Changing the 8 mm projector speed with Arduino PWM

Other Hardware Motors, Mechanics, Power and CNC
1

Hi,

I'm making a Arduino controlled 8 mm film projector for conversion do digital. At this point I had a working prototype that was able to hold set rpm with the precision of approx. 0,07 of a revolution using a PID algorithm. The 25 volt DC motor was powered by L298N module, which had power (25 V) from the projector.

However, things got a bit sketchy when I was trying to implement rotation direction sensing. I realised, that all can be done a lot easier, but my electronic knowledge is not good enough at this point.

The projector has a variable speed system, which sets the speed of the projector with a potentiometer and couple of transistors. Now I though I would be possible to loose the L298N module completely and just connect the arduino PWM output pin directly to the projector. This is the projector schematic:

The R2 and R3 are trimmers for minimal and maximal projection speed, the R5 is the potentiometer for setting the speed.

What should I measure, where and how could I connect the arduino directly to the projector? I'm guessing between the transistors V1 and V2 and the resistor R1 but I'm not confident enough to try it.

Thanks in advance
With regards
Daniel Krnac

2

Well the motor appears to be field-wound rather than permanent magnet, there is some speed
sensing device (DC tachogen?) plugs into the 7-pin connector. The direction seems to be mechanically
switched in an H-bridge configuration (both for motor and tachogen I think?). The motor has rotor
and stator mislabelled in the diagram BTW.

Some photos might be useful here.

And do tell us exactly which projector model, don't expect us to guess that.

3

Yes, it is field wound. This is the original schematic plus my commentary - the motor was just labeled by an "M" and a the coil above had no commentary whatsoever so I measured motor outputs and figured out what is what.

The coil from the schematic should be armature based on resistance and continuity measurements. I may have switched stator and rotor but it makes no difference whatsoever function wise.

The difference is, however, that the coil has a +35 voltage over it all of the time and it doesn't change polarity.
The M part has a +10 to +25V (depending on what speed is set by the R5) and it does change the polarity - like you said, with the mechanical rotational switch.

The switch has 3 positions - forward, off and backward. It's mega complicated (10 cm long cylinder that has 14 in and outs) but the important thing is, that it switches the polarity of the motor (M) poles. That' how it changes direction of the motor.

When I looked at the switch, the parts that are relevant look like this:

On the input side:
contact 1 is + V (+10V to +25V)
contact 5 is 0 V

On the output side:
2 and 6 are shorted together and act as first output pole for motor
7 and 8 are shorted together and act as second output pole for motor

Example:
1 shorted to 2+6 and 5 shorted to 7+8 is forward direction
1 shorted to 7+8 and 5 shorted to 2+6 is backward direction

The projector itself had no speed sensing device, that was my added part. Like I said, I had a working prototype where an arduino was sensing speed (with an infrared groove sensor and the projector shutter disc) and setting the PWM modulation values accordingly (with a PID algorithm).

So at this point I basically need to know, where into the schematic should I connect the arduino output pin and thru what resistor to directly control the projector speed and if it's even possible (which it should be).

The projector is Meolux Meopta Meos Duo. It was made in Czechoslovakia in 1979.

2000×1500 255 KB

2000×1500 304 KB

Thanks for the help
With regards
Daniel Krnac

4

In general you cannot switch stator and field winding, as the field winding has orders of magnitude
more inductance than the armature and a lot less current.

So what plugs into the 7-pin connector at the top of the circuit diagram if not a tachogen?

5

I switched the labels in the scheme, that's all I meant.

What connects into the connector is a special device, that synchronies the projector with a special tape recorder (I guess you could call the whole thing a sort of a tachogenerator). Since my films are without sound tapes and I don't have the peticular device nor a tape recorder, it's not of much interest.

Can I maybe use it to connect the arduino? Any idea how to connect it to the projector?

Or maybe I can use the arduino to somehow simulate the 3k3 potentiometer?

Thanks again
D.K.

6

Oh, I see. Perhaps K in the diagram (right hand side) is a reed relay sending sync pulses to the other unit?

7

At first I thought it's a mechanical switch that measures the speed or something but it turned out to be a sort of mechanical fuse that breaks the ground, when something stalls the motor (or at least it looks like that). Quite weird, never seen anything like that - thin transparent tube with two metal cylinders barely touching.

So that won't help with the arduino connection, unfortunately. What now?

Thanks again
D.K.