The goal was to design a low cost motor driver for my HO turntable that would run high speed while the direction pushbutton is held down, switch to low speed when the pushbutton is released and continue to run at low speed until the turntable reaches the next track position.  To compensate for backlash and motor momentum one detector is used for clockwise rotation with a separate one for counterclockwise.

Flip-Flops “AA” & “CC” control the have an additional NOR gates “B” &”D” that inhibit the “push button up” reset until IR detector turns on.  I am using a simple IR detector diode than conducts about 3.5 uamps in the on state.  This is more than enough to pull 10 Meg to a low input state.

The output of flip-flop “AA” also drives the direction relay using a transistor in an emitter follower mode. Almost any NPN transistor is fine.  I am using a 2N3904.  With a minimum gain in the 60 to 100 range it only requires about 0.5 ma to drive the 33 ma relay coil.

The two flip-flops are cross coupled using two 0.01 mf capacitors. These capacitors provide a negative pulse on the output of the other IR detector. This resets the other flip-flop & allows a change in direction before the turntable has stopped.

NOR gate “E” plus “F” gate wired as an inverter control the speed selection by always holding down one of the reset inputs of the 556 dual timer.

The output of gate “G” is low when either of the flip-flops is set.  The output of gate “H” is the inverse of the output of the active side of the 556 dual timer.  When both inputs to gate “I” are low the high output turns on MOSFET and provides the PWM voltage to the motor.

The 556 dual timer provides a Pulse Width Modulated signal at two different duty cycles.  Using the steering diodes provides a constant frequency with a PWM range from virtually 0% to 100%. The 47 ohm resistor prevents the zero ohm path from the output to the timing capacitor when the potentiometers are at the end points.

Snubber diodes and a small value capacitor are used to protect the MOSFET from and spikes produced by the motor.

The status diodes are optional but they make adjusting the IR detectors and motor speed much easier.

Almost any general purpose diode is fine.  I am using 1N4004 since 400v and 1 Amp far, far, far exceeds all requirements and cost $0.02 each.

The MOSFET is also not critical.  Depending on your motor, a device in the 3 Amp range should work fine but you might want to consider using a high current (10 Amp\+ ) as the very low ON resistance and therefore the low heat generated may eliminate the need for a heat sink.

I purchased the parts and proto board from futurlec.com with a total cost of less than $10.00.