This was by far the coolest machine I came across when deciding what to build for my final project, even though it really is the most useless machine in the world. I remember seeing this concept when I was a kid and always wanting to build one of my own. It is a very basic idea, you flip a switch on and a motorized arm comes out of the box to turn itself off. This involves a micro server or geared motor, two switches, and a battery pack. The switches consist of a toggle switch and a micro switch. The micro switch will be wired to work opposite of how it usually works and the toggle switch starts in the reverse direction. When the machine has no power, the micro switch is being held in the off position by the servo arm and it isn't until you flip the toggle switch that it is released, causing the arm to rotate toward the toggle switch. When the servo arm turns itself "off", it is really causing the motor to reverse and head back to rest on the micro switch. That is when the machine is fully powered down. The build that I decided to go with is a more complex one, but you will be much prouder of the product once finished. I was not able to get mine to work how I wanted it to and I will explain the problem later, but I hope that you can take my work and improve upon it.

Supplies

Including digikey part numbers:

Servo 900-00005-ND

Battery Holder 4-AA Cells BH24AAW-ND

NE555P Chip 296-1411-5-ND

C1 0.1 uF Capacitor 478-1831-ND

R1 220K resistor CF1/4220KJRCT-ND

R2 27K resistor CF1/427KJRCT-ND

R3 15K resistor CF1/415KJRCT-ND

D1 1N4148 Diode 1N4148FS-ND

DPDT Toggle Switch 450-1533-ND

SPDT Micro-Switch EG4544-ND

And lastly, the box I decided to go with was a 3D printed box found here:

https://www.thingiverse.com/thing:646414/makes

Step 1: Circuit With 555 Timer

I went with the 555 timer for this Useless Machine because it is a useful precision timing device which can act as a simple timer to generate single pulses, which is what we need to run the servo. You will see below that I test the servo below with an arduino, producing a square wave with single pulses of a certain width. If you look at the 555 timer in the circuit diagram, you will notice that the Voltage pin is connected to the Reset pin because a voltage applied to the Reset pin will cause the output to go low. You can also see that the Trigger is connected directly to the Threshold pin. A negative pulse on this pin "sets" the internal flip-flop when the voltage drops below the threshold voltage causing the output to switch from a low to a high state. Other important details to keep in mind are the direction of your diode that connects from the Threshold pin to the Discharge pin of your 555 timer. Putting the diode in the wrong direction will cause your Useless Machine to malfunction.

Other key features to keep in mind:

15k Ohm resistor connects pin 1 and pin 4 on the toggle switch
27k Ohm resistor stretches diagonally from pin 3 to pin 4 on the toggle switch
220k Ohm resistor goes from Threshold to Discharge of the 555 timer
0.1u Farad capacitor goes from Ground to the Trigger of the 555 timer
Power (red wire) from the 4 battery pack goes straight to pin 1 of the micro switch
- Black wire goes to ground
Pin 2 of the micro switch connects pin 4 of the toggle switch, which connects to Voltage on the 555 timer (this cuts power when the micro switch is pressed and held)
Servo has brown, red, and orange connections
- Brown to ground, red to voltage, and orange to PWM signal (output on 555 timer)
Pin 2 of the toggle switch goes to Discharge on the 555 timer

Step 2: Test the Circuit on Breadboard

If you choose to use a 555 timer for this project like I did, it will be very wise to test everything out on a breadboard before you construct your Useless Machine. My photos above show exactly how it should be wired up, but I ran into some minor barriers that can be fixed with a little bit of component trial and error.

Step 3: Testing Your Servo

If you are having trouble getting your servo to move at all and you feel your circuit is perfect, you may want to set up your servo with an arduino to test if it is still good. I have included a picture of my setup, a picture of the square wave output you should get, and a video of the square wave changing as the servo rotates.

I have included a link below to the Proto Supplies website page for the SG92R Micro Servo. I used this page to test my servo because it includes a description about the product, key features of the servo, motor connections, an evaluation of the product, and some test code for you to run as I did above.

https://protosupplies.com/product/servo-motor-micr...

This servo has a limited range and that range is usually 180 degrees. As you can see in the video, the servo starts to vibrate when it tries to go to a position farther than it can go. The website above suggests that you try to keep the range between 20-160 degrees to avoid stripping the gears and damaging the motor. This servo expects to see a pulse on its PWM pin about every 20 milliseconds. The width of each hash mark on the oscilloscope is 8 milliseconds so you can see that the pulses come about very 2.5 hash marks. The width of the pulse determines the position (angle) of the servos shaft. The pulse can vary between 1 milliseconds and 2 milliseconds. A 1 milliseconds pulse positions the shaft at 0 degrees. A 1.5 milliseconds pulse positions the shaft at 90 degrees. A 2 milliseconds pulse positions the shaft at 180 degrees.

Step 4: Useless Machine Simulation

As you can see in the videos I have shared, my signal is very faulty and is causing some slight jitter in my servo. I have tried many different things to fix this problem (ex. switching out components). I have even tried switching some of the wiring out for newer wire just in case some of it was old and the connections were bad. I hope that nobody runs into the same problem as I have. The videos do show some good things though. I start the process of running the machine just like it would if it were constructed by holding the micro switch, causing the servo to be completely powered down. Then, I flip the toggle switch and release the micro switch, which causes the servo to rotate to the right toward the toggle switch. I simulate the arm hitting the toggle switch by flipping it again manually back to the "off" position and the servo moves to the left back toward the micro switch. I press and hold the micro switch because the arm would come to rest on that switch.

As I said at the beginning, I was not able to finish this project in time. These small barriers kept me from completing my Useless Machine, but I hope that you can learn from this and find where I went wrong. I will be continuing to get this project to work perfectly. Once you get it working, last steps are to solder it to a PCB board and assemble everything into your box.