What would be the circuit equivalent be of a uniselector, aka rotary relay, aka stepping relay?

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Uniselectors were commonly used for phone switch boards (and thus why we used to hear individual pulses). The uniselector:

  • has high-voltage galvanic isolation between inputs (easily hundreds if not thousands of volts)
  • carries bi-directional, high current at low resistance
  • incrementally steps between inputs, guaranteeing input pairs stay exclusive
  • has a floating ground (it really has no ground at all)

Potential equivalents

  • An array of bistable relays, controlled by a decade counter and an overlap protection circuit, is the closest I can come up with. However, this feels very unsatisfactory as its needlessly complex in part count.

  • Transistors and FETs do not provide isolation, so are not a replacement.

  • Analog switches are not an equivalent as they have relatively high internal impedance (>1ohm), and thus cannot carry much more than signal current.

  • \$\begingroup\$ An analogue cross-point switch might be the term you are looking for. \$\endgroup\$
    – Andy aka
    Commented Apr 2, 2021 at 15:13
  • \$\begingroup\$ The cross-point switches I saw are all variants of analog switches, which suffer from (relatively) high internal resistance. Do you know of one which could carry more than a few hundred mA? \$\endgroup\$ Commented Apr 2, 2021 at 15:16
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    \$\begingroup\$ Telephony audio routing (whatever the method) requires no more than 5 mA peak for the audio signals. Your question made the comparison with the old Strowger relays and, subsequently, cross point switches put Strowger relays on the scrap-heap for those applications. So, before you go any further, add the full technical details of your requirements to your question. \$\endgroup\$
    – Andy aka
    Commented Apr 2, 2021 at 15:22
  • \$\begingroup\$ I used the Strowger relays as the easiest form to find clear pictures. However, those are just a single example of uniselectors. In general, they are capable of carrying as much current as standard COTS mechanical relays, which can go into the tens, or even hundreds, of amps. i.ebayimg.com/images/g/6lkAAOSw8gVX67k4/s-l1600.jpg is an example of one used in MiG-29 aircraft. \$\endgroup\$ Commented Apr 2, 2021 at 15:33
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    \$\begingroup\$ If you want a replacement, you must provide specifications of what you want to stop people guessing. Add that to your question else the answer becomes, replace X with another version of X made by somebody else (and note that shopping questions are off-topic). \$\endgroup\$
    – Andy aka
    Commented Apr 2, 2021 at 15:38

2 Answers 2


There is no equivalent. If you allow electromechanical, then the equivalent of a uniselector is a uniselector. There simply is no other component with sequential operation and mechanical contacts. Perhaps there are units with extra stuff tacked on, and sold under different names (e.g., perhaps someone makes a manual selector switch that can also be remotely advanced), but in as much as the basic functionality is there, I mean, it's just defining a tautology.

It's a complicated mechanism; computers have been built using them, plus relatively fewer relays in addition. That is to say, uniselectors save a fair number of relays compared to an all-relay design (where "relay" means ordinary say SPST to 4PDT types, latching or regular), and so we should expect to need a fair number of relays to replace one in general. They're logically powerful devices.

If you disallow electromechanical, then there is no exact substitute, fairly obviously, but even just in signal terms. Mechanical contacts are unmatched in power handling, bandwidth and on/off ratio. Transistors work in limited circumstances: there are SSRs of various types that are capable of significant power at low frequencies (usually mains frequency or including DC), but are limited in Rds(on) (or similar metrics); or at high frequencies, but are limited in signal level (e.g. RF and logic switches in the say 0dBm to 5V range). Even these are limited to the low GHz, while a good RF relay is limited only by its geometry / physical size. Likewise, at DC, an on/off ratio of some billions is typical for a transistor, but thousands to millions of times higher for mechanical switches.

Conversely, there are generally-undesirable traits that mechanical contact have, that transistors don't: contact bounce; slow operation; contact resistance; breakdown (arcing); and the need for wiping (mechanical motion, often while carrying load current), or manual cleaning.

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    \$\begingroup\$ "Mechanical contacts are unmatched in power handling, bandwidth and on/off ratio." That's a really great comment. \$\endgroup\$ Commented Jul 9, 2022 at 15:00

Opto-MOS type photo-voltaic opto-isolators (some are rated for 2 A), driven by a string of CD4017-type Johnson counters.

  • \$\begingroup\$ Thanks for the idea. Wouldn't the Opto-MOS internals allow current to flow backwards even when the isolator is de-energized? \$\endgroup\$ Commented Apr 2, 2021 at 15:43
  • \$\begingroup\$ No.. there are AC rates ones which have back-to-back Mosfets. \$\endgroup\$
    – tobalt
    Commented Apr 2, 2021 at 16:54
  • \$\begingroup\$ Thanks, looking around I found the TLP241B, mouser.com/datasheet/2/408/…. Mouser places it in the opto-isolator category, but digikey lists it under SSR. Good to know! \$\endgroup\$ Commented Apr 2, 2021 at 17:52

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