Here is circuit if you want an alarm to be sounded when somebody touches some thing. If any body touches say, the door handle, buzzer inside rings. But if want to keep the intruder away, keep the circuit on in the night or when you are away. It can be configured for use in a museum or display exhibition where you do not want visitors to touch the exhibits.
Alarm sounds from speaker for 10 seconds after anybody touches the touch plates. This circuit using CD4011 can be used as a doorbell. The pin out and truth table are given Figure 32 and Figure 33 respectively.
Gates are some of the fundamental building blocks of digital electronics. This CMOS IC, CD4011 has four two input NAND gates in one package. The truth table for this type of gate is given below. Inputs are taken as A and B. If both or any one of the inputs goes low, the output goes high. On the other hand, if both inputs go high, the output goes low. Converse happens in the case of AND gate. It can handle voltages from 3 to 15 volts.
Two NAND gates are used for touch sensing and other two gates are used for sounding alarm. Gates consisting of U1A and U1B act as a timer for 10 seconds which is activated by touching the touch probes. These can be conveniently concealed in a door handle or any other special object. If any body touches input gate at U1A and U it results in a high signal for 10 seconds at its output. The other two gates comprising of U1C and U1D make a high frequency oscillator. This is activated by a high signal at the output of U1A. It is amplified by Darlington pair consisting of BC 147 and BEL187 which drives the speaker.
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