[sdiy] non-mechanical switching solutions

Rick Jansen rick.jansen at xs4all.nl
Tue Jun 28 09:19:34 CEST 2016

There are very very small reed switches, and if you wind a coil with enough threads the current is quite low. It was just a thought. 


> On 27 Jun 2016, at 22:46, jays at aracnet.com wrote:
> One thing to keep in mind when using relays is unless you are using latching relays you will need current to keep them in the proper state. Latching relays usually take a pulse, go to a state and stay there without the current until the next pulse puts them in the next state.
> Jay S.
>> On 2016-06-27 13:21, Rick Jansen wrote:
>> Miniature reed switches? Sort of mechanical, but hey.
>> rick
>>> On 27 Jun 2016, at 16:15, Pete Hartman <pete.hartman at gmail.com> wrote:
>>> I have a problem I'm trying to solve....
>>> ...and let's just assume the problem has to be solved, I can't just
>>> redesign around it.  In a perfect world ... but that's not where
>>> this sits.
>>> I'm working with a ladder filter, switching 3 different capacitor
>>> ranges (Moog 904A, yes?).  But I don't have room for a multipole
>>> rotary switch of the necessary size.  What electronic solutions
>>> might I use for this?   (and note that while step one is to solve
>>> four poles for the 904A, step 2 will be to solve for the 904C which
>>> has a *9 pole* switch, which hopefully can use the same solution)
>>> If I use an on-off-on switch, pull ups and ground connected to the
>>> common, I have three positions giving me 01, 11, and 10 which can be
>>> decoded to 3 out of 4 positions on a mux or a decoder.  A Mux acting
>>> as a switch to put in or out the various capacitor values....
>>> First thought: CD4052, but that doesn't work well in practice at
>>> all.
>>> Second thought: DG409, better characteristics, but that still
>>> doesn't work sufficiently well.
>>> Dig around a while for why.... aha, the on resistance for both is
>>> significant, especially for the capacitive stages of a ladder
>>> filter.  The DG409 has on the order of 100R - 120R.  Too much, and
>>> confirmed to be the issue by comparing physical connections with
>>> wire to physical connections with 100R resistors.
>>> Third thought: MOSFETs have low on resistance....  And an initial
>>> test of VN0104's (on resistance on the order of 3R - 5R, much
>>> better) shows somewhat more promise.  A little concerned about on
>>> capacitance and also whether I can get an appropriate Vgs at the top
>>> stage of the ladder (where the source voltage will be on the order
>>> of 10V), but so far so good; may need a MOSFET with a lower range
>>> for Vgs-on, but the VN0104 is pretty darn good with a max of
>>> 2.4V.... that's just at the edge of the spec though, I think,
>>> something better would be nice.  Don't want to be in the position of
>>> having to "select" working transistors for the top stage.
>>> So the questions for the assembled brains bigger than mine:
>>> 1) can anyone suggest a better (yet still reasonably common) mosfet
>>> with low Vgs and low rds-on?
>>> 2) some other (electronic, not mechanical) solution that would work
>>> in this specific instance?  Note that while the above description
>>> assumes an SPDT switch, I can go up to 2 poles, but not beyond.  An
>>> on-on-on would also be an option.  Toggle switch is a requirement
>>> though.
>>> Just to repeat; hopefully this isn't necessary in this audience, but
>>> some other fora where I occasionally ask questions, I often get
>>> offered answers that ignore the constraints I'm trying to fit within
>>> (in this case, no mechanical solution, redesigning to use a physical
>>> rotary switch is not an option).  "Your constraints are stupid" is
>>> not a helpful suggestion, yet you might be surprised how often it
>>> comes up :).
>>> Thanks...
>>> Pete
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