by awright » Sun Jan 07, 2018 1:21 am
You have not described clearly what the mechanical aspects of your winding up string device is, but presumably if the string can wind up to the extent of stalling the motor it is also either pulling tension on the string or torquing the far end of the string. If feasible in your application, can you rig up a set of contacts to sense the tension or torque and activate a relay or circuit to reverse the motor? Sensing torque would be best, since that could activate contacts specific to the direction of motor rotation.
Ideally you would use microswitches activated by the torque or tension to achieve positive transitions rather than simple contacts pushed together that can have noisy, tentative contact.
The simplest approach would be to have the switches activate a "latching relay" with DPDT contacts to reverse polarity of the power applied to the motor. You can purchase latching relays that rely on permanent magnets or mechanical latches in the relay or you can create your own by using a plain relay with a third contact to hold the relay closed. Circuit details are simple but would vary depending upon the type of relay and configuration of actuation contacts you employ.