Warning! One Million Ohms

I shared this little project with a few folks here on the forum and got good feedback and suggestions that I could sell it. So I thought it'd be fun to try. It's listed on tINDIE.com for anyone that's interested. Guaranteed to keep the zombies away from your workbench!

That's an awfully (physically) large resistor! Isn't there an exercise question in Horowitz and Hill that says something like "Why will you never practically exceed the rating of a 1/4 watt 1Mohm resistor?" ?

AWOL: That's an awfully (physically) large resistor! Isn't there an exercise question in Horowitz and Hill that says something like "Why will you never practically exceed the rating of a 1/4 watt 1Mohm resistor?" ?

LOL, it's an especially large resistor considering it's not actually part of the circuit. It's a 2W unit, so something like 1400V would be needed to generate dissipate that much power.

Jack, you gotta share the story behind this. I don't think I can type it up well, and can't find a link for it.

One Million Ohms is for wimps: http://www.silverlight.ch/resistors.php ;)

CrossRoads: Jack, you gotta share the story behind this. I don't think I can type it up well, and can't find a link for it.

Evidently the joke goes back quite a ways. Engineers and other techie types would tape a 1M resistor to a piece of paper and write, "Warning! One Million Ohms", then post it on the lab door, etc. In some cases this was successfully used to keep the janitorial staff or others that didn't "get it" from messing with a project.

The incredible thing is that even at my advanced age ;) somehow I had never managed to run across the joke until last year, when Grumpy Mike mentioned it here on the forum. I had a good laugh, and knew right then that there had to be an electronic version of it. I'm hoping others will want one, too. I gave some out last Christmas to friends and co-workers, at least one of which was questioned about the danger of having it on his desk at work!

Not long after I had the first boards made, the joke turned up as a question in my son's physics textbook, with the query, "Why do physicists and engineers find this amusing?" :D

This makes me wonder when dihydrogen monoxide (DHMO) will make it into the textbooks ;)

Why all the components on the bottom...smoke and mirrors?

It is an attiny that is intended to provide the "action".

RPCoyle: Why all the components on the bottom...smoke and mirrors?

The 1M resistor is smoke & mirrors, everything else is part of the circuit. Four dropping resistors for the LEDs, a power supply bypass capacitor, a button to change the flashing speed & pattern. And an ATtiny85.

Of course! IT BLINKS! Now all you got to do is add an IR sensor and a audio module that cry's DANGER! DANGER! when someone gets to close.

Whenever I encounter resistances of one million Ohms or more, I just use my DMM as a shield. It can measure several million Ohms, so I know I'm safe behind it. ;)

RPCoyle:
Of course! IT BLINKS! Now all you got to do is add an IR sensor and a audio module that cry’s DANGER! DANGER! when someone gets to close.

Ha! Interesting idea! Someone also suggested having it turn itself on at random intervals. That might raise some eyebrows. I might have to work on alternate firmware at some point.

I've never heard of this joke before but as soon as I saw the board I laughed and laughed, it's hilarious. As an "oldie" I feel an microcontroller is a little bit overkill for the flashing though as a good ol' 555 would've done just fine. in fact with a few added components and a 556 it should be quite possible to get some cute random actions into the circuit while getting the impressionfactor up a bit. :grin:

karlboll: I've never heard of this joke before but as soon as I saw the board I laughed and laughed, it's hilarious. As an "oldie" I feel an microcontroller is a little bit overkill for the flashing though as a good ol' 555 would've done just fine. in fact with a few added components and a 556 it should be quite possible to get some cute random actions into the circuit while getting the impressionfactor up a bit. :grin:

I certainly am glad I'm not the only one that hadn't heard it before! :D From a purely economic standpoint, the ATtiny85 makes sense. Advantages are individual control of the LEDs (several flashing patterns and speeds), no additional timing components, no power switch, automatic shut off (sleep mode) and re-programmability. But I know what you mean, certainly this circuit does not take advantage of much of the MCU's capability.

I have a lot of computer-literate friends that don't really know microcontrollers or much about electronics, and I like to explain to them that the thing is a complete computer, is handling 1000 interrupts/sec, has three kinds of memory, an ADC, can run 10 MIPS, costs $2, etc.

From a purely economic standpoint, the ATtiny85 makes sense.

eh? Assuming simple blinking it the desired functionality, a 555 is $0.41, an ATtiny85 is $1.18 (qty 1, digikey).

The 555 needs a few passives (less than $0.77 worth, I'd wager) and requires more assembly work (which could be viewed as an advantage, for a kit).

-j

kg4wsv:

From a purely economic standpoint, the ATtiny85 makes sense.

eh? Assuming simple blinking it the desired functionality, a 555 is $0.41, an ATtiny85 is $1.18 (qty 1, digikey).

The 555 needs a few passives (less than $0.77 worth, I'd wager) and requires more assembly work (which could be viewed as an advantage, for a kit).

-j

I meant to duplicate the same functionality as my circuit. There are six flashing patterns, three speeds, automatic power-down after five minutes...

Ah, so not "simple blinking". :)

-j

kg4wsv: Ah, so not "simple blinking". :)

-j

Haha, yeah maybe just a little fancier blinking ;) Anyway I figure that given the somewhat frivolous nature of the project, maybe frivolous use of a microcontroller sorta fits in :D

Here's mine, thanks Jack! These boards are a lot of fun. I took mine to work and everyone there got a big kick out of it.

Regarding the kit: It's very easy to assemble. It took me about 10 minutes. The PCB is very sturdy and the silkscreen printing is clear and sharp. The hardware is excellent. I didn't expect a battery holder or power terminal blocks, but they were included. Everything you need is right there in the kit. The instructions are more than adequate with photos and wiring diagram on the downloadable PDF. You can add the headers for ICSP programmer if you want to tinker with the code, or re-program it all together. Jack has put the code up on GitHub for anyone who wants it. Nicely done Jack!

The use of button input and sleeping the ATtiny85 are nicely implemented in the code and I learned from reading through it. But there are already several different blink patterns with different timing, so I haven't felt the need to tinker with it yet. It's really cool as is!

I've ordered more of these as gifts for friends. I have several friends that are near impossible to find holiday gifts for. A couple of them are into electronics too. So these make a unique and affordable gift that is certain to entertain.

Lastly, Jack is a pleasure to deal with. Many of you already know of him from his posts in the forums here. I have asked Jack a couple of follow-up questions regarding things I didn't understand (in the microcontroller forum) and he went out of his way to help me. Thank you Jack!

Here is a short video I made with my iPhone that shows some of the blinking patterns. You simply press the button to change the pattern, which I do several times in the clip. Press and hold the button to power off, or it goes to sleep after 5 minutes:

YouTube: http://youtu.be/GAgEru5UjlU

Vimeo: https://vimeo.com/49014569