This simple circuit was suggested by the Electronics Handbook Volume XIII 1993. LED D1 emits light for R3, a photo-resistor, keeping the circuit cut. When it ceases to focus light, the transistor saturates and the Buzzer fires. B1 must have tension according to the buzzer (Sonalert). Rx depends on the monitored voltage according to the following table:
| Voltage | Rx |
| 5 V | 330 Ω |
| 6 V | 470 Ω |
| 9 V | 820 Ω |
| 12 V | 1 k Ω |
| 15V | 1k5 Ω |
R1 is 10 kΩ and R2 is 1 kΩ. The transistor can be BC548 or equivalent.
