This circuit was submitted to the 555 Timer contest.

This circuit, page 1 Automatic Motor Reversal Circuit Page 1 (493) and page 2 Automatic Motor Reversal Circuit Page 2 (411) will automatically reverse the direction of the motor.  In applications where a motor might be releasing rope or wire, it may be desired to have the motor reverse the direction for a very short period of time in order to take up some slack or to release tension on the rope.  This circuit will do this by using four LM555 timer devices in monostable multi-vibrator mode.   This circuit is designed to work when a relay device is used for activating separate coils for controlling the direction of the motor.  I originally designed and built this circuit using 74HC4538 one shot devices for a similar application.  VL is assumed to be a low voltage need for the LM555 and the logic gates in the circuit, typically 5 or 3.3V.  VH is the high voltage if needed for the motor and relay voltages most likely 12 volts.

Typically the LM555 timer needs an active low trigger with a pulse that is much shorter than the output pulse.  In this application the trigger pulse duration most likely be longer than the output pulse and the trigger will be active high.  With the help of the LM555 notes on the website: http://homepages.westminster.org.uk/electronics/555.htm, the 4 networks of R3, R4, C2, D2, and R8, R9, C7, D4, and R13, R14, C11, D5, and R18, R19, C16, D6, convert long active high trigger pulses into short active low trigger pulses.

Press the first switch, S1, to connect the VH supply to the relay coil to activate the motor in the forward direction.   This could be held for a short time close to 1 second or for several seconds, but can be held for as long as needed.  The signal will be passed through the AND gate U1A, since the other input to the AND gate is normally high, and activates the trigger network to the the LM555, U2, which will output a pulse to the reverse coil of the relay and activate the motor back for the desired amount of time set by the variable resistor, VR1 and C3 (which results in a maximum of 1 second for 50K ohms and 10 uF).  Placing a resistor in series with VR1 can be used.to limit the minimum time of the reverse pulse.

This output from U2 is also input into U4 through the triggering network described earlier.  The output pulse of U4 is inverted through the NOT gate, U3B and input into the AND gate U1B.  This pulse is active low and disables the reverse pulse from triggering the forward pulse which was just completed with the key press, and leading to an oscillation.  U5 and U6 work respectively for a keypress from S2 for the Reverse direction.

The low voltage, Vl can easily be generated from the high voltage, VH, 12V, with a linear regulator.  If power consumption is very important, then a buck converter could be used.  The inverter gates could also be replaced with LM555s in Schmitt trigger mode or an NPN transistor.