Touch alarm circuit

Touch alarm circuit TouchAlarm

Circuit Principle

The circuit of the touch alarm is shown in the figure, which is mainly composed of four parts: power supply circuit, touch delay circuit, thyristor switch circuit and negative resistance oscillator.

The power supply circuit is composed of VD1, VD2, C1, C2, etc., which provides about 12V DC working voltage for the touch delay circuit. The touch delay circuit is mainly composed of VT1 and other components. Usually VT1 is in the cut-off state, and the thyristor VS is in the off state due to no trigger voltage. The subsequent circuit does not work without power.

N is a touch electrode. When a person touches it, the leakage current of the human body is injected into the emitter junction of VT1 through resistors R1 and R2, making VT1 conduct. The 12V DC voltage on C2 provides a positive gate current to the gate of the thyristor VS through VT1 and VD3, making VS turn on quickly. The two ends of capacitor C4 can obtain a DC high voltage of about 300V. The negative resistance oscillator composed of VT2, R4, R5, RP and B starts to vibrate immediately, and the piezoelectric ceramic piece B emits a loud alarm sound. The tone of the alarm sound can be changed by adjusting the variable resistor RP.

After the hand leaves the electrode sheet N, the transistor VT1 quickly returns to the cut-off state, but at this time the electric energy stored in C3 can continue to provide a positive gate current to VS through VD3, so VS will not be turned off immediately, and the circuit will still alarm. After the charge of C3 is discharged, VS loses the positive trigger current and turns off when the AC passes zero. Since the sound-generating device of the circuit is a piezoelectric ceramic sheet, the circuit power consumption is very small, and the charge stored in C4 can still maintain the negative resistance oscillator for a period of time until the charge of C4 is discharged, and the circuit stops the alarm. If the electrode sheet N is touched again, the circuit can alarm immediately. From the above analysis, it can be seen that the circuit has two levels of delay, so it is not necessary to use a large-capacity capacitor to obtain a longer delay time. Each time N is touched in this circuit, the alarm time can be maintained for about 3 minutes. Since the negative resistance oscillator works in a high-voltage state, the peak-to-peak value of the output waveform is high, so the alarm volume is relatively loud.

Component selection and production

VD1 uses a 12V voltage-stabilizing diode, VT1 can use a common 9013 type silicon NPN transistor, and VT2 is preferably a 3DK4 type silicon switch transistor. VS should use a unidirectional thyristor with a small trigger current, such as 2N6565, CR106, etc. C1 requires a CBB type polyphenyl capacitor with a withstand voltage of more than 400V, and C4 requires an electrolytic capacitor with a withstand voltage of 450V. The touch electrode N is connected to the circuit with two high-value resistors R1 and R2, the purpose of which is to increase the safety and reliability of the circuit. B uses a ∮35mm piezoelectric ceramic sheet. In order to increase its sound volume, a suitable resonance cavity should be configured. If a high-loudness piezoelectric ceramic alarm horn can be used, the effect will be better.

If you want to change the delay time of the circuit alarm, you can change the capacity of capacitors C3 and C4. The larger the capacity, the longer the delay time. Increasing or decreasing the resistance of resistor R3 also has a great influence on the delay time. The larger the resistance of R3, the longer the delay time. However, the resistance cannot be too large, otherwise VS will always be in the on state and the circuit will not work properly. In addition, the trigger sensitivity of the thyristor also affects the length of the delay time. These factors can be adjusted through experiments.

When installing this circuit, the phase and neutral lines of the 220V AC power supply must be connected as shown in the figure. If they are connected incorrectly, the circuit will not work properly. If the negative resistance oscillator cannot work after the thyristor VS is turned on, the circuit can start oscillating by properly adjusting the resistance value of the potentiometer RP, and the alarm sound will be adjusted to the maximum volume and the most satisfactory tone.