What is your dumbest design mistake?

[GonzoEngineer]

Many years ago, I worked for a company that set up computers and peripherals in hotels.

They wanted to save money, so the point of sale terminals were RS232 'dumb' terminals, sitting on top of an electric cash drawer.

I build a circuit that would watch for an ASCII code string and drive a solenoid to open the cash drawer.

It worked great until the first day the hotel lost power.....and all the cash drawers flew open and went "ding ding ding" like slot machines paying out!

 

[mramos1]

If that was a building access controller, it would be OK.

I have etched a couple boards at home and forgot the laser is the mirror, ticks you off after you wait for all the copper to fall, waste the acid, stuff in chips and wonder why the pins do not match the part.

When I worked in R&D, we generally checked, re-checked, and past the design around. Now we did play with the thermatron (temp cycler) in ways not covered in the manual.

 

[dknguyen]

An SMA connector that had four 1mmx1mm square pins...they do not fit into a PCB with 1mm diameter holes.

 

[3v0]

I have etched a couple boards at home and forgot the laser is the mirror, ticks you off after you wait for all the copper to fall, waste the acid, stuff in chips and wonder why the pins do not match the part.

I have seen more then one board like this. You can make dip parts match if you can bend the pins 180 degrees.

 

[mramos1]

I have seen more then one board like this. You can make dip parts match if you can bend the pins 180 degrees.

Done that on a single chip boards My eyes are too bad to even thing about SMT.

 

[3v0]

Done that on a single chip boards My eyes are too bad to even thing about SMT.

It is all in having the righ tools. I use use a 2 lens + loop jewelers headset/magnifier every time I pick up a soldering iron. What made SMD possible for me was the QX3 microscope. The 10X setting works well enough and the bottom light works well for aligning SOICs, if you do not have too much copper on the other side of the board.

3v0

 

[Leftyretro]

Well I once designed a analyzer instrument installation for a processing plant. I utilized and specified it be powered by the closest avalible field AC power panel. It turned out that the closest AC power panel was controlled by a remote main breaker that had a photodetector input such that all the plant lighting in that area fed from that field panel would be automatically turned off during the day and on a night. I guess that is why there were only plant lights being powered from that panel. Needless to say I got teased about that for quite a while.

Lefty

 

[GonzoEngineer]

I can remember four hours trying to figure out why a microprocessor wasn't working......It was a 40 pin DIP.....took me four hours to figure out that I had plugged it in backwards!

I guess those three Magaritas at lunch were the problem!

 

[Nigel Goodwin]

Nothing to do with me - but years back my younger brother worked for Nat West bank, and he told me the story.

The larger banks in the UK had an agreement where you could use your cash card in any of the different banks machines.

Unfortunately, the person who wrote the software for the interaction between the banks forgot about leap years.

Anyone who put a card in a different banks machine on the 29th February had the card confiscated!

Now that's what you call a mistake!.

 

[mramos1]

Yea, imagine the call from all the people that needed new cards and the cost. Ouch.

These days, I help run commercial "smart" buildings.

I would like to get rid of daylight savings time, no leap year problem yet. I can not tell you what a pain M$ and other vendors cause me when DST flips back and forth and for some reason they changed when it is to happen.

I get calls, elevators are not working, doors are locked, my email is off by an hour.. The clock on my desk phone is off. You want to say "stop looking at the clock on the phone and get to work".

I have all the system set to no DST so twice a year I just deal with it by hand (remote access helps and I am up early). I know M$ fixed it, but I still check them all.

Gonzo, if I did that and it did not run, I would have to put a finger on it, see it's hot, and think it must be in backwards, if the right chip!

Funny the things we check over and over as we make mistakes in life.

 

[Marks256]

the dumbest thing i've done so far is accidentally running a 24vac coil relay off of 120vac. i swore i read that it could take that... but i guess i was reading the contact voltage. It ran fine for about 2 seconds... then it started shorting out and burning...

oops

 

[Mikebits]

I did a major screwup once. My company was planning a demo of our gear to the telcommunications group in China. One of my task was to select a international Power supply/UPS converter to interface our gear to the power in China.
Once in China and starting to hook up our gear in some dingy telecom building, I discovered that I had specified the incorrect power plug for our UPS. We could not plug in our equipment because I selected the wrong plug.

