Abstract: An alarm circuit for detecting the opening of any door of a semi trailer. The alarm circuit has power supply 11, negatively grounded with the positive terminal electrically connected to a keyed rotary switch 16. A double pole double throw relay 14 is electrically connected at terminal five to the output terminal of the keyed rotary switch is electrically connected at its terminal five to the output terminal of the keyed rotary swtich 16. A green indicator lamp 19, which is grounded is electrically connected to a normally open single pole single throw momentary push button switch 17 and to terminals three and seven of the relay 14. The other end of the normally open single pole single throw momentary push button switch 17 is electrically connected to the output terminal of the keyed rotary switch 16. An alarm horn 13, which is grounded, is electrically connected to terminal number one of relay 14.