The 4.21MHz crystal oscillator is built using a Series Resonance circuit with 74LS04 TTL logic ICs. I abandoned the analog design and rebuilt the PWM circuit around digital TTL ICs. This design did not last long because it required too many calibrations and did not give a linear PWM Duty Cycle in response to the digital control lines. The circuit around the timer functioned as an a-stable oscillator with a center frequency of 20KHz, and by feeding the NE555 pin 5 with an analog voltage I controlled the Duty Cycle and the PWM signal. The control lines from the processor drove a 3 bit D/A converter, which in turn fed a NE555 timer IC. The first implementation I used for a while was an analog circuit.
I'll start out by briefly describing the "evolution" path that led me to this specific solution, and the different considerations that made me drop other solutions in favor of the one I'm using.īecause I used a permanent magnet DC motor and wanted to control it's speed and direction, I designed a PWM signal generator: current, voltage and switching characteristics, which will highly depend on your specific implementation and choice of motors. When selecting the FET transistor note max. This setup will prevent the motors from running if no power is applied to the CPU/logic circuits. Note that as a second fail-safe mechanism, the Q1 (2n2222) transistor will not allow the FET to conduct if no signal is applied to the opto-coupler/isolator.
When selecting the coupler, pay attention to the switching characteristics, and make sure that your drive current into the IR side and the Collector current limiting resistor drive the unit into it's saturation region.
The separation is done with an opto-coupler/isolator, which is driven by a 7405 Open Collector driver through the 82ohm resistor. For simplicity, I chose not to use an H-Bridge and used a single Power-FET and a DPDT relay for reversing the motor rotation direction.īecause I used the battery packs that came with the hacked power-driver, the motor power is separate from the CPU and logic power source.