Automatic speed control circuit again

[DrZarkov]

At last I found some time to continue, and I struggled over a new problem:

this is the circuit from the NBTV handbook:


And this is the PCB:


Problem: You see that 680 Ohm resistor connected to the photo-LED on both, the diagram and the PCB. On the PCB there is a second 680 Ohm resistor which is not in the diagram, between the photo LED and the 4046, connected on the right to a 10 n cap and to the left??? for me it's looking like connected to nothing? What does it mean? I'm nearly ready with soldering, but I will wait until I know what it means before I'll connect the circuit to power...

[Steve Anderson]

Problem: You see that 680 Ohm resistor connected to the photo-LED on both, the diagram and the PCB. On the PCB there is a second 680 Ohm resistor which is not in the diagram.

I think that the second 680Ω resistor is to be used with the 'floating' red LED shown, but the wiring is missing in the layout. It's just a 'power on' indicator. An alternative is to put the red LED in series with the IR emitter and reduce the 680Ω resistor to 560 or 470Ω. This will save about 15mA of supply current. Or simply don't bother with it!

There is also a 10k resistor between pins 3 and 5 of the 4046 which is also missing from the circuit. This would largely affect the function of the 47k trimmer and I would suggest omiting it, at least at first.

I've mentioned this before...make sure that the chip is a 'true' 4000 series device, labled HEF4046, CD14046 or MC14046....make sure there is no HC or HCT in the model number. The HC(T) devices will go 'pop' at 12V.

I have also never understood the function of the 10n cap connected to pin 2, it's on the output of phase comparator I which isn't used in this application. Can anyone enlighten me?

Steve A.

As an aside, there is a related device to the 4046 called the 7046, but it's only available in HC or HCT versions, so a redesign would be needed for this application. Its advantage is that it has a 'lock detect' output, this could drive a LED telling you when it has aquired lock. The pinning is not exactly the same so caution is needed.

Gone is the 7V Zener in the 4000 series devices and a third phase detector is added in the HC(T)4046...a legacy of the 4000 series chronic power supply sensitivity of the VCO...which isn't used here, so no problem.

[DrZarkov]

Everything is ready, except the HEF 4046. I've controlled my chips here, "of course" I've got the wrong ones in the shop! That means another day of waiting...

[Steve Anderson]

That means another day of waiting...

I did mention elsewhere that the 'true' 4000 series chips (HEF and the like) are being phased out as the demand has dropped right off. You should be able to find them for some years yet, but it's best not to start a new design with them.

The HC(T) series have largely superceded them being far faster, having a much greater output drive capability and still use virtually no current unless close to their maximum speeds. Even so, they are now considered a 'mature' product, also due for retirement soon.

The downside is the HC(T) series can only be used on supplies from 2-6V (typically 5V), whereas the 4000 series can be used from 3 to 15V. So in due course most of the NBTV 'standards' will need redesigning.

Steve A.

[DrZarkov]

I've ordered three (2 for spare or other projects) of them in the club-shop, the only source I could find. So it will take some more time until they are here in Germany...