pic: How Not To Do It: Electronics Edition



Was rooting through an old electronics bin, found…this…lying around.

Now, I don’t know what’s more disturbing - the fact that this was made in the first place, or the fact that it was in a bin of stuff that clearly had been used before.

Thought some people around here might get a chuckle out of this. Or a cringe. Or something.

I particularly like that whoever made this bothered to shrinkwrap the battery leads.

Not too disturbing, if used properly.

It is a great visual example of how an H-Bridge works. Granted there is no speed regulation with it, but with the proper motor and gearing, it is perfect!

Besides, the spark show would be quite an attention getter!

I like this!

Control of dc motors is based on ideas from the 19th century. The device pictured is a double-pole, double-throw “knife” switch, made somewhat famous by its appearances in several movie re-creations of Dr. Frankenstein’s laboratory.

An FRC speed controller is just a switch like this, improved so that it can be thrown back and forth very rapidly without wearing out, heating up, taking up a lot of space, or requiring much effort.

Just to clarify, I think the idea of building macro-scale demonstration pieces is great. What bugs me is the utter lack of insulation on pretty much everything (seriously, it’d be almost impossible to touch the actual ‘switch’ part of this assembly without shocking yourself).

How would one shock themselves if the demo is for a standard FRC motor and battery? Unless there’s a lot of saltwater involved, its unlikely you’ll feel anything.

Now, protecting the battery from shorts is a whole different concern…

Ever had a cut on your finger? Especially when working in a shop?

Yes, it’s only a 12V battery, but that doesn’t mean it’s not extremely bad practice to have massive pieces of exposed circuitry.

I suspect that the counter EMF from the motor could tickle your insides.

I’m diggin’ the copy of Munkres in the background. :cool:

Hah, was wondering if anyone would recognize it, though to be honest it wasn’t my favorite math text. It wasn’t bad, but a lot of the early point set proofs which ought to have been done in rigor consisted of just pictures and handwaving (claiming “we can construct a homeomorphism” but never actually demonstrating one, etc).

I still keep it around, though, for want of any other good references for point-set topology.

That’s Munkres. Even later “proofs” on covering sets and homotopy are fuzzy. Working through his “Elements of Algebraic Topology” presents similar frustrations. He works on intuition.

And cuz it’s a nice looking book :wink:

Bang-bang speed controller Rev. 1.0

Now all you need is some pneumatics, and you have, well, a spike.

LOL…not really a speed controller though. It’s an H-bridge switch.

“Bang-bang speed controller” is just another name for an h-bridge.

Technically, you don’t need an H-bridge for a bang bang speed controller, just a single on/off switch. But yes, you’re right.

My favorite part of this is the huge gauge wire on the battery leads and the tiny gauge everywhere else.

It is a perfect example. In reality, an H-Bridge is just transistors performing the same operation, just solid-state.

I guess the spark could be a possible recruiting tool – that, only if the correct precautions are taken! It’s not the best idea to try this at home, at least if the voltage is greater (or lower) than 0v :smiley:

I think that that is actually quite a typical setup to introduce the system.

However, it’s pretty cool how I still use that setup all the time. Just take a DPDT switch. Connect the middle pins to the motor. Connect one side of the switch to the battery. Connect the other side to the battery too. However, on the second side, reverse the polarity. If the switch is on one side, the motor will spin in one direction. Switch the switch and the motor will switch directions!

I am wondering what that was even meant for. Is that supposed to be a motor tester of some sort? It seems quite dangerous!

Not exactly an H bridge since the legs are not independent. However, please add a fuse or breaker to this in the future. Not the 120 amp either, it must be safely sized for the smallest wire.
While it is unlikely to cause any harm at 12 volts, this is a good demo to show what was considered “safe” 100 or more years ago. I am reminded that when the IBEW was founded in 1891, the average life expectancy of a line worker was 36. It was common practice at that time to string power cables on poles that contained live wire using horse drawn wagons with ladders mounted on them.
To use this for household line voltage please remember that 120 volts is the RMS value not the peak, which is over 300 volts.