Made this one for Nathan Latona of Tera Melos.  I never miss a show when they come to town and they never fail to impress. Check em out, especially if you're into Mathrock/prog/technical rock/actual talent.

Etching is always a rough process.  The GIF goes quick, but there is lot of work happening in between those pictures.  Every little thing has to be scraped just so with a dental pick.  Talk about tedious.  You can see that the image didn't transfer correctly the first time so some steps had to be repeated (Lost 3/5 Words in the first transfer).  In the end, it' turned out fine and I'm happy with the results. 

Resistors - Which to use?

Here’s the main types of resistors, that you’ll need to know about for anything really.  I’m mainly talking about SMD resistors, but this also applies to hole mount resistors. 

- Carbon resistors - Thick film resistors - Wire wound resistors - Metal film resistors - Metal foil resistors

Carbon resistors are rarely used and mainly found in dated equipment in hole mount packages. They are noisy, non linear and have poor temperature coefficient. Non linear? Yep!To put it simply, the relationship between voltage and current isn’t linear, as it should be! This means that when this kind of resistor is used in amplifiers, it does not only introduce noise, it also distorts. 

Thick film resistors are dirt cheap. They are better than carbon resistors, in terms of temp-co and linearity, but still noisy and not as linear as we’d like for precision linear applications such as measurement equipment or audio. They are great for anything but that, so use these for your house keeping circuitry, such as DC detector, overtemp shutdown, softstart timing, and all that stuff that isn’t directly related to audio quality.

Wire wound resistors are typically high power resistors, as the name suggests, made from a piece of wire. These are extremely linear and noise free. Use them in your output stage between emitters and for current measurement shunts.  - Or any other high power application, There’s not a lot of other choices over 1W. 

Metal film resistors. These have a few names, but to the best of my knowledge, there’s no real difference between them.   - Thin film SMD chip resistor  - Metal film SMD chip resistor  - MELF metal film resistor  - MELF thin film resistor  - Metal film or thin film hole mount resistor They’re all basically the same, a thin film of metal that has been cut to the right value. These resistors are as linear and noise free as wire wound resistors. 

Metal foil resistors are for extreme precision purposes and they are ridiculously expensive. Prices start somewhere around 5€ at 100ps quantity! It’s used for extremely critical measurement equipment. They have excellent ageing characteristics and wickedly low temp-co, along with of cause low noise and high linearity. 

A word on resistor noise

When I say “noise free” I of cause mean that there is only very little noise except from the thermal noise. See my article on resistor noise for details about this. 

I have ALWAYS wanted to be able to build my own robots. However I’ve much less interest learning to do complex programming. I’m happy with just being able to code in HTML and not crash a website.

Mark Arnott created the Smart Servo Controller to help control servo motors, relays, lights and motion algorithms (and more)

“Stumped on how to make your robotics project move? Now you can create complex motions and control logic without writing any code.”

Here’s a little more detail on all this.

The device has 8 inputs and 14 outputs that can be configured for different type input and output types. The operating voltage can be change from 3.3v to 5v for compatibility with other electronics.

The board runs from a 5 volt power supply. A selector switch sets the input and output operating voltage. There are disconnects that allow the input channel and output channel to operate from a different voltage supplied by the user. This way you can supply the servos with your own voltage source and not rely on the board power supply.

Current feedback - It’s not that difficult at all!

A current feedback has an input that consists of a unity gain buffer and two current mirrors in the configuration you see here: 

The buffer marked Buf1 has a quiescent current running in through +V terminal and out -V terminal. This current is mirrored so the same current runs from Q3 collector to Q4 collector. 

When +IN is higher than -IN, a current will flow out of the -IN terminal (this is the error current, hence the name current feedback). This current causes the current in terminal +V to be higher than the current in terminal -V. This in turn causes the upper current mirror to source more current than the lower one sinks. This difference in current is flowing to capacitor Cdom and so the voltage on Cdom rises and the output voltage follows and rises too. And so +IN > -IN = Vout rises, and vise versa. And so when you connect the output to the negative input you get a circuit that functions very much like a conventional opperational amplifier. 

This also explains why the feedback resistor value from out to -IN affects the bandwith of the amplifier. A lower resistor value will cause a higher feedback current to flow at the same error voltage and so the Cdom charges faster and voltage rises faster.