coil tuning

"... it is CMOS, so not insensitive to ES..."-FredM

Looking at the TI TLC555 datasheet a little closer, I see "ESD Protection Exceeds 2000 V Per MIL-STD-883C, Method 3015.2".  I'm no ESD expert and so can't say if this is better or worse than 4000B CMOS, but it sounds good.

Fred, have you ever tried the littelfuse SP721?  Specs:

ESD Interface Capability for HBM Standards:
- MIL STD 3015.7 ................................................. 15kV
- IEC 61000-4-2, Direct Discharge,
- Single Input .......................................... 4kV (Level 2)
- Two Inputs in Parallel ............................ 8kV (Level 4)
- IEC 61000-4-2, Air Discharge ...............15kV (Level 4)
High Peak Current Capability:
- IEC 61000-4-5 (8/20μs) .......................................±3A
- Single Pulse, 100μs Pulse Width ........................±2A
- Single Pulse, 4μs Pulse Width ............................±5A
Designed to Provide Over-Voltage Protection:
- Single-Ended Voltage Range to ........................ +30V
- Differential Voltage Range to ............................ ±15V
Fast Switching .............................................2ns Rise Time
Low Input Liakage ............................1nA at 25ºC Typical
Low Input Capacitance .....................................3pF Typical

You can basically stick that sucker on anything that might overvoltage, including differing logic level interfaces (probably with a series resistor) and stepped down (the AFE uses a capacitive divider) Theremin antenna waveforms.

"Fred, have you ever tried the littelfuse SP721?" - Dewster

No, I havent - But seeing the spec, I will look at buying some when I get back to the lab.. Only, I have about 1k neons which would end up without a use.. And neons have the advantage that I could advertise my theremins as having a tube in them!

LOL ;-)

Regarding ESD, 4000 series can tolerate 400V from the standard HBM (Human Body Model - 100pF via 1.5k resistor) whereas the TS555C can stand 1500V (not 15kV) from the HBM - so it is nearly 4x more robust..

But, I do not think even 1.5kV is anywhere nearly safe enough, I think it is way too lax - A person walking over a carpet can easily develop a potential of 20kV, and this can easily kill both the 4000 series and the more robust TS555C (and, for that matter, it can damage a BJT even though it may not kill it - it can reduce its operational life and the device can fail unexpectedly at some future time). Limiting the ESD peak to some managable voltage (90V for example) and then limiting the current which can be provided by this voltage to a level the device can stand is, IMO, the safest option.

I am a probably more pedantic than most, having worked in two critical areas, medical life support equipment and monitoring equipment for explosive atmospheres (oil rigs, refineries etc) where any failure can cost lives and lots of money! - Theremins are a different animal altogether, but old habits die hard!

Fred.

(as an aside, I built a neon tube theremin once just for the hell of it - two simple neon relaxation oscillators.. The real fun was when I added an "equalization" coil and got "lightening bolts" jumping from the antenna to the nearest ground! - As a theremin, it was hopeless! ;-)

A lot more wind about ESD for obsessive engineers: ;-)

"You can basically stick that sucker on anything that might overvoltage, including differing logic level interfaces (probably with a series resistor) and stepped down (the AFE uses a capacitive divider) Theremin antenna waveforms."

This, alas, is the most overlooked aspect of ESD protection - We only tend to think about protecting semiconductors! ... But think about it - What is the voltage rating (and, for that matter, current rating) of the capacitors in your divider? .. Under ESD conditions, any "down-line" protection device can even put more stress on passives which are "up-line". Damage to ceramic capacitors from ESD can be really nasty - you wont see it, it wont show up without doing some quite fancy testing - ESR, temperature etc, because the dielectric properties can be permanently changed by a large brief overvoltage event. Resistors are also subject to damage.

