John's Junk Drawer - Experimenting

Open vs FreeBSD -- Night and Day

nospam@example.com (John) — Thu, 19 Mar 2015 13:36:00 +0000

I have switched to FreeBSD after spending a few months banging my head against OpenBSD. OpenBSD is a nice O.S., and I really didn't want to switch, but now that I'm trying FreeBSD, here are some first impressions of the differences.

Reasons to Choose FreeBSD

Would you like clear, well-written user documentation, instead of curt citations of man pages, and faq entries? You want FreeBSD.
Would you like to experiment with hosting MS-Windows (or Linux, or another BSD, etc.) as a guest O.S. in a virtual machine, right out of the box? You probably want FreeBSD.
You have an older machine with only 1/2 gig of ram, and you think you should be able to run firefox without hitting the swap file? Free is the O.S. for you.
You have an older computer that runs at just under 1 gighz, and you think it's reasonable to expect an O.S. to deliver decent performance from that? You need FreeBSD
You've invested a lot of time switching from MS-Office to OpenOffice and you don't want to have to now learn the latest other OTHER office suite? Definitely, you want FreeBSD.
You're an experimenter who wants to install BSD on your Raspberry Pi? Most seem to agree, FreeBSD is a better fit for you.

Reasons to Choose OpenBSD

You think PF (Packet Filter) is a really cool piece of software and you want to run it? OpenBSD is for you.

And Now, for Something Completely Different

Are you a masochist who is excited by the idea of being beaten to a pulp, and left naked in the middle of the ethical, legal, and financial mine-field that is the GPL?. . . You want linux.

Water Level Indicator

nospam@example.com (John) — Wed, 11 Sep 2013 15:09:00 +0000

An experimenter in a facebook group that I'm a member of put up the following schematic. They asked for group members' help to give a simple explanation of the circuit. I've put the schematic here, so that I can give some feedback with a little space and some links.

The above circuit is a fluid level meter that converts the level in the container on the left into a single-digit number, from 0-9 on a seven-segment display. In this case, 0 (zero) represents the lowest level, and 9 (nine) is the highest level.

. . . . . . .
The Sensor

On the left, the level sensor is a set of ten conductors. Nine of the conductors are held high using high-resistance (560K) pull-up resistors, and one is connected directly to ground. When the water reaches a certain level it conducts between the respective conductor and ground.

Water, in its pure form, is an insulator, but most water (including drinking water) has impurities that make it slightly conductive. It is still not a good conductor, and this is why the pull-up resistors need to have relatively high resistances. In this case, when the water covers a given conductor, the conduction path to the ground pin is enough to pull the voltage down from high to low.

. . . . . . .
74147 (74HC147)

Chip name: 10-Line Decimal to 4-Line BCD (10-Line to 4-Line Priority Encoder)

This is a 74xxx (say: "Seventy-Four Hundred Series") chip that converts the "level" signal on its 10 input lines to a binary number representing the highest line active.

Important: The inputs are "active low" meaning a low value (near ground) represents an active (true) line, while a high level (near 5v) represents false.

As the water rises, it covers higher and higher pins, grounding each one it covers. Remember that, normally, these were held high by very high-value resistors. When the water provides a conductor to ground, it pulls the line down close to zero.

Again. the 74147 places a binary number on its output, which represents the number of the highest of its input pins that are at ground. You can see this in the truth tables, here:

[pdf] 74147 (74HC147) datasheet

. . . . . . .
What is 'HC'?

The 74xxx series of chips were originally a very popular series of TTL (Transistor-Transistor-Logic) chips. These chips, their pin-outs, and their functions have since been remade using High-speed CMOS. This is what the HC in the number signifies. It is the old TTL chip's function and pin-out, just implemented with CMOS technology.

Though the logic functionality and pin-outs are exactly the same as the old TTL logic chip, the HC may be important in this circuit. This is because one advantage of CMOS is that it has very high input impedance, and that may be needed to make this particular circuit work with the high resistances provided by water-conduction.

