Why the door ring can't work?

[tony_lth]

Hi, ALL,
I tested the TX board with a 100M/1GSa/s oscilloscope, and noted the voltages on the SCH.
It seems that the crystal freq isn't right. Pls comment, many thanks.
If it's the crystal issue, I will contact the vendor to get a right crystal, maybe they gave me the wrong crystal, who knows.

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Best,
Tony Liu

 

[FvM]

Q2 collector should show a 32 kHz signal, I guess a square wave. If it doesn't, it's not necessarily a wrong or defective crystal. It might be a marginal circuit dimensioning problem.

The transmitter output signal can't be seen with a 100 MHz oscilloscope.

 

[d123]

Hi,

Pardon my ignorance. Why would that circuit have no variable capacitor for the 32k crystal? - Which component there ensures it can be made to oscillate? Is it supposed to be one of the connected resistors (R2, R3, R4)?

 

[betwixt]

It is a simple multivibrator with the crystal being the feedback capacitor. I would expect to see something like 6V of signal at 32.768KHz at the collector of Q2. The DC voltages seem reasonable.

I would guess either the crystal is faulty or the gain of the transistors is too low. You can increase the amount of feedback by reducing the value of R3 but it's difficult to predict a minimum value you can go down to without knowing the crystal drive level. It should certainly be safe to try dropping it to 100K to see if it starts it oscillating. The exact crystal type is unimportant and the frequency doesn't even have to be precise, you can use almost any 'wristwatch' crystal in that position. I assume the same type is used in the receiver so if one is faulty, perhaps the other is too.

Brian.

 

[tony_lth]

Hi, ALL,
I tested the RX board, and found the RX crystal always not worked, but seldom the RX crystal is 32KHz.
I don't know how to trig the crystal, and found when power on, the RX crystal is 32K rarely, mostly the RX crystal not works.
I assume that when power ON, the RX crystal should always work.
Attachment is the note for the RX SCH.
Pls comment.
Best,
Tony Liu

 

[FvM]

How do you determine "RX crystal doesn't work"? The RX crystal works as filter rather than oscillator. There won't be a 32 kHz signal without active transmitter.

 

[Audioguru]

I made a proper schematic from the CD4069 "box":

 

[betwixt]

Tony, the crystal in the transmitter is to produce the 32KHz oscillation. The crystal in the receiver is to ensure only 32KHz is recognized. That's how the receiver knows not to chime when other signals are picked up by the receiver. You should NOT see any signal after the crystal in the receiver unless the transmitter is sending it out.

Brian.

 

[tony_lth]

Hi, ALL,
I see, many thanks.
So the main problem is that the TX crystal not works.
I will try to fix that.
And I will let you know the process any further.
Best,
Tony Liu

 

[tony_lth]

Hi， ALL and Brian,
I followed Brian comment, and changed R3 of TX to 100K, then at TX Q3 C can see stable 32K signal.
Now the door ring can work.
Many thanks, everyone, especially to Brian.
Best,
Tony Liu