on the other hand the transistor tr2 has its base connected to the collector of the transistor tr1 thus the moment a voltage across the terminals of the condenser c1 reaches a predetermined level the transistor tr2 is rendered conductive thereby blocking the transistors tr3 tr4 the solenoid 11 is connected to the collector of the transistor tr4 and is maintained excited only while the transistors tr3 tr4 are in conductive condition and is de energized simultaneously when the transistors tr3 tr4 are blocked the resistors r2 r3 are used for stabilizing the operation of the schmitt circuit the double exposure preventive circuit 21 consists of switches s1 s3 s4 s5 a condenser c2 a discharge circuit consisting of resistors r4 r5 for dividing the voltage of a battery or cell e and a semi conductor control circuit consisting of a thyrister or scr 22 the condenser c2 is connected by way of parallel connected switches s3 s4 across the resistor r 6 and constitutes a self discharging circuit with resistor r4 and switches s1 s5 thus by closing either one of switches s3 and s4 the condenser c2 is charged and by closing both switches s1 s5 the condensor c2 is discharged on the other hand the scr 22 is connected by way of the solenoid 11 to the base of the transistor tr4 and its gate is connected by way of switches s1 s5 to the positive terminal of the condensor c2 the switch s1 is adapted to be closed by the depression of the release plate 9 while the switch s4 is a manually operable switch for use in multi exposure picture taking and adapted to be freely opened and closed from the exterior of a camera the winding.