R1_______________4M7 1/4W Resistor R2_____________100K 1/4W Resistor R3_______________2M2 1/2W Trimmer (Carbon or Cermet) (Optional, see Notes) R4_______________1K 1/4W Resistor (Optional, see Notes) C1_____________470pF 63V Ceramic Capacitor C2,C4___________47µF 25V Electrolytic Capacitors C3_____________100nF 63V Ceramic or Polyester Capacitor D1_______________LED Any color, shape and size (Optional, see text) IC1_____________7555 or TS555CN CMos Timer IC BZ1____________Piezo sounder (incorporating 3KHz oscillator) SW1_____________SPST Toggle or slide switch (Optional, see Notes) B1______________1.5V to 9V Battery (See Notes)
The proposed circuit is simple to implement, has a very low cost and can be used in dozen of occasions. Connecting it to the door handle, it constitutes an effective as economic alert for your room, office, lab, or even drawer. But we are confident that you will be able to think of other applications and uses of this gadget.
You can choose an LED or beeper, or both as a warning signal and a battery supply in the 1.5 to 9V range, according to your needs.
Unlike other similar circuits, this one does not require any connection to the mains.
IC1, a CMos 555 timer, is wired in a monostable configuration. Pin #2 of the IC is tied to the positive rail but its voltage will fall when the wire connected to Trimmer R3 is touched, determining a ground connection through the human body.
Capacitor C1, initially charged to supply voltage, is discharged to the ground through R3 in series with the resistance of the human body and the voltage on pin #2 drops to zero.
This starts the monostable: pin 3 goes high for a time (about 5-6 seconds) determined by the network R2, C2, the piezo sounder will beep and/or the LED will illuminate.
- Trimmer R3 adjusts the sensitivity of the circuit, but in most cases it can be omitted.
- The monostable delay time can be varied by changing the values of R2 and/or C2
- If not using the LED, R4 is obviously omitted.
- Since the stand-by current of this device is negligible, SW1 can be omitted.
- If the circuit is powered with battery voltages lower than 9V and the LED is used, the value of R4 should be reduced accordingly.
- Using a suitable piezo sounder, the circuit will work well even at 1.5V, but to allow an LED to illuminate, the supply voltage should not drop below 3V.