After panic and anxiety subsided, a couple field engs and I set off running around the markets of Tianjin China in search of plugs. After a half day of searching with no success we had our taxi driver take us to our hotel where we had one of our chinese marketing guys explain to the taxi driver what we needed.
Luck would have it and we found the plugs.
I spent the whole night reworking power cables. But in the end it all worked out.

 

[duffy]

After desiging the blinking lights in the rim, my partner and I agreed that the frisbees would look better in the new opaque plastic we got samples of instead of the translucent ones we had been testing. To get it under the deadline before Chinese New Year, we called them up and gave the go-ahead to manufacture our first run of a thousand in these lovely pastel shades.

The opaque plastic is made by adding a white powder to the pellet mix. When the guy from Toys R Us threw one of the frisbees up in the air, it came straight back down on the sidewalk and cracked in half at his feet.

 

[Roff]

This wasn't exactly a design, but it was definitely a dumb mistake!
About 30 years ago I decided to replace the wimpy little radiant wall heater in my bathroom with a fan-forced heater, controlled by a separate 24 hour timer. After installing the timer and the heater, I turned the breaker back on. The timer made this loud growling noise, the fan put out a gale-force wind, and after a few seconds, wind-driven flames came out of the heater! In a panic, I turned off the switch, and after a little head-scratching, it dawned on me that the old heater was powered by 220V. DUH! **broken link removed** Why I had assumed it was 110V, I don't know.
Of course, the new heater was wired for 110V, and the timer was 110V. The timer and the fan motor were destroyed. I managed to salvage the heater by moving the resistance-wire terminals around so that they were in series instead of in parallel, I found a 220V fan motor at Grainger, but it had a smaller diameter shaft, which I fixed to accept the fan hub by making a shim out of some Mylar sheet stock. I gave up on the timer, because I couldn't find one which ran on 220V. I was tempted to get another new 110V unit and run it off one phase of the 220, but the cable in the wall was 2 wires plus a bare ground. I would have had to use the ground as the neutral for 110V, which I was not willing to do.
I'm a pretty good EE, but a lousy electrician.**broken link removed**

 

[Papabravo]

Sending bare boards and blank OTP processors to the assembly house. They did not work so well and the processors had no provision for programming after they were on the board.

 

[Mikebits]

Sending bare boards and blank OTP processors to the assembly house. They did not work so well and the processors had no provision for programming after they were on the board.

Lol, that's a good one. How many boards?

 

[GonzoEngineer]

Sending bare boards and blank OTP processors to the assembly house. They did not work so well and the processors had no provision for programming after they were on the board.

Ouch!

 

[0xFF]

I was tempted to get another new 110V unit and run it off one phase of the 220, but the cable in the wall was 2 wires plus a bare ground. I would have had to use the ground as the neutral for 110V, which I was not willing to do.
I'm a pretty good EE, but a lousy electrician.**broken link removed**

Stepping down is easy... 220V (which is now referred to as 240V) is red, black (both hot) and ground (bare). All you need to do it remove the red (hot) from the breaker (at the panel end) and put it with the neutrals (put white tape on it if it makes you feel better). You now have single phase 120V.

Stepping up is the exact reverse (just make sure you have a big enough wire gauge for what you're running) -- remove the white (neutral), put red tape on it and attach it to a breaker (making sure you're on the opposite phase (leg) of the panel). You now have 2 phase 240V.

Of course, you'll have to change your receptacle either way.

(EDIT: just noticed you said 2 wire... so they should already be black/white... where I said red in the first paragraph, sub it for white.)

 

[Hero999]

I suspect he already knows that and that connecting the heating elements in parallel is not an option because it would draw too much current.

The problem he has is that he has a 240V heater and a 120V timer and doesn't want to use the earth as a current return as it would violate the electrical code.

What about using an auto transformer to get 120V?

It would only have to be rated for 10VA or so and wouldn't have to be very big.

If you can't find a small 240V to 120V auto-transformer then you could use a small mains transformer with a twin 120V primaries connected in series and connect the timer from the centre tap to phase.

The timer probably uses a small mains transformer so another option would be to see if you could replace or re-wire it for 240V operation.

 

[0xFF]

The problem he has is that he has a 240V heater and a 120V timer and doesn't want to use the earth as a current return as it would violate the electrical code.

Ah, I read

I was tempted to get another new 110V unit and run it off one phase of the 220...

as "I was tempted to get another new 110V heater and run it off one phase of the 220..."