The Ineffective Halloween Machine – Construct Digital Circuits

This yr’s Halloween challenge is a ineffective machine constructed with 555 timers. It’s a coffin with a swap. While you flip it off, a darkish pressure rises from the coffin and flips it again on.

Controlling a Servo with 555 Timers

The thought for this challenge got here after we printed the straightforward 555 PWM circuit that controls a DC motor utilizing a PWM sign created by a 555 Timer. I began fascinated about the thought of controlling a servo as an alternative.

After some experimentation, I noticed that the frequency modified an excessive amount of after I modified the obligation cycle. So my servo would solely transfer at random occasions.

I made a decision to make use of two 555 timers as an alternative: one in astable mode to set the frequency to a gradual 50 Hz. And one in monostable mode that may set the obligation cycle. With this setup, I used to be capable of management the servo with none points.

Two 555 timers controlling a servo motor

The worth of the resistors R3 and the capacitor C2 units the usual obligation cycle. This place is when the arm is hidden.

While you push the pushbutton SW1, R4 is related in parallel with R3 which reduces the obligation cycle and strikes the servo to its different place. That is the place that ought to make the arm flip off the toggle swap.

I used a pushbutton for this experiment. However within the ineffective machine setup, SW1 could be a toggle swap.

Including a Delayed “button push”

The switching between the 2 positions occurred a bit too quick. I used to be barely capable of take away my finger earlier than the servo arm moved into place to flip the swap again off. So I wanted a approach to delay issues earlier than the arm switched positions.

I attempted just a few various things earlier than I ended up with the next: A pMOS to exchange SW1. And a capacitor and resistor so as to add a delay from the toggle swap is flipped to the pMOS activates.

Circuit for delaying R4 being placed in parallel with R3.

Mainly, what this circuit does is that it locations R4 in parallel with R3, similar to SW1 did earlier than. Nevertheless it does so after a brief delay of a few seconds. The values of C3 and R5 units the delay time.

The Last Circuit

The delay circuit labored completely. And beneath you may see the total schematic for this challenge:

Schematic of useless machine with 555 timers

The one a part of the circuit that isn’t proven above is a 7805 voltage regulator that converts my 9V battery to 5V, which was the VCC voltage I used to make the whole lot suitable with my servo.

Elements Wanted

  • Servo motor (I used SG90)
  • Toggle swap
  • Field to place the whole lot into
  • 2 x 555 Timer
  • 1 x pMOS transistor
  • 5 x Resistors (See beneath for values)
  • 3 x Capacitors (See beneath for values)

To decide on values for the primary 555 timer, use this 555 calculator in astable mode and take a look at completely different values till you discover the frequency you need.

Be aware that the time low you get would be the lowest doable obligation cycle.

To decide on values for the second 555 timer, use this 555 calculator in monostable mode and take a look at completely different values till you discover the resistance you want for the 2 completely different servo positions.

Use the best resistance worth for R3. Work out what R4 must be with the intention to get the bottom resistance worth (this calculator is likely to be of assist).

Drawing of useless machine circuit soldered


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