Experiment 33: Moving in Steps
I’ve used a 10K resistor to pull up the input to each 555, so that the timers are naturally in their quiescent state. A 0.01 μF capacitor links the output from one timer to the input of the next so that they are electrically isolated from each other, and the capacitor just conveys a “spike” of voltage when one timer finishes its “on” cycle, and its output goes low, which triggers the next. On the righthand side, I’ve used 10K resistors and 22 μF capacitors to generate a cycle of about a quarter of a second—except that the topmost timer has a 8K2 timing resistor. The reason for this is that when power is first applied, the timers will all be waiting for each other to begin, and timers 2 and 4 or 1 and 3 may fire together. By giving one timer a shorter cycle than the others, I minimize this problem. The LEDs are included just to give you some visual verification of what’s happening. Without them, if you make a wiring error, the motor may turn to and fro erratically, and you won’t know why. Initially you can run your circuit with only the LEDs connected, just to make sure it works. Figure 5-109 shows the breadboarded circuit before the motor is plugged in. Then add the motor by plugging its wires into the breadboard, where you’ll make connection with the outputs (pins 3) of the timers. See Figure 5-110.
Figure 5-109. To test the control circuit for errors, four LEDs show the outputs from the four 555 timers. The loose yellow wire at the righthand side connects to pin 2 of the first timer. Touch the free end of this wire to the positive side of the power supply to reset the timers, and then, if necessary, make a brief negative connection with the free end of the wire to restart their sequence.
Apply power, and you should see the motor turning in steps, in sequence with the LEDs. If the LED sequence isn’t stable: 1. Connect a wire directly from the input (pin 2) of the topmost timer to the positive side of the voltage supply, and wait for the timers to calm down. 2. Restart the sequence by disconnecting the free end of this wire, or (if necessary) touch the free end of it briefly to the negative side of the supply, to trigger the first timer. One thing you may have noticed, if you’re paying very close attention: the common terminal of the motor is connected to positive. Therefore, when each timer flashes positive, that positive signal isn’t actually powering the motor. The low outputs from the three timers that are not firing at any given moment are sinking current from the motor. It seems quite happy with this arrangement. You’ll need some theory to understand why.
What Next?
Figure 5-110. After the circuit has been tested, the motor can be added by hooking its control wires to the outputs of the four 555 timers.
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