The simple circuit shown in figure 1 may be an excellent solution for a system of audible warning for machines, opening of doors, violation of space, presence of objects in a line that must remain free, etc. The circuit has low power consumption in the standby condition which makes it possible to feed from batteries or battery and also uses discrete components that are easy to obtain.
The circuit activates an LED when the two inputs go high. The sensors connected to the inputs can be of the reed-switch type and for the protection of more than one point, several of them can be connected in series.
The two sensors must be open for the alarm to fire by turning on the LED.
In figure 2 we have the printed circuit board for implementation of this small sound alarm of general use.
Material list
Q1 - BC548 - NPN general purpose transistor
D1, D2-1N4148 or equivalent
LED - common red LED
R1 - 3k9 k ohms x 1/8 W - resistor
R2 - 120 ohms x 1/8 W - resistor
Several:
Printed circuit board, power supply, wires, solder, etc.
