Priority Switch (MEC185E)

This project was found in my book Robotics, Mechatronics and Artificial Intelligence (sold out) published in the United States. It shows how to build a priority switch suitable for mechatronics or robotics projects with microcontrollers or as a building block for simple projects.

An important block, with many applications in robotics and mechatronics, is the priority switch. This circuit can detect which sensor (or switch) was activated first and turn on a circuit to disable the other.

Our circuit of a priority switch is shown in Figure 1 and uses SCRs as basic elements. Assuming that both SCRs are off and S1 is closed before S2, the SCRs are activated first, providing current to Load2.



Figure 1   Priority switch.
Figure 1 Priority switch.



With the SCR2 turned on, the voltage in its anode falls to about 2 V, which is not enough to turn on SCR1 if S2 is closed.

Note that this circuit is self-latched, meaning that to turn the triggered SCR on or off, the power must be cut momentarily. The SCRs must be mounted on heat sink if the loads draw more than 500 mA.