# wpmcnamara

Member Since: January 6, 2010

Country: United States

• You can cut the price differential to \$0.12 per part if you can figure out how to read a potentiometer without using an ADC. But that requires the same math that you have to know to use a 555. :)

• Take your IRF510. It’s good for up to 5.5A or so. Taking one of the precision gear motors that Sparkfun sells, we find that its stall current at 12V is 1A. So, no immediate problem there. The rDS(ON) is 0.54 ohms. Call it 0.5 ohms for easy math. Fully on, the IRF510 will dissipate about 1/2W at full motor current. By the time the FETs resistance equals that of the motor (12 ohms), the FET will be dissipating 3W due to the voltage drop. Well within it’s capability, but pretty wasteful. This, by the way, is why motor control is PWM, whether with FETs or BJTs. Handling large currents through any sort of variable resistance results in a lot of heat.

Now, for the professional reason its a bad idea. Assuming you burn one staff day using a 555 for your design and one hour for a microprocessor solution. Totally unrealistic, but it will help prove the point. Using Jameco, because that’s where i was looking at the IRF510 datasheet, the cheapest micro with ADC capability (gotta read that potentiometer) is \$0.89 qty 100. The cheapest standard 555 is \$0.17 qty100. Let’s assume the quantity discount stops there. Lets assume your engineering time costs \$120 an hour. It cost you \$120 to design in the microcontroller and \$900 to design in the 555. Wow, it’s a lot cheaper to use the micro right? What happens when you make 100,000 of the widgets you just designed. Each micro controller widget costs \$0.72 more to make. All of the sudden you spent \$72000 more in parts to save \$780 in engineering costs.

The above is a very quick lesson on why something that makes sense in the hobby realm or as a one off solution makes no sense, and may very well doom a product, that will be commercially produced. There is absolutely nothing wrong with using a microcontroller to solve a problem that could be done with a slightly more complex and time consuming, if cheaper job, when you are doing it for yourself. But claiming that the cheaper solution has outlived its usefulness because its old and a bit more complex to understand (depends on your point of view) won’t get very far when presented to people who do design work for commercial projects.

• I can’t quite make it out in the picture, but who makes that power strip in the last image? I could certainly put a few of those to good use.

• Perhaps a bit nit-picky as the data sheet calls x8+x2+x+1 a CRC-8, but the oh so helpful table you link specifies it as CRC-8-CCITT with a plain CRC-8 being x8+x7+x6+x4+x2+1.

• I first learned about charge pumps many moons ago. I built a high voltage charging circuit for a photoflash cap – something like 220uF@450V. I started out with an inverter turning 12VDC into ~100VAC and then running it through a four stage Cockcroft-Walton voltage multiplier. At the time, I didn’t access to a good, high current flyback transformer, nor the knowledge to build the drive circuit. The output of the photoflash cap was used to drive each stage of a coil gun. Took me to state level science fair competition.

In reference to the timing jitter from the RedBoard. The Atmel timer/counters can be configured to generate complementary PWM signals with dead time. See section 3.1.3 of App Note AVR447 for an explanation. Since you are using a fixed duty cycle, the concern for updating duty cycle values in parallel goes away. This does, of course, require programming the AVR at the chip level and not using the Arduino environment. Tuning the dead time should allow you to increase efficiency some too, though moving to FETs instead of BJTs may help more.

• I see great utility is some of the things that can be done with networked devices. I can monitor power usage in my house. I can control individual vents in my HVAC system, not only to make sure I’m not heating and cooling rooms needlessly, but that I am also getting enough volume through the system for it to be working efficiently. All that kind of stuff is great. What is not great is that I have to open the whole thing up to the outside world to use the products. Somewhere along the way, products stopped being an end unto themselves and started being a vehicle for mining data about users that could be further monetized. Having to sign up with some random company storing data in “the cloud” just so I can set my thermostat is a non starter.

The, we get to the command and control part. If you put something on the internet, it will be hacked. Just a matter of time. Someone will break in and make it do something unintended, just because they can. I’m not talking about malicious actors. There are over 7 billion people in the world. The combination of brilliant, bored, internet connected, and curious factors out a lot of folks, but even one hundredth of 1% is still a lot of people poking around just to see what they can do. Unfortunately, the side effect is not to make products more secure and tamper proof, or only connected to the outside world if absolutely necessary. The side effect is knee jerk laws and regulations that make doing the same things on stuff you own illegal.

In short, I see a number of positives from connected devices and a tsunami of negatives. Now, get off my lawn!

• I have to say that MicroCorruption is a really cool idea. I actually use the MSP430 for projects and it is neat to see the simulated environment they have put together. I’ve only gone through the tutorial and the first two challenges and… well, its pretty addictive.

• You guys might consider stocking the RFD21813. I just ordered three from Mouser because I found out about them after reading this post. I would have much rather ordered them from SparkFun. While lacking a few useful things, like a low power receiver mode, they look to be very useful modules for mess and many-to-one sensor nets where high speed data rates aren’t needed.

• This particular model only switches at the zero crossing of the AC input. This means that you would, at best, only have 120 PWM steps at a frequency of 1Hz down to three steps (0, 50, 100) at a PWM frequency of 60Hz. Each step would correspond to one half cycle of the AC input. For those with 50Hz power it would be 100 steps @ 1Hz and three steps @ 50Hz. It might work for incandescent lights, though I would expect you to perceive the flicker at half power with anything more than a few steps.

The zero crossing switching is also why you can’t switch DC with these. Technically, you might be able switch DC if the zero crossing doesn’t require the output voltage to swing negative to switch, but it will behave quite oddly. The output would latch the current state of the input when DC is applied to the output and would stay that way until DC is removed from the output. If the input was ON when DC is applied, then DC would flow so long as it is applied to the output, regardless of changes to the input. If the input was OFF when DC is applied, then DC wouldn’t flow regardless of input change. Depending on the design of the zero-crossing detection circuit, you might be able to switch PWM’d DC so long as your PWM signal off width was long enough for the zero crossing to be detected and so long as the zero crossing circuit doesn’t require the voltage to go negative to trigger.

• No need for razor blades. Just use APC electric props. They are pretty near razor sharp at the tips to begin with. Given what one will due to a finger, I doubt a balloon would stand a chance.

No public wish lists :(

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