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This is for NES clock circuit. I need to find a way to switch the crystal between PAL and NTSC. I have used a normal switch to switch those with no problem. Same goes obviously with a relay, but I'm trying to think how to switch it with some transistor or something else.

This is a working video switch. Very elegant. I tried same approach with crystals, but doesn't work:

video switch

So any idea what to use to switch crystals on this circuit? I can double the whole circuit and switch that, it works, but too many parts when I could swap only the crystal itself.

clock generator

I have tried already 4066 but it doesn't work. I was thinking about optoisolators but their frequency is not enough. Maybe transistor didn't work because it's inverting, but how to do non-inverting properly? I tried something like this(only one crystal to test):

non-working non-inverting abomination

I have already ordered some small relays (G6K-2F-Y) just in case I cannot come up with anything. They are surely to work.


Does this look like it could work? Based on what @GodJihyo said. Compare to the original post. Those wires going off-picture would be the enabling voltages.

enter image description here

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    \$\begingroup\$ When trying to use a transistor as a switch, consider what happens to the base current flowing through that transistor and how it affects the rest of the circuit. You can't just pretend that a transistor can switch things like a relay can. \$\endgroup\$
    – brhans
    Jan 27, 2022 at 17:47
  • \$\begingroup\$ Yea i know it's not just a switch. It's basically an amplifier that can be used as a switch. I can do only very basic transistor/fet circuits. I'm thinking that the crystal signal passing though would be the same, just a little amplified in the process. \$\endgroup\$
    – Quezacotl
    Jan 27, 2022 at 18:02
  • \$\begingroup\$ Could try a double-pole single-throw relay, but note three things: 1. Some relays have a "wetting current" minimum (such as 5mA) and will not work reliably with the tiny currents of a crystal. 2. The extra wiring (clock signal through the relay contacts) will introduce parasitic capacitance and inductance which might affect operation. And 3. This long length of clock trace (now going through a relay) might make an excellent antenna if it's length is close to the wavelength (MHz) being used. \$\endgroup\$
    – rdtsc
    Jan 27, 2022 at 18:22
  • \$\begingroup\$ Didn't think of that... My experiments with some heavy many-amp relays were working mostly but crashing occasionally. I though i was because i had like 15cm unshielded wires going to the NES board and they are rated for 230V switching. Of course that, but also what you said. \$\endgroup\$
    – Quezacotl
    Jan 27, 2022 at 18:40
  • \$\begingroup\$ Just use 2 ILS switches and a magnet ... \$\endgroup\$
    – Antonio51
    Jan 27, 2022 at 18:56

1 Answer 1

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One method of switching crystals involves using diodes. A diode is put in series with each crystal and a voltage is used to bias the diodes on or off. RF chokes are used to keep the bias voltage from loading down the signal from the crystals.
This was done in things like police scanners and CB radios quite a bit, so you might look for schematics of those for some ideas on how to implement it.

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  • \$\begingroup\$ Like this? radiomods.co.nz/xtalswitch.html Then to figure out how to implement it :/ \$\endgroup\$
    – Quezacotl
    Jan 27, 2022 at 18:37
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    \$\begingroup\$ @Quezacotl Yes, that's the basic idea. Of course it's going to be easier with some oscillator configurations than others. Optimally the oscillator would be a type where one side of the crystal is grounded, but it's possible with other types. In radio circuits diode switching is also used for selecting different filters and for transmit/receive switching. \$\endgroup\$
    – GodJihyo
    Jan 27, 2022 at 19:40
  • \$\begingroup\$ Yes, it's best to use a common-collector or common-base oscillator if you're going to use diode switching. The buffer stage will have to change, and you'll need to have individual trim capacitors for each crystal. \$\endgroup\$
    – TimWescott
    Jan 27, 2022 at 20:23

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