Figure shows a thief sensitive security alarm. When the human hand touches the metal piece A, it gives a sensor signal to gate. So, the drain current of VT1 decreases, which increase the resistance between drain and the source. When C becomes the high point of potential, VT2 starts to conduct. Along with that, the light-emitting diode VD1 starts to glow brighter. This in turn triggers the music IC KD482. The IC drives the transistor VT3, and the music will be heard at voice-driven speaker BL.
When the human hand is taken back, the drain current of VT1 becomes higher, their leakage resistance between source reduces, C-point into a low end, VT2, VD1 gets OFF, D-point into a high-voltage, BL will automatically stop.
This circuit can also be used for anti-theft, will VT1 gate of the contact line installed in the door handle or door locks and other metal parts, when thieves can touch the hands of an alarm.
The change of the circuit can be used to touch the children’s toys. At the terminals of KD-482,ie, at E, E ‘ connect two light-emitting diodes VD2, VD3 (as shown), while the light-emitting diodes VD1 installed in the middle of the forehead just gives a look like 3 eyes, not only for voice. VD1 which is used to control the light and shade of, VD2. VD3 will be frequent blinking along with the music.