Multiple cyrystals?

This is just an idea but do you think that it may be possible to modify the controller so that you can have 2 crystals and an external switch to switch between them? There would obviously be 2 radio receivers with differnt crystals on the robot itself.

I only ask because it would be nice to have 12 channels or 6 channels for say adjustments and then another 6 primary channels for the control of the robot.

It should work, though you’ll want to be careful to keep the wires as short as possible and use a very low-noise/low-resistance switch. The crystal oscillator circuit is carefully designed, and stray capacitance and added resistance may throw it off enough to become unreliable.

I remember seeing a project in an old electronics magazine that used an edge-card connector to hold several crystals and a rotary switch to select between them. I don’t recall what frequency range the rig was intended for, so I don’t know if it is suitable for the 75MHz band that Vex uses.


  • Dean

I recently Designed a circuit that did something very similar to what you are doing, only with headphone cords. I’ll draw out a modified version and post a picture. The circuit would let you send a signal through one crystal, the other, or both.

I’ve drawn and posted that circuit. it uses 2 DPDT switches to coose between one crystal, the other, or both. if you don’t need to do both, you can redo the circuit with just one DPDT switch.
Diagram is here
Photo is here (replace the push button on my breadboard with another switch)
when you finish it, it should be about this size

You definitely don’t want to connect both crystals at the same time. It won’t allow you to transmit on both freq’s at the same time. Most likely, it just won’t transmit at all. But if it does, it’ll probably be some third frequency that isn’t useful.

Otherwise, the circuit looks about right. I’d use one DPDT switch, but use both circuits (sides) so that whichever crystal is in use has both contacts routed to the transmitter, and the other crystal has both contacts disconnected.


  • Dean

Hmm… never thought of that. you can probably figure out how to modify it with just one switch, but just in case…

I tried this on my remote for my lifesize R2D2 and yes it does work.
I placed 2 crystals inside a box I bought from radioshack and the crystals in there factory housing fit inside the box nicely.
I used a single 2 way switch to alternate between the 2 crystals.
The problem I had was: My bad soldering to the switch and the wires were too long and got some interference.

If you could make a better design and much better soldering than mine.
I think it would work GREAT!!
I needed the 2 freqs. because I have 2 Vex in R2 and needed to switch between them.
I’ve resently moved on to a different setup for R2.

I’m glad it works, although I wish I had found a box like that for mine, I just used the packaging that the switch came in, which is extremely flimsy. If anyone knows where I could find a mounting box that’s about 1x1 inches, that’d be great. (I can sell the headphone version for like 15-20 bucks:D )

All Electronics has a good selection of random small enclosures. Here is nice one for 90 cents that is 1.5" x 1" x 2/3".

Also, Radio Shack sells very small enclosed AAA and AA battery holders that I’ve had good luck using as tiny project boxes. All you have to do is pull out the spring clips with a pair of pliers. Since they are for batteries, they have a nice slide-off cover for easy internal access. I’ve used these to make cables that needed active components in-line, but were a bit much to cover in heat shrink.

Hope this helps,

  • Dean

thanks, I think this will work.