A thyristor circuit and an apparatus using the circuit for the prohibition of operation of a motor vehicle by an intoxicated person may be fabricated by combining an ignition switch with an engine locking and unlocking means, timing means, sensing means and an indication means. The normal ignition circuit of a motor vehicle is interrupted by an engine locking and unlocking means. A timing signal is generated by the timing means by selectively charging a circuit in turn is responsive to the performance of the operator in a motor ability test such as the ability to maintain a predetermined amount of steady pressure on a brake pedal. If the operator is successfully performing the test, the charged node achieves a preselected potential and an indication means produces a first cognizable signal which indicates a successful performance. After the operator has successfully performed the test for a predetermined duration, a second cognitive signal is produced by the indication means which indicates that the operator is qualified to operate the vehicle. The normal ignition circuit is then completed by the engine locking and unlocking means in response to the timing signal. The motor vehicle may then be started in the conventional fashion.