Automatic Vacuum Controller (Part One)

I have been working for some time on a way to automatically open the vacuum port of a machine when it is powered on and automatically close the port when it is powered down.

This is the first of two parts which includes a servo controller, servo, and the mechanics to open and close the port. Part two will include the electrical triggering interface between the machine and the servo controller.

Step 1: Initial Design

I chose a Tower Pro MG996R Digital Servo for about $10.00 on line and drew a rough plan for the circuit utilizing the servo tester idea I found on the internet.

I started with a NTE3040 optoisolator with NPN transistor output, Q1. When 6 volts is applied to the input of the optoisolator, the transistor conducts applying a voltage to the input of the cmos 40106 inverting hex schmitt trigger Q2, Q3 and Q4, inverting three times and presenting an active low to the trigger input of Q7, 7555 cmos timer, starting the timing process. I determined that the time it took for the servo to rotate 180 degrees was 2.42 seconds. I configured the 7555 timer Q7 as a monostable multivibrator to turn on for that length of time. Q7 turns Q8 on and off. The timer Q8 is configured as an astable multivibrator at 84.368 HZ, the frequency that energizes the servo to rotate 180 degrees left or counter clockwise. (looking from the top)

I am using a 2N3904 NPN transistor to drive the servo. You can use any similar one such as a 2N2222, I just happen to have this one in stock. I started out powering the circuit with a 9 volt battery and 5 volt regulator but later moved to 6 volts and a power supply as the circuit grew.

Conversly, when power is taken away from the optoisolator, the input to Q5 goes low, is inverted two times, and triggers Q9, which in turn energizes Q10 which is configured to oscillate at 272 HZ which reverses the servo 180 degrees.

I chose the cmos 40108 for the inverters and clean transitions it provides.

This probably can be done with a microprocessor. I am not a programmer and I wanted to use as much from the stock parts that I had.

I added some photos of the process from prototype board to populated, tested and powered circuit board. This is still a prototype so it is big and clunky and still going through changes.

Step 2: The Mechanics Step 1

I started with a piece of plywood 16"x 9 1/2"x 5/16"

Cut a 2" hole in the bottom matching the blast gate and mounted it with a couple of screws.

I used a small aluminum piece 4x3/4x3/16 and bent 1 inch up, drilled a hole in each end and mounted the bent end to the blast gate handle with a small bolt and nut.

Step 3: The Lever Step 2

The lever consists of two 1" x 3/8" acrylic plastic from Tap Plastics

I cut a piece of 9"x 1"x plastic and drilled 2- 5/8" holes to accommodate 2 bearings and one hole for a 10-32 bolt for the bearing on the second piece at the elbow. 3 1/2" between centers. I drilled and tapped 4-40 threads into the sides of the plastic and used 3/32" set screws to hold the bearings in place. The middle bearing comprises the fulcrum of the lever. The second plastic arm is 6 1/4" long. 5 inches between centers. The last piece of aluminum is 3 x 3/4 x 3/16" with 1 1/2" between centers which I mounted on the star lever that came with the servo. I don't have any formulas for the lever construction, it was trial and many errors. It has been operating smoothly for a month without any problems. Next I will be designing the interface with the machines. For now I am using the switch to open and close it.

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