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I'm making a circuit including a 555 timer controlled by an RC combination, where R is fixed and C is one of several capacitors (values around a few pF to a few hundred pF). I would like to connect the capacitors to the timer circuit using an analog-friendly multiplexer IC (like the 4051). This way, the 555 frequency can be set using any of the capacitors, depending on which is needed at the moment.

Would this actually work? Is this a correct use of a multiplexer chip? Does anyone have any recommendations on how to do this?

Edit: it needs to be controllable by a microcontroller, so a rotary switch won't work (at least rotary switches as I know them). Is there a better way--maybe a bunch of relays?

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    \$\begingroup\$ Isn't the simplest way just a rotary switch? \$\endgroup\$ – Ryan Griggs Feb 28 '17 at 3:07
  • \$\begingroup\$ yes with a little fiddling you can, It depends on the type of multiplexer though. \$\endgroup\$ – Trevor_G Feb 28 '17 at 3:37
  • \$\begingroup\$ Just connect all the capacitors to the '555, but only ground the other terminal through MOS switches. The '4051 multiplexer might work, but isn't intended for the (for instance) high discharge currents. \$\endgroup\$ – Whit3rd Feb 28 '17 at 11:15
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    \$\begingroup\$ Ok, the obvious question you probably didn't want to hear: Since you are using a microcontroller anyway, why not generate your frequencies direct from that? \$\endgroup\$ – Tut Mar 6 '17 at 19:08
  • \$\begingroup\$ The microcontroller's job is to measure the frequency of the 555 in order to measure capacitance (the capacitors are variable). \$\endgroup\$ – Vulcan Mar 7 '17 at 19:43
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You should read the datasheet of the analog multiplexer and look for capacitance values, especially input capacitance. Those unwanted and unavoidable capacitances will influence the frequency of the 555 timer, especially if the capacitor connected to the 555 is very small, about a few pF. The unwanted capacitances are not constant and will influence the stability of the generated frequency. For good stability, the smallest capacitor used should be much larger than the unwanted capacitances of the multiplexer.

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    \$\begingroup\$ I think you mean capacitance and not capacity. \$\endgroup\$ – Oskar Skog Feb 28 '17 at 9:03
  • \$\begingroup\$ You are right, thank you for the comment about capacitance. \$\endgroup\$ – Uwe Mar 6 '17 at 19:04
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Would this actually work?

yes.

but much easier to simply use a switch on the low-side to switch in / out a capacitor.

if two such switches + two capacitors give you four combinations of capacitance. ...

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  • \$\begingroup\$ A switch like a relay? It has to be microcontroller-controlled. \$\endgroup\$ – Vulcan Mar 1 '17 at 15:19
  • \$\begingroup\$ He means an MOS analog switch. \$\endgroup\$ – StainlessSteelRat Mar 6 '17 at 19:33

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