This sensor circuit is like an electronic watchdog for people.
It uses electrical parts to know if someone is nearby.
These circuits are often used in things like security systems, lights that turn on automatically and smart home devices.
The circuit works by noticing changes around it especially those caused by a person being close.
Then it does something based on that, like setting off an alarm or turning on a light.
Circuit Working:
Parts List:
| Component | Value | Quantity | Notes |
|---|---|---|---|
| Resistors | All resistors are 1/4 watt unless specified | ||
| 22k | 2 | ||
| 1k | 2 | ||
| 10k | 2 | ||
| 100k | 1 | ||
| 8.7M | 1 | ||
| 2.2k | 1 | ||
| Potentiometers | |||
| VR1, VR2 | 25k | 2 | Presets |
| VR3 | 1M | 2 | Presets |
| Capacitors | |||
| Ceramic C1, C2, C4, C6 | 120pF | 4 | |
| Ceramic C3, C5 | 100nF | 2 | |
| Ceramic C7 | 330nF | 1 | |
| Electrolytic | |||
| C8 | 100μF 16V | 1 | |
| Semiconductors | |||
| Transistors | |||
| BF494 | 2 | ||
| BS170, BS250 | 2 | ||
| Diodes | |||
| D1 to D4 | 1N4148 | 4 | |
| Others | |||
| Relay | 1 | ||
| Coils L1, L2 | 470uH | 2 | |
| Metal plate | 1 |
This capacitive proximity sensor circuit also known as a presence detector responds to the presence of any conductive object including humans.
The sensitivity can be adjusted using P1, which is positioned at a significant distance from the main presence detector circuit.
While it does not detect object movement it functions effectively as a proximity sensor.
Adjusting the sensitivity with P1 allows customization of the detection distance.
An application of this circuit is its automatic door opening capability when placed on the front of the door.
The presence detector comprises a clapp oscillator with T1 and a monostable.
The clapp oscillator is chosen for its frequency stability operating at around 1MHz when the surface of the sensor acts as a capacitor for the oscillator circuit.
The switching time can be fine tuned using P2.
It is crucial to avoid introducing metallic objects in close proximity to the circuit as this may cause the relay to remain closed.
Additionally, this circuit serves as a detector for aggressive liquids offering the advantage of the sensors surface not coming into direct contact with the liquid.
Formula:
Usually, a transistor based capacitive proximity detector circuit makes use of the change in capacitance that occurs when an item gets close to a sensor electrode and ground.
To identify these variations in capacitance, an oscillator or amplifier arrangement use a transistor.
The following formula may be used to predict the oscillators operating frequency, f which is dependent on the capacitance C proximity of the detecting electrode and is affected by:
f = 1 / 2πLC
where:
- f: is the oscillator frequency.
- L: is the inductance in the oscillator circuit.
- C: is the total capacitance in the oscillator circuit, including the sensing capacitance Cproximity.
The oscillator frequency of a transistor based capacitive proximity detector is changed by the capacitance change brought on by closeness.
The presence or absence of an item close to the sensing electrode can be determined by measuring or detecting this frequency shift.
In real world applications, the proximity detectors sensitivity, stability and range will depend on the precise values of the resistors, capacitors and transistor type utilized.
Depending on the demands of the particular application and the surrounding circumstances, adjustments can be required.
How to Build:
Building a capacitive proximity sensor circuit involves several steps and requires a basic understanding of electronics.
Circuit Diagram:
- Connect the components as per the following schematic diagram:
Oscillator Section T1:
- Connect the components R1, R2, C1, T1 to form the clapp oscillator.
- Ensure the capacitor C1 is in close proximity to the sensor area.
Monostable Section:
- Connect the components R3, C2, D1 to create the monostable section.
- Adjust P2 for desired switching time.
Preset P1:
- Connect P1 to allow adjustment of sensitivity for detecting proximity.
Relay Connection:
- Connect the relay in such a way that it controls the desired output such as automatically opening a door.
- Connect the power supply to the circuit ensuring it matches the requirements of the components.
- Power on the circuit and test its functionality.
- Adjust P1 and P2 to achieve the desired sensitivity and switching time.
Important Note:
- Carefully follow the circuit diagram and double check your connections.
- Exercise caution when working with electrical components and ensure that the power supply voltage is appropriate for the components used.
- Avoid introducing metallic objects near the circuit during testing as it may interfere with the sensors operation.
Conclusion:
These capacitive proximity sensor circuit contribute to automation, energy efficiency and improved security in various environments.
The specific design and components may vary based on the intended application and the desired features of the sensor circuit.
This is a basic guide and you may need to adapt it based on specific component availability and circuit requirements.
If you are unfamiliar with electronics consider seeking assistance from someone with experience or consulting with a professional.