Designing a Rega Rim Drive

Hopefully today will see the frequency counter circuit doing something, and if successful it will be counting the frequency the motor is running at.

Now that's OK for a techie but not for an end user who wants to know the rotation speed of the platter. 50Hz relates to 33.33 rpm only when using a turntable designed for 50Hz (230V usually) with the belt set on the appropriate pulley.

So what I need the counter to do is indicate 33.33 when the motor is receiving a frequency of 50Hz. OK, so if I clock the counter every second it will read 50, and if I extend the clock by 1.5 so it counts every 1.5 seconds it will indicate 33.33... No!

To indicate 33.33 to show what's to the right of the decimal point the clock period would have to be 150 seconds and nobody would put up with such a slow count - that's crazy!

So, to do this we have to do what all electronics engineers do under the circumstances and that's cheat!

Now I will tell you where I'm getting my 50Hz from in the first place, and it's from a much higher frequency - 51,200Hz to be precise - and that's divided down 1024 times to give 50Hz using quite a clever chip which is known as a binary ripple counter. It has lots of divide by two outputs and being binary it doesn't take many stages to reach 1024 (2, 4, 8, 16, 32, 64, 128, 256, 512, 1024).

So what I have to do is use a much higher frequency into the frequency counter - and I have plenty available from the ripple counter from which to choose - and then I can set an appropriate much shorter clock period to get it to display 33.33 rpm, and then just apply a factor of 1/1.5 to get it to indicate 50 Hz.

Anyway, that's what I hope to achieve today one way or another. While I'm here I will mention another thought I had (which many others have thought about I'm sure) and that was to use a DPM chip and display. Basically a volt meter, and drive it with a frequency to voltage converter. The big problem here is scaling accuracy which depends on component accuracy, and I decided it was too much of a fiddle. It is much easier for an analogue guy to understand (like me), but as I've done the logic (dare I say digital) side of electronics (most of my early career) I'd best reawaken the grey matter!

To recap, what I want to display is 50.00Hz when the unit is outputting 50Hz to the motor, and I want a switch to put it into r.p.m. so as to indicate 33.33 rpm when it is outputting 50Hz to the motor.

For 60Hz operation, I want to display 60.00Hz when the unit is outputting 60Hz to the motor, and I want the switch to put it into r.p.m. so as to indicate 33.33 rpm when it is outputting 60Hz to the motor.

Hope you're following?

Now, 50.00 is simply 5000 with a decimal point after the second digit, and so are all the other numbers I want it to read out.

So when it's doing 50.00Hz I need a signal that's doing near to 5000 oscillations a second, and in fact if it's a little less than a second all the better because the display could flicker if the time interval is too long.

My master clock runs at 51,200Hz and is divided down 1028 times for 50Hz. So if I divide by 8 I get 6400 and if I set the time interval at 0.78125 seconds it should read out 5000 (6400 x 0.78125 = 5000). Great!

So looking for the divide by 8 output I realise I bought the wrong binary ripple counter.

But never mind, there is a replacement (a 4040) which will do both of the functions I need.

But for the time being to get this working I need a divide by 8 circuit, and I'll have to make this by using 3 of 4 available flip-flops from a 4013. In fact I could replace the binary ripple counter with 3 4013s if I were so minded, but a 4040 does it all in one chip.

So now I need to make a clock to do 0.78125 seconds with a switch to make it do 0.52078125 seconds for it to read out 3333 (with the decimal point hard wired after the second digit so it looks right).

And for 60Hz use the frequency is simply relative, so the 0.78125 seconds still applies, but for 3333 the time interval of 0.52078125 is divided by 1.2.

And that's what I meant by "cheating".

So here's my mock-up of the ac motor speed controller and digital readout. More on this later.