TTL To RS-232 DB9 M&F

There is also a write up of this project at ECN Magazine.

The title is not quite correct. Actual supply levels using the TI SN74LVC2T45DCTR range from 1.65 volts to 5.5 volts. Nominal supply voltages are 1.8V to 5V. Signal input levels will vary with the supply voltages. If you use the NXP 74AVC2T45DP chip the inputs will operate from .8 volts to 3.6 volts. However due to differences in packages you cannot use the NXP 74AVC2T45DP on the board as designed. The pins on that chip are .5 mm spacing. The TI chip has .65 mm spacing. Baud rates to 230,400 are possible but I have only tested the board which uses the TI SN74LVC2T45DCTR at speeds up to 115,200.

You can buy the bare boards Male DB9 or Female DB9 for $8.60. The boards come from OSH Park whose motto is "We're fab." And they are. Gold plated.

You can get the full documentation (schematics, parts list, and chip placement) here.

The board outputs full RS-232 levels (+/- 10 volts nominal about +/- 8 volts actual) for TxD, RxD, DSR, DTR, CTS, and RTS. These signals are available in a jumper field allowing you to jumper the board (with jumper shorts) for DTE or DCE use. RI is also available, but you have to wire jumper it according to usage. Since RI is so seldom used these days that should not be a serious impediment.

One point about the documentation. The schematics for the DB-9 Male board and the DB-9 Female board are identical. So is the parts placement. The only difference is how the DB connectors are wired on the PCB.


Building the Board

Install the 22uF, 6.3V capacitors C1, C2, and C8. Install all the .1uF capacitors C7, C9, C10, C11, C12, C13, C15, C17, C19. Install the 4.7uF 16V capacitors C3 and C4. Install the 2.2uF 16V capacitors C5 and C6. Install ICs U1 thru U5. Install the rest of the surface mount parts. Install ground pin GND0. Install the .1" ctrs headers and jumper fields. Install the DB-9 connector.

Update: 18 March 2014

I have modified the boards so that they can be powered from the device they are plugged in to. For that you use JU2. JU1 has been removed. In addition capacitors C3, C4, C5, and C6 have been reduced in capacitance so that the board powers up faster. You can find the documentation for the board at TTL to RS232 DB9 M&F 25 Sept 2013 Documentation. OSH Park has the TTL to RS232 DB9F 25 Sept 2013 available for $8.60 ea. The TTL to RS232 DB9M 25 Sept 2013 is also available for $8.60 ea.


Engineering is the art of making what you want from what you can get at a profit.

Hot Air Iron

You can read more about how I developed this design at ECN - DIY hot-air iron.

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The comments to an article I posted to ECN Magazine a while back got me interested in making my own hot air soldering iron. A look around the web connected me with a do it yourself project to make just such an iron.

First off you will need a suitable iron. I checked the Variable Temperature Soldering Station I bought from Amazon for about $20 and found after checking it out that it was enough like the iron in the project to proceed so I did.

First you need to gather the project materials. For the iron conversion you will need about six inches of silicon rubber model airplane fuel tubing. It should be rated for 3/32" fittings. I liked this racing orange tubing. But blue/clear tubing would probably also work. This type of tubing is size rated according to the outside diameter of the metal tubing it will mate with. We will be needing such tubing in brass. One foot each of 3/32" (to mate with the fuel line) and 1/8" (to mate with the air line). You can pick it up at your nearest hubby shop. I got mine at Royal Hobby. I also bought a K&S Tubing Cutter in order to avoid ragged hacksaw edges on the cut tubing.

The first step is to open up the iron by removing the three screws that hold the heating element to the handle.


Find a suitable spot on the iron hand and drill a 3/32" hole pilot between the wires. I drilled my first hole straight into the iron (90 degs to the barrel) and then using that one as a starter hole drilled a 9/64" hole from the same point at 45 degrees to the barrel. It may be a good idea to reinstall the heater with at least one screw to keep the wires from absorbing excessive vibration during drilling. I hand drilled the final 15/64" fuel tubing hole using a pin vise/hand chuck to hold the drill. The point is to go slow so you are not drilling into the power wires at the end of the hole.


