This ultra-simple circuit can be mounted in a box the size of a cigarette pack or slightly larger and installed in several places. In your room, for example, with the sensor plugged into the lock, you'll know when someone touches it by lighting an LED. The circuit is very sensitive serving to monitor the touch on remote objects.
In the standby condition, with the LED off, the consumption of the device is very low, which allows it to be permanently connected without affecting the battery life.
The power can be made by either 4 common batteries or a 9 V battery for more contact mounting. For the battery case, resistor R2 should be increased to 1k to reduce power consumption. In figure 1 we have the complete diagram of the alarm.
Welding of the main components can be done on a small terminal bridge, as shown in figure 2.
The wire for the sensor must be at most 5 meters long and be of the hooded type. At its tip we can connect an alligator claw to facilitate the connection to protected objects, which must be made of metal. We can also use a sensor, a metal plate 10 x 10 cm to 20 x 20 cm.
The ground wire should be used if the need for greater sensitivity is found. It can be attached to any metal object or to the aluminum windows and doors.
If you turn the unit on, it is a signal that the protected object is making contact with the ground. It must be isolated.
Material list
Q1 - BC548 - NPN general purpose transistor
Q2 - BC558 - general purpose PNP transistor
LED - common LED of any color
B1-6 or 9 V -pins or battery
R1 - 47k ohms x 1/8 W- resistor - yellow, violet, orange
R2 - 470 ohms x 1/8 W - resistor - yellow, violet, brown
Miscellaneous: sensor, terminal bridge, battery holder or battery connector, wires, solder, etc.
