This Capacitive Proximity Sensor Circuit works like an electronic watchdog that detects when someone comes nearby using electronic components.
Also, these kinds of circuits work in smart home devices, automatic lights and security systems; the circuit detects changes around it, especially when a person comes near and then it blinks a light or sounds an alarm.
Circuit Working:
Parts List:
| Components | 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 |
To begin with, this presence detector also called capacitive proximity sensor react when metal thing or person come close.
Here, P1 preset stays far from main detector circuit and uses to change how sensitive it is, even it does not see object moving still work good as proximity sensor.
We can change detect distance by turning P1 preset and one big use of this circuit is to open door automatic when put in front of door.
Furthermore, this presence detector have two parts: one monostable and other one is clapp oscillator with T1 transistor.
Sensor surface act like capacitor in oscillator circuit and clapp oscillator work around 1MHz because it give good frequency stable.
Also, P2 preset uses to set how long the switch stays ON.
Important not to put metal things near the circuit because it can make relay stay closed.
Hence, this circuit also can find strong liquids and good thing is sensor does not touch liquid directly.
Formula:
Normally transistor type capacitive proximity detector use change in capacitance when object come close to ground and sensor plate.
Transistor work like oscillator or amplifier to catch this change in capacitance and oscillator work at frequency f and we can guess this frequency with this formula:
f = 1 / 2πLC
where:
- f is frequency of oscillator.
- L is coil inductor in circuit.
- C is total capacitor value also include sensor capacitance (Cproximity).
When capacitance change because object come near it also changes oscillator frequency and by watching this frequency change we know if something is close to sensor or not.
Which resistor, capacitor or transistor we use, it all changes how good the sensor work.
Sensitivity stable working and how far it can detect all depend on these parts and maybe we need to adjust some parts to make it work better for our use.
How to Build:
To build a Capacitive Proximity Sensor Circuit follow the below mentioned steps:
Circuit Diagram:
- First, connect parts like in the schematic above.
Oscillator Part T1:
- Next, put R1, R2, C1 and T1 together to make clapp oscillator and keep capacitor C1 near the sensor place.
Monostable Part:
- Join R3, C2 and D1 to make monostable section and use preset P2 to set how long switch stays ON.
Preset P1:
- Then connect P1 so we can change how sensitive the sensor is.
Relay Connection:
- Also, before we connect power be sure voltage is right for the circuit and all parts.
- After that, turn ON the circuit and check if it working and use P1 and P2 to adjust sensor distance and time delay.
Important Notes:
- Follow the circuit diagram carefully and also double check all wire and part connections.
- Be careful with electricity and use correct voltage and during testing, do not put metal things near sensor it can mess up the working.
Conclusion:
To conclude, this type of Capacitive Proximity Sensor Circuit help in better safety, save energy and do auto work.
Also, parts and design may change based on what we need, so this is just a simple guide maybe we need to change things depending what parts we have.
Lastly, if anyone is not sure how to do, then better ask expert or someone who know electronics.
References:
Datasheet Transistors BF494 Datasheet Transistors BS170 Datasheet Transistor BS250