This little PC Watcher circuit will help you to prevent unauthorised access to your personal computer. After construction on a small piece of veroborad, enclose the whole circuit in a tamper proof plastic box and connect its input to a vacant USB port at the rear of your Desktop PC, using a standard USB cable.Now hide the unit in a suitable place. If anyone tries to switch on your PC, the circuit starts beeping to raise an aural-alert. The alarm function can be disabled by flipping the SPST toggle switch S1 to its ON position. The circuit is “self-powered” ie no additional power supply/battery is required!
This alarm circuit is designed using the 14-stage ripple counter CD4060 (IC1). When computer starts running, 5 Volt dc available from its USB port is fed to the circuit through polarity protection diode D1. When IC1 gets power from the PC, it resets through C3 and R1 and starts oscillating. Reservoir capacitor C1 maintains the voltage to IC1 stable so that the oscillation is not affected by slight variations in the USB power line, while capacitor C2 suppresses any unwanted high frequency noise.
Resistor R2 and capacitor C4 maintain the oscillation of IC1, as indicated by the beeping of piezo-buzzer connected to its output (pins 3&5) through an npn transistor switch T1. With the components values shown, the buzzer beeps intermittently (after an initial delay of 30 secs) for near 30 secs, then stops for the same period of time and the cycle repeats until the input condition is reversed.