Cut off about 1" of a copper clad scouring pad (no soap please), twist it up and stuff as much as you can into the heater barrel being careful not to disturb the power wires.


You can now snake your fuel tubing through the handle and insert the (squared off) tip into the bottom of the heating element between the wires. Create an air seal where the tubing enters the heater with Permatex High Temp Red RTV. It is rated for one hour at 650°F. The area of the iron it is being used on probably gets no hotter than 300°F so the seal should last a while. The RTV will skin over in about an hour and be fully cured in 24. Let it sit.

Now put it all together. To do that you will need to cut one 1" (2.5cm) long piece of 3/32" brass tubing and another equal length of 1/8" brass tubing. Overlap them about 1/8" (3 mm) and solder, epoxy, or super glue them together. Be sure to ream the tubes out with a suitable drill if the tubing cutter necked them down. You may also have to sand down the end of the 3/32" tubing to make it fit. That little fitting will connect your iron tubing to the air line tubing.


The nozzle for the hot air iron is a Stimpson GS5-12 backed up by a 4-40 nut drilled out to 5/32".

Now we come to the question of pumps. I had an Whisper 10 (old square model) laying around and by itself it was barely adequate to give sufficient flow to melt solder paste. In tandem with an AQT3001 I also had around it was better. But the best was the Petco AC-9903 which has a flow/pressure adjustment on top. The AC-9903 is rated at 4.5 liters a minute and .014MPa. You can read more about my adventure with air pumps at DIY hot-air iron.

I used a ProsKit 9303C meter for the temperature measurements I did. I was looking for better/worse more than accuracy. And for that it was adequate.

Power Supply Digital WW

I needed a "digital" power supply (+5V and +3.3V) for a number of my small projects. I also wanted a low noise supply that didn't require building a power line interface. I did build one of those for projects requiring more power. You can read about that project at Power Up. The Power Up supply can deliver 800 mA per supply. The supply described here is only good for 150 mA per voltage and it runs a little hot at that current. So think of the 150 mA as a design limit with a more practical limit (longer life) of 100 mA or less. That is still plenty of current for today's low power designs. In addition the supply uses a Wall Wart (the WW in the name) for its AC power connection. No power entry construction required.

One more important point before you get started: the TI TPS7A4901 (data sheet) chip used for both regulators is a low noise regulator. This can come in very handy when powering precision crystal oscillators where you want to keep the noise down to maintain short term accuracy. It is not the lowest possible noise. But it is quite good for a one chip solution.

The design files (schematic, board layout, and parts list) for the board can be found at Power Supply Digital WW - design files. If you want to build one OSH Park has the boards for $11.00 each.

So lets start the build. First put the 22uF 6.3 V capacitors (C9 and C10) on the board. Then solder the U1 and U2 pins to the top side of the board. Next turn the board over and squirt some Chip Quick SMD291NL flux into the U1 and U2 grounding/heat sink holes on the back side of the board and apply your iron and solder to those holes (I use 63/37 tin/lead) until they are filled. That grounds the chips and also acts as a heat sink for them.

Now solder the rest of the surface mount components to the board. At this point I like to test the board because trying to work on the regulator chips with all the other components mounted is very difficult (read impossible) without removing parts. To do that testing you have to short out RV1 and RV2 and apply power to the chips. About 6 volts is OK for both chips because no significant power will be drawn - this is only a voltage test. The voltages should read about 2.9 volts for U1 and 4.9 volts for U2. If you don't get those voltages check U1 and U2 (the most likely culprits) for shorted or open pins. Since I plan to make a lot of these boards I built a pogo pin rig for this test using a bare board. The holes for the pins that go into RV1 and RV2 positions need to be drilled out (.040" - #60 drill) if you are using the Mill Max 0856-0-15-20-82-14-11-0 pogo pins to make your temporary connections. It will look like this with the board mounted on the test jig:
This is the test jig with the pogo pins:
I used brass washers between the nylon nuts to adjust the height of the jig.

