Sequential controls are used where continuous remote control of the mechanical installation, such as rotating antennas or valves, is required.
That shown in the diagram offers a setting accuracy of 2.5%, although it has only a few components.
The control motor is in series with a bridge rectifier across the secondary mains transformer Tr2. The rating of the transistor former must accord with that of the motor.
The other two main transformers are light-duty, 12-V types from across whose secondary a small alternating voltage may be taken by means of P1 and P2.
The wiper of P1 is connected to the gate of T1 via a resistive network R3-R5. The wiper of P2 is connected to the source of T1. The source-gate junction of the transistor serves as a null-point detector. When the circuit is balanced, the potential difference between the two wipers is zero, so T1 is switched off. No current can flow through the motor since the current loop through the rectifier bridge is broken for each half cycle. When one of the potentiometers is adjusted the circuit is no longer balanced and T1 is switched on during either the positive or the negative half cycles, depending on which of the potentiometers was adjusted.
Current then flows through the motor, D4, T1, and D1, or through the motor, D3, T1, and D2. In other words, the motor can rotate in either direction. If the motor is supplied mechanically with P2, P1 may be used for remote control of the motor.
The circuit as shown is intended for 12 V motors; if different motors are to be used, bear in mind that they are operated from half-wave rectified voltage, which means that the transformer must be rated at 1.52 times the motor voltage.