Some delicate sensors such as reed-switches and even sensors made with small metal plates do not withstand the high currents of the motors and tend to burn out quickly no longer operating or causing intermittent operation problems. One way to avoid this problem is to use a transistor as a power switch and activate it with a low current switch, as shown in the circuit in the figure. This S1 circuit can be a reed-switch, a homemade blade switch or a limit switch that will operate with a few milliamps of current only. The used transistor must have a small heat radiator. We recommend the BD135 for motors up to 500 mA and the TIP31 for motors up to 1 amp. An important point of this circuit is that several sensors can be connected in parallel to control the motor.
