MC2100 Dashboard Replacement Control Circuit Update 2
In response to Terry’s comment on my previous MC-2100 post, I got the breadboard circuit hooked back up for some testing. In doing so, I found a couple things missing in my previous schematic. I also may be backing off calling this version of the circuit done. It looks like there may still be a bit to learn :).
For starters, I had previously switched the output of the circuit to a PNP transistor, rather than being driven directly from the comparator. I’d run the circuit like this for a while on the lathe, but forgot to add it to the schematic. I also used a different combination of resistors to get to 14k in the 555 circuit. These changes are included in the schematic above.
The second thing I’ve changed is the addition of a capacitor from the voltage divider output to ground. I didn’t have this in place until tonight. I’ve got the MC-2100 sitting on my desk in my office, with a motor clamped to the table top and hooked up. The circuit that had worked fine in the garage when hooked to the lathe would only pulse the motor at a low speed in my office. The MC-2100 indicator LED would blink a few times, the motor would spin up, and then the LED would go solid again, indicating a loss of signal.
I started poking around with my oscilloscope, and realized there was a lot of high frequency noise on the output when the motor was running. I probed the power supply from the MC-2100, and it checked out, nice and flat. The regulated 5v in my circuit looked good as well. However, the output of the voltage divider potentiometer was pretty noisy. Adding the capacitor between ground and the pot output cleaned things up nicely. Probing the output resulted in a much cleaner square wave.
There’s still a little noise in the circuit, but I haven’t been able to pinpoint it yet. I assume shielding it would help, but noise and interference is a little outside my experience range at this point.
For a point of reference, I plugged the arduino with the PWM code flashed into the MC-2100, and the output looked better than the simple circuit, but still had a small amount of noise at the 60 usec period that the motor is driven at.
In summary, both circuits seem to be picking up a little of the PWM frequency that drives the motor (60 usec). Prior to adding the buffering capacitor to the simple circuit, the motor noise was enough to break up the 50 ms period signal being sent to the MC-2100. This caused the MC-2100 to cut the voltage to the motor, removing the interference, and allowing the 50 ms control signal to reach the MC-2100, starting the cycle over again.
If anyone else is out there trying this control method, I’d love to hear if it’s working for you. So far I’ve had a couple rounds of thinking I’ve got everything sorted out, then realizing there’s more to learn.