Capacitive-sensor-alarm
The unit is built around a balanced-bridge circuit that utilizes both capacitance and inductance. The bridge comprises capacitors C2 and C3, along with the center-tapped winding of transformer Tl. One end of the bridge is connected to ground through capacitor C4, while capacitance changes are introduced via capacitor C1. A slight change in capacitance unbalances the bridge, generating an AC signal at the base of transistor Q1. Transistors Q1 and Q2 are configured to form a modified Darlington amplifier. The collector load for Q2 is a separate winding of transformer Tl, which is connected out-of-phase with the incoming AC signal. This configuration produces a significant, distorted signal each time the bridge becomes unbalanced. The distorted signal is extracted from the bridge circuit by a third winding of transformer Tl. This signal is subsequently rectified by diode D6 and applied as a DC signal to the base of transistor Q3. The applied signal activates relay K1 as soon as the unbalanced condition occurs, and the relay deactivates when the circuit balance is restored. For standard alarm applications, the relay should be designed to latch, ensuring that the alarm condition persists until the system is reset. An audible alarm device, such as a bell or klaxon horn, can be activated by the relay. Alternatively, if a silent alarm is preferred, a light bulb may be utilized. Transformer Tl is available for purchase as part #6182 from Pulse Engineering, P.O. Box 12235, San Diego, CA 92112.
The balanced-bridge circuit serves as the core of this unit, leveraging the principles of capacitance and inductance to detect changes in capacitance. Capacitors C2 and C3, along with the center-tapped winding of transformer Tl, establish a balanced state under normal conditions. The introduction of a capacitance change through capacitor C1 disrupts this balance, leading to an AC signal generation at the base of transistor Q1. The modified Darlington amplifier configuration, formed by transistors Q1 and Q2, amplifies this signal effectively.
The collector load for transistor Q2, being a separate winding of transformer Tl, is critical in maintaining the proper phase relationship with the incoming AC signal. This out-of-phase connection is essential for producing a substantial, distorted output signal whenever the bridge is unbalanced. The third winding of transformer Tl extracts this distorted signal, which is then rectified by diode D6. The rectification process converts the AC signal into a DC signal, which is fed to the base of transistor Q3, allowing it to control the relay K1.
Relay K1 plays a pivotal role in the alarm system. It is designed to activate immediately upon detection of an unbalanced condition, providing a timely response to changes in capacitance. The self-latching feature of the relay ensures that the alarm remains active until a manual reset is performed, thus enhancing the reliability of the alarm system. The choice of alarm output can vary depending on the application; an audible alarm can be implemented using a bell or klaxon horn, while a light bulb serves as a silent alarm option.
Overall, this circuit design efficiently integrates various components to create a responsive and reliable alarm system, suitable for a range of applications requiring capacitance monitoring. The availability of transformer Tl as part #6182 from Pulse Engineering facilitates ease of assembly and implementation for those looking to construct this circuit.The unit is constructed around a balanced-bridge circuit. using both capacitance and inductance. The bridge consists of capacitors C2 and C3, and the center-tapped winding of Tl. One end of the bridge is coupled to ground by C4, while capacitance changes are introduced through Cl. A small capacitance change unbalances the bridge and produces an ac signal at the base of Ql. Transistors Q1 and Q2 are connected to form a modified-Darlington amplifier. The collector load for Q2 is a separate winding of Tl that is connected out-of-phase with the incoming ac signal. That produces a large, distorted signal each time the bridge is unbalanced. The distorted signal is taken from the bridge circuitby a third winding of transformer Tl. That signal is then rectified by D6 and applied as a de signal to the base of Q3. The applied signal energizes the relay, K1, as soon as the unbalanced condition occurs, and the relay drops out as soon as the circuit balance is restored.
Of course, for normal alarm use, the relay should be made self-latching, so that the alarm condition remains in effect until the system is reset. An audible alarm, such as a bell or klaxon hom, can be operated from the relay. If a silent alarm is needed, a light bulb can be used. Transformer Tl can be purchased as part #6182 from: Pulse Engineering, P.O. Box 12235, San Diego, CA 92112.
The balanced-bridge circuit serves as the core of this unit, leveraging the principles of capacitance and inductance to detect changes in capacitance. Capacitors C2 and C3, along with the center-tapped winding of transformer Tl, establish a balanced state under normal conditions. The introduction of a capacitance change through capacitor C1 disrupts this balance, leading to an AC signal generation at the base of transistor Q1. The modified Darlington amplifier configuration, formed by transistors Q1 and Q2, amplifies this signal effectively.
The collector load for transistor Q2, being a separate winding of transformer Tl, is critical in maintaining the proper phase relationship with the incoming AC signal. This out-of-phase connection is essential for producing a substantial, distorted output signal whenever the bridge is unbalanced. The third winding of transformer Tl extracts this distorted signal, which is then rectified by diode D6. The rectification process converts the AC signal into a DC signal, which is fed to the base of transistor Q3, allowing it to control the relay K1.
Relay K1 plays a pivotal role in the alarm system. It is designed to activate immediately upon detection of an unbalanced condition, providing a timely response to changes in capacitance. The self-latching feature of the relay ensures that the alarm remains active until a manual reset is performed, thus enhancing the reliability of the alarm system. The choice of alarm output can vary depending on the application; an audible alarm can be implemented using a bell or klaxon horn, while a light bulb serves as a silent alarm option.
Overall, this circuit design efficiently integrates various components to create a responsive and reliable alarm system, suitable for a range of applications requiring capacitance monitoring. The availability of transformer Tl as part #6182 from Pulse Engineering facilitates ease of assembly and implementation for those looking to construct this circuit.The unit is constructed around a balanced-bridge circuit. using both capacitance and inductance. The bridge consists of capacitors C2 and C3, and the center-tapped winding of Tl. One end of the bridge is coupled to ground by C4, while capacitance changes are introduced through Cl. A small capacitance change unbalances the bridge and produces an ac signal at the base of Ql. Transistors Q1 and Q2 are connected to form a modified-Darlington amplifier. The collector load for Q2 is a separate winding of Tl that is connected out-of-phase with the incoming ac signal. That produces a large, distorted signal each time the bridge is unbalanced. The distorted signal is taken from the bridge circuitby a third winding of transformer Tl. That signal is then rectified by D6 and applied as a de signal to the base of Q3. The applied signal energizes the relay, K1, as soon as the unbalanced condition occurs, and the relay drops out as soon as the circuit balance is restored.
Of course, for normal alarm use, the relay should be made self-latching, so that the alarm condition remains in effect until the system is reset. An audible alarm, such as a bell or klaxon hom, can be operated from the relay. If a silent alarm is needed, a light bulb can be used. Transformer Tl can be purchased as part #6182 from: Pulse Engineering, P.O. Box 12235, San Diego, CA 92112.