. . . . . . .
The Transistors and Pull-Up Resistors

As far as I can tell these four transistors (T1-T4) are merely used as inverters, and in this circuit, they can probably be replaced with inverters. Essentially, the 74147 chip has an active low output (see datasheet linked above), meaning that 0 represents true.

For example: in active-high (what most would consider 'normal') representations, the binary output '1100' is a binary representation of the decimal number '12' (8+4). In active-low logic, the low signal (0) is the number, so '1100' represents 3 (2+1).

The input to the next chip is active-high (i.e., 'normal'), so the output lines have to be inverted. This is the purpose served by the transistors and 33K pull-up resistors.

. . . . . . .
CD4511

This chip converts the BCD encoded number on its input to proper patterns needed to drive each segment LED in a 7-segment, common cathode (common ground) display.

Not only does this chip convert the BCD input to the correct set of segments for each digit, it also provides high current on its outputs, in order to drive the LEDs. The resistors are current limiters and serve to protect the chip from short-circuit conditions.

This chip is also implemented in CMOS (Complimentary Metal Oxide Silicon). Here is the data-sheet:

[pdf] CD4511B datasheet

. . . . . . .
Caveats

I've not built this circuit. I've just produced this quick analysis, by eye-balling the diagram, so that I could answer my fellow experimenter's call for feedback about it. Again: This is quick and dirty. I have not tested the circuit or my advice about it.

-djr

2012-11-01 11:09

Update on the $80 Oscilloscope

nospam@example.com (John) — Wed, 16 Nov 2011 14:18:22 +0000

Finally got the intensity problem fixed on the new (used) scope. It was a couple of bad diodes and some supporting components. I tried to get the originals, but that was not going to happen in any decent time frame.

. . . . . . .
The Fix.

Trusty ol' 1N4004 diodes don't have the fancy low-capacitance, high-switching-speed of the original diodes, but they have more than enough reverse voltage and power handling. . . Hmmm. . . would they work? YES! The scope is all calibrated now, and works like a charm. Now that the brightness works I can see what the whole Inten'D thing is for (my old Heathkit scope didn't have delayed triggering). Pretty cool!

. . . . . . .
Summary: $80 + a little elbow grease = 35 MHz Dual Trace Scope

The $80 price includes the shipping cost!

Model: JDR Instruments 3500

Found and downloaded a free manual (images scanned into a PDF). It was needed for the schematics to fix the intensity problem, but it also included the calibration procedures.

The guy who sold it was a bit of a sleaze bag (imo). He said, directly, that he wasn't aware of any problems, though he took a picture of it with the traces showing pulses. To me, that means he would have fiddled with the focus and intensity controls and seen that the intensity control had no effect.

So, it had a problem with the intensity that turned out to be two different diodes in the intensity circuit (go figure).

Now I've got a scope that probably cost >$1K new, and would cost $200-500 used in good working condition today. Not too shabby.

-djr

A New (well, for me) Oscilloscope for the Bench!

nospam@example.com (John) — Thu, 27 Oct 2011 16:31:10 +0000

The scope has arrived!

It came in the mail yesterday. A used, JDR Instruments, 35 MHz, dual trace scope for $80.00 (Shipping included) on eBay!

It had two obvious problems.

No horizontal positioning — I was able to fix that by following wires and traces back from the front panel.
The brightness is at full all the time — not able to fix that without schematics.

I spent a bit of time with it, and while it needs some calibration, I haven't been able to find anything else wrong. Also cool, it has a Z input in the back, which means I'll be able to play asteroids on it

-djr

Another Inch Out of the Ditch

nospam@example.com (John) — Thu, 08 Sep 2011 12:18:15 +0000

Finally managed to slosh enough junk around to clear the table in the foreground and then clean off the old workbench. The last time it was out from under a quarter inch of dust you needed a t-square to produce PCB layouts (see it there). The old scope smoked out as soon as I turned it on. It smelled up the whole basement.

All the bread-boarding sockets had been left completely uncovered under all the dust and in the dampness for the entire time. Nonetheless, I decided to assemble a small, experimental, logic circuit on the old breadboards and surprisingly, the circuit worked!

So, what does it mean if, while other guys are spending time and money building man-caves, you start building a maker cave? Well, you might just be a geek my friend.