The Sorcerer's Progress (Page 6 of 62)

Over to amy, I'm still clueless!  LOL  At least I've only read one chapter, so I believe I have a solid excuse for my ignorance  hehehe

(Over to amy, I'm still clueless!  LOL  At least I've only read one chapter, so I believe I have a solid excuse for my ignorance)

Nah.  New Jersey hoards his clues.  I pick on him all the time for this.  Doesn't mean that I won't keep trying to figure it out because I like him.  It's just when he complains about having to 'dumb it down' for the average reader, he doesn't seem to realize he's talking about me.  (sticks out tongue, blows raspberry into the ether.) 

To give him credit, I have learned a lot about outline chapters because of reading his material.  He is building the foundation of a great story.  Once these chapters are in place, he has the ability to flesh them out and add the pieces that are missing.

I recognize that I'm a hypocrite.  Truth.

A

Just keep walking njc!

Sigh.  Circuit changes aren't enough ... but I think I see ones that are.  I have to see whether they would make the voltage detector too sensitive to the battery voltage.  The L07011 and TN0620 mosfets can take the extra voltage.  Hmmm.

I see I have to explain.  'mosfet', also MOSFET => Metal Oxide Semiconductor Field Effect Transistor.  More properly Insulated Gate Field Effect Transistor (IGFET) because metal oxide is just one of the things used for the gate insulator.  Mosfets belong to the FET family.

There are two big families of transistor-type devices, junction-based and field effect.  FETs are used for dense logic circuits because they can be made very small and the insulated gate variety draw power only when the logic state changes.  Eack kind has its strengths and weaknesses, but the preponderance of logic circuitry means that many more FETS are manufactured.

BJTs (Bipolar Junction Transistors) operate by a current controlling a current, and their gain numbers are unitless.  FETs are voltage controlling current, and their gain numbers have units of inverse resistance, which is conductance, so they are called transconductance devices.

I'm using a mix of them, trying to use each for its strengths.  You can get the datasheets for them via Google: ksc1845, ksa992, lp0701, tn0680.

My device has multiple modules: voltage detector, timer, flasher (with three multivibrator stages), a low-voltage detector for the battery, and a pilot light module (the three-way pigeon flasher).  There's also an input filter that faces the phone line, blocking the ringing signal, providing protection from voltage transients, and presenting a high impedance so that the box cannot affect the phone line.  The output from the filter goes to the voltage level detector, which is concerned with detecting on-hook and off-hook voltages.

The voltage detector, the timer, and the control parts of the flasher operate on three volts, provided by three-terminal regulators (commodity semiconductor parts) from the battery voltage, which starts as high as 6.4 volts with new cells, and drops to about 3.8 volts.  The undervolt detector (a commodity device with an adjustment provided by a BJT) turns on between 3.7 and 3.8 volts, and sends a signal to the 3-way pilot flasher to flash red, one very brief pulse roughly every six seconds.  The box as a whole will keep working to a bit under three volts, but the batteries should be replaced long before then.  It should be able to run the red flasher and the other circuitry for weeks before it stops working.

The voltage detector sends the off-hook signal to the timer, but it also sends off-hook and on-hook signals to the pilot flasher, causing it to flash yellow or green roughly every 20 seconds.  The rate will depends a little on the battery voltage, and also on which LED is being fired.

The problem is that the pigeon flasher is driven directly off the battery (to avoid wasting energy in the regulator) and the logic on and logic off levels it needs are not those provided by the voltage level detector.

Both the off-hook detector and the on-hook detector use a BJT as the first stage, surrounded by resistors that bypass them until a certain current (measured in the nano-amps to micro-amps) makes it through the high-impedence input filter (DC impedance over 30 million ohms).  What follows involves going from first-order approximations in theory to second-order approximations.

The low-level (off-hook) detector (which is bypassed and shut down when the high-level detector goes active) turns on and draws current from the positive rail through a resistor, causing the voltage between the resistor and the BJT to drop away from the positive rail.  This turns on a MOSFET that (a) drives a hysteresis circuit and (b) operates another MOSFET to invert the logic and drive the timer.

Now, I may be able to drive that resistor from the unregulated (battery voltage) rail instead, if two conditions hold.  First, the MOSFET has to be able to stand the gate going more positive than the source terminal.  It can, within limits.  I would be within those limits.  Second, the voltage drop would have to be enough to turn the MOSFET on in spite of the extra voltage to be overcome, and that extra voltage would effect the circuit's turn-on voltage to some degree.  Right now the resistor is 910 kilohms (all resistors having 5% tolerance in their values) and the turn-on current is about a third of a microamp, which means that I need about a three quarters of a nanoamp at the base of the first transistor (assuming a gain of 450 in the transistor at those current levels--see the datasheet).  Its base-emitter bypass resistor is about 7 megohms, and at those current levels the base-emitter turn-on voltage is around four-tenths of a volt, so it needs about 55 nanoamps  before it turns on.

