I bought an old fashioned Singer sewing machine at a garage sale. I want to make a throttle for the AC motor. I want to use a potentiomete?

Here is what the microcontroller has to do: - Read the ADC to determine the pot position - Wait for AC zero crossing - Wait for 0 to 8 milliseconds based on ADC reading - Turn on a SCR with a short pulse (it will turn off at the next zero crossing) - Repeat forever This method of power control is often called Phase Control.

Most sewing machines are set up with a foot operated throttle, replacements are on ebay. If you want to make a sophisticated version then adding a tachometer to the motor should do that but does it actually enhance the product? Users tend to look at the work, not a speed display.

I want to use a potentiometer on a foot pedal. I think the pot is wither 5 or 10k, cant remember now but I need help with the "throttling" of the AC (120V) coming in..." Not sure what pot you're referring to but definitely don't try to limit 120VAC with just a simple pot. Light dimmer switches don't even do this, they pulse width modulate.

Think of it this way; if you've ever seen a high speed video of a light bulb lighting up, it takes a few milliseconds to get to full brightness. Let's say it takes 30 ms. If you could flip a light switch on and off every 15 ms the light bulb would only ever get to half brightness.

A couple reasons for this: 1) Efficiency; if you just used a resistive load (and it would have to be a big one) such as a pot then a 100 watt bulb would always use 100w regardless of where the pot was set. The power that isn't utilized by the bulb is wasted as heat at the pot. 2) Heat at the pot; Heat melts stuff.

I'm not familiar with what method a sewing machine uses for motor modulation but if there was a 5 or 10K pot in there before, it was probably just being used as some kind of slave/relay setup to control a higher power circuit. Anyway, the moral of the story is; don't stick a pot in a light socket ;).

You could modify a light dimmer switch. Just put the guts in your foot pedal, move the original pot from the switch, to replace the one in the foot pedal. Another option is to have your uC do double duty; have it display the stats and control the motor.

Depending on how eleaborate you want to get, we can fill in more details or give other suggestions.

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