Theremins which have an equalizing inductor are quite well protected by this inductor - one is, after all, putting a charged (to say 20kV) 100pF onto the antenna via 1k5 ohms, this resistance with the inductance takes all the speed off the edge of the pulse, and by the time this pulse has transversed the inductor and got to the oscillator, it can be managed with a transient supressor of some kind, or even be safe without protection - Even though the voltage will greatly exceed the insulation resistance of the coil wire, there is unlikely to be enough current to cause major damage to the coil if it arcs to a lower potential point .. For theremins, the above is 'safe' - Quite different if one had the theremin in an explosive atmosphere though...

Fred said: "Christopher, unfortunately, No!, I think it is about the words we use, and the fact that we understand different things by them. I am starting to think though that perhaps you are a stubborn genius who enjoys teasing people."

Genius no, there is such wisdom in your words that I guarantee after 5 minutes of personal communication between us you would think it might be so.

There was a time you terrified me, trashing something about every theremin ever designed. Today I recognize you as gruff but good-hearted. (-'

Christopher

"There was a time you terrified me, trashing something about every theremin ever designed. Today I recognize you as gruff but good-hearted. (-' "

Awww - Thanks Chris, one of the nicest things anyone has said to me in a long time! - And sorry I terrified you! - Truth is, I also terrified myself! - For every judgment I made on others, I felt duty-bound to be twice as hard on myself..

Those silly days are over - from now on im going to be twice as hard on everyone else as I am on myself! LOL! ;-)

Fred. 

"A lot more wind about ESD for obsessive engineers: ;-)" -FredM

For one looking to build a bunch of something and not be buried in returns and bugs and whatnot, there's almost no such thing as too careful.  Thanks much for your ESD insights!  When I worked in telecom they had discharge tubes as the front line defense (against lightning and "power cross" - power lines coupling to telephone lines) followed by lower and lower voltage breakdown devices the closer it got to silicon.  The main thing they were trying to avoid was a fire at the central office or electrocuting anyone, but anything above and beyond that was gravy.  When the layoffs hit they lost decades of hard won talent and knowledge.  Modern industry is kind of nuts.

"For one looking to build a bunch of something and not be buried in returns and bugs and whatnot, there's almost no such thing as too careful." - Dewster

Well, there is a trade-off.. It is actually impossible to make anything totally reliable, all one can do is strive to improve reliability and immunity... There comes a point where the cost of implementing improvements and protection against extremely slight risk is not commercially viable or justified.

Getting the risk / cost balance right is an art - one I am not particularly good at. I tend to add more protection than is justified.. Fine if lives depend on it, not so clever if one adds a lot of cost to prevent a one-in-a-million chance of failure on something like a theremin!

I was quite fortunate in the first year of my theremin R+D, as I had access to a testing lab (ESD, EMC etc) and was able to do evaluation of some of the risks - Alas, I was something of a novice on theremins at the time - there are other tests I would do now if I had "free" access to the lab.

I did get enough data though to give me a fair idea about requirements for both ESD and EMC with theremins, enough to give me confidence that I am competent to self-certify my theremins (if I ever produce anything for manufacture again) to CE and possibly FCC compliance following a brief inspection by a lab (which shouldnt cost more than a couple of hundred £).. If I hadnt had the expierience at the lab, testing would have cost several thousand £.

Fred.

"When the layoffs hit they lost decades of hard won talent and knowledge.  Modern industry is kind of nuts."

More than "kind of" - Old geysers (anyone over 50, LOL) are not wanted - new grads are employed instead - Grads are fine if they have some old geyser to show them how to use a screwdriver and ACTUALLY teach them REAL electronics, by the time they get into their 30's some of them, under this guidance, actually become useful engineers!

But without us, it takes them a long time - and they end up doing things like using high speed expensive op-amps to do a job that a fast BJT costing 1/100th of the price, can do better!

(in fairness, there are some bright young Grads - I am talking about the "bulk" of engineering grads above - I must have interviewed  hundreds in my career (at one point I was seeing so many idiot grads, that i wrote a simple exam which taken by all interviewees, and they only got to see me if they passed), and remember 3 who were good - but I remember many technicians on the shop floor who I got promoted because they were really good and interested - hobbyists who had not gone to further education for financial reasons or whatever, and who were now ready to pick up books and get themselves a degree while working)