Now, if instead of .4 volts to turn on the second-stage MOSFET, I need that .4 volts plust the possible 3.4 volts between the battery rail and the regulated rail, I will need about eight times the current at both output and input of the first BJT--that's six to seven nanoamps instead of three quarters after the 55 nanoamp bypass, a change from less than one percent to about ten percent, and -that- will change the input voltage threshold by the same ratio.

It appears that I can raise that 910Kohm resistor by a factor of five or so, reducing the current needed from the first stage transistor proportionally, and thus reducing the input current by nearly the same proportion.  (Refer to the datasheet; at these collector (output) currents, the gain drops as the current drops.)  This will reduce (but not eliminate) the load on the current through the input bypass resistor, and reduce the sensitivity to battery voltage.

The question to answer is what third-order effects occur.  At lower battery voltages, the input transistor will be saturated, which will reduce the effect of the hysteresis circuit and might make the stage resist turning on at all.  The quickest way to find out is to try it.

The added resistance will slow the turn-off of the detector output when the BJT turns off.  The output-side MOSFET's insulated gate input acts like a capacitor, storing charge (but not so linearly; see the datasheet) and must discharge through the resistor.  A larger resistor means a slower discharge, although the driving voltage will be higher.  So long as I'm under two hundred milliseconds, I figure I'm fine.  I can even tolerate a one-second with the box in use, but such delays make it very hard to measure the thresholds.

The high-level (on-hook) detector has similar issues, but they are magnified by the use of two BJT current amplifiers, and a third to bypass and shut off the low-voltage detector.  I'm not going to drag that out here unless you insist, but there are also interactions with the low-voltage detector to consider.

So New Jersey, you like the thrashing? Or are you using me to point out areas that need amplification so you don't waste your time on the ones that dont?

Here's the Little Fugue in g-Minor on the harpsichord.  The plucked timbre is a little bell-like and might appeal to Collin.  It's a slower recording.  This organ recording uses mostly sweet stops, somewhat bright and very articulate, with distinct chiff.

You can find youtubes of this piece played on guitars and by tuba ensembles!

Okay, Cesar Frank.  Hmm.  One of the 'problems' of classical music is that you have slow intros.  Still ...

Okay, here's a very characteristic recording of the Cantabile.  Not the first piece I would have chosen, but there are interesting harmony things going on.  It starts moderately, but does get a little louder.  It's possible that some of the stops will be too strident, but it shouldn't be outrageously so.

Here's a moderately registered version of the Toccata from Widor's Organ Symphony nr 5.

Liszt, Hungarian Rhapsody nr 2..

The Mahler Symphony nr 8.  I suspect its dynamics will be too much, at least for the first minute thirty-five.

HTH, as they like to say.

I probably should've asked this question sooner because if you agree (which would be a first! LOL  just for the record, I still like you njc despite this trait of yours where you refuse to budge, without it you won't be you so don't change - as if you'd change just because of my opinion, but thought I'd just mention it to be safe - so please keep it in mind if you agree and agree slowly and carefully as not to cause me falling off my chair or anything), I've just allowed you to go through a lot of pain and effort for not much gain.  So here my suggestion goes. 

You are trying to somehow connect this into the phone's "circuit" to detect when it goes off-hook if I understand correctly.  What type of phone does your mother have?  Wouldn't it be easier to install an independent (to the phone's workings) button that would also needs to be depressed by the handset otherwise the warning LED would activate.  Now, the issue with this is, how do you know your mother or someone else isn't using the handset?  I'd suggest having a timer based on how long she's usually on the phone before the LED's would flash to avoid this scenario where the LED would flash even though the phone is in use.  This is of course assuming this time would be acceptable before the LED needs to indicate the handset isn't placed correctly.  But it's not a biggy I think, as it would just help to make sure the handset is placed back correctly too.

Or is this really an over-simplified way of looking at the issue at hand?  Or what am I missing?  I'm really trying to help out here!  Unlike someone else who's name I won't mention who keeps on plastering silly Simpson's pictures all over the site that looks a lot like his book cover colour scheme *ducks*  please don't cry, K, we also like you just the way you are! 

Idea #2

On one of our projects at work we needed loading arms to be replaced before tanker drivers were allowed to disconnect. The instrumentation guys used some or other connector switch with a spring loaded mechanism. The same concept could be applied to a handset I guess .... Should be heaps easier than playing with voltage ranges if you can find a connector that would do the trick?

K, you would make a terrible henchman.

NJC, consider a soup can and a very long string.

Janet, you're a dear.

At least no fingers have been harmed in making this picture ......