This sensor circuit functions similarly to a human electronic watchdog.
It senses if someone is close by using electrical components.
These circuits are often found in smart home appliances, automatic lighting and security systems.
The circuit detects changes in its environment particularly those brought on by a person nearby.
Based on this it then takes action such as flashing a light or sounding an alarm alert.
Circuit Working:
Parts List:
| Component | Quantity |
|---|---|
| Resistors (All resistors are 1/4 watt unless specified) | |
| 22k | 2 |
| 1k | 2 |
| 10k | 2 |
| 100k | 1 |
| 8.7M | 1 |
| 2.2k | 1 |
| Presets VR1, VR2 25k | 2 |
| Preset VR3 1M | 2 |
| 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 |
| Transistors BS170, BS250 | 2 |
| Diodes D1 to D4 1N4148 | 4 |
| Relay | 1 |
| Coils L1, L2 = 470uH | 2 |
| Metal plate | 1 |
A presence detector another name for this capacitive proximity sensor circuit reacts when any metallic item including people are present.
P1 preset which is positioned far from the main presence detector circuit is used to change the sensitivity.
Despite being unable to detect movement of objects it works well as a proximity sensor.
The detecting distance is modify by adjusting the sensitivity using P1 preset.
One main use for this circuit is its capacity to open doors automatically when it is mounted on the front of the door.
Two components make up the presence detector: a monostable and a clapp oscillator with T1 transistor.
Since the sensors surface serves as a capacitor for the oscillator circuit the clapp oscillator which operates at about 1MHz is chosen for its frequency stability.
P2 preset is used to fine tune the changeover time.
Avoiding placing metallic things near the circuit is essential since doing so might keep the relay closed.
This circuit also functions as a detector for harsh liquids with the benefit that the surface of the sensor does not come into direct contact with the liquid.
Formula:
Normally a transistor based capacitive proximity detector circuit uses the capacitance move that happens when an object gets closer to a ground and a sensor electrode.
A transistor is used in an oscillator or amplifier configuration to detect these capacitance fluctuations.
The operational frequency of the oscillator f is estimated with the following formula which depends on the capacitance C closeness of the detecting electrode and can be affected by:
f = 1 / 2πLC
where:
- f is the oscillator frequency.
- L is the inductance in the oscillator circuit.
- C is the oscillator circuits total capacitance which includes the sensing capacitance Cproximity.
The capacitance changes triggered by proximity modifies the oscillator frequency of a transistor based capacitive proximity detector.
By monitoring or detecting this frequency change one can figure out if an object is near the sensor electrode or not.
The exact values of the resistors, capacitors and transistor type used will impact the proximity detectors sensitivity, stability and range in actual use.
Adjustments can prove necessary depending upon the specific applications requirements as well as the surrounding conditions.
How to Build:
To build a Capacitive Proximity Sensor Circuit follow the below mentioned steps:
Circuit Diagram:
- Connect the components as shown in the above schematic diagram:
Oscillator Section T1:
- Connect the components R1, R2, C1, T1 to form the clapp oscillator.
- Ensure the capacitor C1 close by the proximity to the sensor area.
Monostable Section:
- Connect the components R3, C2, D1 to create the monostable section.
- Adjust P2 preset for getting the 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.
Source of Power:
- When the power supply is connected to the circuit make sure it fulfills the component requirements.
- Turn the circuit ON and check that it is functioning.
- To get the correct sensitivity and switching time adjust the presets P1 and P2.
Important Information:
- Pay close attention to the circuit schematic and be sure all the connections are correct.
- When handling electrical components take care and ensure the voltage from the power source is suitable for all the circuits parts that is used.
- During testing stay away from placing metallic items next to the circuit since this might disrupt the sensors functionality.
Conclusion:
These circuits for capacitive proximity sensors improves security, energy efficiency and automation in a number of situations.
Depending on what is being used and the required characteristics of the sensor circuit the exact design and components can differ.
This is only a basic tutorial and one might need to modify it by depending on the circuit needs and the availability of particular components.
If not comfortable with electronics think about visiting a professional or asking someone with experience for help.