Next mount all the 470uF 10 volt capacitors (C3 through C8). Now add in the transistors with thermal pads mounted loosely to the heat sinks. I used 6-32 by 3/8" screws and split lock washers to insure contact of the transistor with the heat sink is solid. Mount and solder Q1 first and then tighten the screw. Then Q2 followed by Q3.

Mount all the rest of the components and test the board under power. I used a 22 ohm 1% 3 watt resistor mounted on a power plug for the 3.3V section and a 33 ohm 1% 3 watt resistor mounted on the same power plug for the 5 V section. Anytime you want to test a supply under load just plug in the load. And that is all there is to it.

I wrote some more about low noise power at ECN Magazine.

Engineering is the art of making what you want from what you can get at a profit.

Space-Time Crystals

This is a little far afield from what I usually blog about. But I couldn't resist.

Physics team proposes a way to create an actual space-time crystal

Earlier this year, theoretical physicists Frank Wilczek, of MIT put forth an idea that intrigued the research community. He suggested that it should be possible to construct a so called space-time crystal by adding a fourth dimension, movement in time, to the structure of a crystal, causing it to become an infinitely running clock of sorts. At the time, Wilczek acknowledged that his ideas on how to do so were inelegant, to say the least. Now another international team led by Tongcang Li has proposed a way to achieve what Wilczek proposed using a far more elegant process. They have posted a paper on the preprint server arXiv describing what they believe is a real-world process for creating an actual space-time crystal that could conceivably be carried out in just the next few years.

Wilczek thought that it should be possible to construct a space-time crystal because crystals naturally align themselves at low temperatures and because superconductors also operate at very low temperatures; it seemed reasonable to assume that the atoms in such a crystal could conceivably move or rotate and then return to their natural state naturally, continually, as crystals are wont to do as they seek a lowest energy state. He envisioned a rotation with a ring of ions that flowed separately rather than as a stream, likening it to a mouse running around inside of a snake laying as a circle. The bulge would flow, rather than the snake itself spinning and would just keep on going, potentially forever. The problem was, he couldn’t figure out how such a crystal structure could be created in the real world.

arxiv paper

H/T DeltaV

Engineering is the art of making what you want from what you can get at a profit.

Testing Telephone Cables With RJ11 Connectors

I'm working on a distributed I2C bus system that uses standard telephone cable and RJ11 jacks for interconnection. Plug and play - if you get the software right. That requires cables that are correctly wired. No shorts of course. But also no twists. Five volts should not appear on the ground connection. And vice versa. To do the testing I have designed a board with three resistors and six LEDs. I use a six volt battery pack of 4 AA cells with a built in switch to power the tester. The batteries should be good for about 120 hours of continuous use. If the cost of batteries irks you the tester can be powered by a 5V wall wart supply. Or a convenient bench supply. The voltage is not critical. Anything from about 4 volts to 6 volts will do. Absolute maximum current possible is about 35 mA at 6 volts. Normal current with a working cable plugged in and a 6 volt supply is about 25 mA.

In use the supply polarity doesn't matter. If you use the supply conventions shown the green power good light comes on with power and if the cable is good you get two green lights in addition. If the cable is reversed you get a green power good light and two red lights. If you reverse the power supply red will be the power good light and two additional red lights will indicate a good cable. Various other faults will give you different light combinations which you can work out from the schematic. You will also find the parts placement and parts list with the schematic.

The only thing that needs special attention during assembly is LED orientation. I used a 5V supply and some clip leads along with two 150 ohm resistors (one for each clip lead) to probe the LEDs before mounting them.

The boards (I2C Simplest Cable Tester 31 Oct 2012) are available from OSH Park for $2.45 each plus shipping.