says a 10 watt rated panel gives

Nah. there is Max handling current and there is Max usage currents/

Normally the lowest!

basically its like this. you really don't want to max out your LED because all your going to do is shorten its life spanned, normally 50% of the max forward current is a good rule of thumb to follow with LED's, then there's the chart that some times is given in spec's to show you where at what point the light output starts to drop off the linear scale which is also a good point of reference. and as far as transistors go, yes it is a good idea to fully saturate the bias plus alittle more to keep the passive resistance in the transistor low so less heat build up will be generated. so if you use a open collector scheme to drive the LED's you should either lower the feed voltage through the common side of the LED's to prevent over current or use a series resistor on each led .. remember that you will lose approximately .6 volts via the transistor. unless your planning on using Power Fets. P.S. there are some lower current fets that will generate enough resistance when fully one to give you the drop you need for your leds thus reducing the part count but remember the heating problem.

FYI -- Just so you know:

An "Under water fan" is normally referred to as a "Wheel" on a boat.

anti protons? what about electrons or even tachyons?

if you have line of sight maybe you could use a modulated laser beam to avoid interference, then you just have to worry about blinding everyone who looks in the wrong direction, or isue lots of safety glasses.

or maybe you cld put the receiver at the far end and just transmit the station you want, you can get video senders wich use a gighz or so, asuming your remote has long range. just have to hope no one else nearby has the same idea..

why do i have visions of it being in a tree house ?? (maybe its the mention of rabbits).

ive often wondered what i wld see if i stuck a scope on the end of my cabletv/modem line, but scared of breaking it, as its NTL it might be a bit fragile and cld be ages before it got fixed...

im also interested in a pc card that wld receive chanels direct from the cable even if its just the unencrypted ones, atm my pc vcr (ATI AIW) is limited cos it cant change the chanel on the receiver box, although there was a thread on pc controled ir remotes ...

Colin =^.^=

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my thoughts exactly but you cld stil use your bs108 in place of the npn transistor John sugested to turn on the PNP transistor. the comon base(or gate) config means the operation is inverted ie high op turns the 18v off.

with comon emiter npn (or comon source bs108) to drive the pnp a high op wil turn the supply on. u stil need a resister from drain to the base of the pnp to limit the curent.

Colin =^.^=

Hey Everyone.

I need to be able to generate a negative voltage from a microcontroller. To be more detailed, when the microcontrollers output is 1 (high) I want to generate a positive voltage of 9V. But when the output is 0 (low) I want to generate a negative voltage of -9V. The positive and negative 9V supply must be with respect to the microcontrollers ground. The microcontroller is

Thanks.

Think of an optocoupler, e.g. ISP847 from

The question is, which LED forward current and voltage would be the best choice, so that the output photo transistor was switched hard on to full saturation but the LED had as long a life as possible?

I've been under the impression that a transistors base current necessary to only "just" saturate the transistor should be increased by a factor of 5 to fully saturate. Therefore as 5mA through the LED would only just saturate the phototransistor, an LED current of 25mA would fully saturate it, but their Vce(sat) current was only 20mA

If you can make sence of the question, then you're a better man than me :-P

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One way I thought I might be able to do it was to use a voltage inverter to turn the 9V into -9V and then use some combination of transistors to switch between 9V and -9V. Does that make any sense? If it does, could someone recommend a circuit?

What's your tolerance on that 9 V? Something like a MAX233 will give you a swing of about +/- 8 V with just one chip and no external components. It also inverts, of course.

Most TTL inputs see anything above about 2 volts as a logic high. See the data sheet for a typical LSTTL gate:

So you can have a tiny bit of leakage and still have a logic high out of the transistor. With a 4.7k pull up resistor (i mA to pull down to ..3 volts) you can have about a half mA passing through it and still have greater than 2 volts out of the coupler.

Another good way to make a fast logic output that is leakage and noise immune is to use two couplers, with their outputs stacked to form a push pull totem pole. Drive their inputs alternately. Most TTL inputs will treat an open circuit input (neither coupler on) as a logic high. You can parallel the pull up coupler with a 10k resistor for extra insurance (adds an extra half mA load to the pull down coupler).