Let’s Consider a Circuit in Which Light Bulb Is Connected to A Battery Using Two Wires.
Positive Terminal of The Battery Is Connected to One Side of The Bulb and the Negative Terminal Is Connected to the Other Side of The Bulb. Now Current Flows from The Battery to The Light Bulb and Back to the Battery. That Is all It Takes to Light the Bulb.
Now If You Want to Switch ON or OFF the Bulb this Can Be Done in Two Ways.
1. The First Way Is to Install Switch on The Wire that Runs from the Positive Terminal of the Battery to the Light Bulb and
2. The Second Way Is to Install Switch on the Wire that Runs from the Negative Terminal of the Battery to the Light Bulb.
Now Switch1 Is Said to Switch the Positive Side of The Circuit, While Switch2 Is Said to Switch the Negative Side of The Circuit.
Here The Bulb Connected in The Circuit Is the Load. By Switching the Positive Side of the Circuit or By Switching the Negative Side of The Circuit the Connected Load Can Be Controlled.
In Proximity Sensors Bipolar Junction Transistor are Used for this Positive Side Switching or Negative Side Switching of the Circuit and this Switching Helps to Control the Load Connected to the Proximity Sensor.
NPN Transistor & PNP Transistor.
Bipolar Junction Transistor Are of Two Types:
An NPN Transistor Is Constructed by Sandwiching a P Type Semiconductor Between Two N Type Semiconductors and
PNP Transistor Is Constructed by Sandwiching a N Type Semiconductor Between Two P Type Semiconductors.
The Transistor Has Three Terminals Namely
PNP Transistor Is Used for Switching Positive Side of the Circuit and
NPN Transistor Is Used for Switching the Negative Side of the Circuit.
Let’s Now Discuss the
Construction of PNP and NPN Proximity Sensors
PNP Or NPN Proximity Sensor Consists of a Proximity Sensor Circuit and a PNP or NPN Transistor Circuit. The Purpose of Sensor Circuit or Proximity Sensor Circuit Is to Detect Any Nearby Object.
When The Sensor Circuit Detects an Object,
It Sends an Output High Signal to The Transistor Circuit. The Transistor Based Circuit May Use PNP or NPN Transistors and It depends on The Application That Sensor Is Used For.
PNP And NPN Has Nothing to Do with Whether the Sensor Is Normally Open or Normally Closed, That Means a PNP Sensor May Be Either Normally Open or Normally Closed. Likewise, an NPN Sensor Can Also Be Normally Open or Normally Closed.
Now Let’s Discuss about a 3 Wire PNP Proximity Sensor.
3 Wire PNP Proximity Sensor
A 3 Wire PNP Proximity Sensor Proximity Sensor Consists of Three Wires in Which,
the Brown Wire is Used to Connect the Positive Voltage,
the Blue Wire Is Used to Connect the Negative Voltage or The Ground and the Black Wire Is the Sensor Signal Output Wire.
In a PNP Proximity Sensor We Know That a PNP Transistor Is Used for Positive Switching, So If PNP Transistor Is Installed Between the Positive Voltage Wire and Sensor Signal Output Wire.
As Negative Voltage Will Be Common Between Sensor and Load, Connect Negative Voltage Wire to Sensor and the Load. Signal Output Wire of the Sensor Is Connected to the Input of the Load. A Load Can Be Any Device that the Sensor Powers Like a Lamp, Pneumatic Valve, Relay or PLC.
In This Circuit Let’s Consider a Bulb as Load Which Need to glow When a Metal Object Is Detected and Switch Off When No Object Is Present.
When a PNP Proximity Sensor Doesn’t Detect Any Metal Object the Signal Pin Output Will Be Low and So the Transistor and The Bulb Will Be Off.
When a Metal Object Is Detected, the Sensor Will Output a Voltage of almost Nearly the Same as The Voltage Fed into The Positive Voltage Pin.
Hence the Transistor Will Be on And So the Bulb Will Be Switched On.
Now Let’s Discuss about 3 Wire NPN Proximity Sensor.
3 Wire NPN Proximity Sensor.
3 Wire NPN Proximity Sensor Consists of Three Wires.
As Discussed Earlier the Brown Wire Is Used to Connect the Positive Voltage, Blue wire is Used to Connect the Ground and the Black Wire Is the Signal Output Wire.
In an NPN Proximity Sensor We Know that NPN Transistor Is Used for Negative Switching, So an NPN Transistor Is Installed Between the Negative Voltage Wire and the Sensor Signal Output Wire.
As Positive Voltage Will Be Common Between the Sensor and the Load, Connect Positive Voltage from the Power Source to the Sensor and the Load.
A Permanent Positive Supply Will Be Connected to the Load that Is to Be Activated. Signal Output Wire of The Sensor Will Be Connected to The Input of The Load.
NPN Proximity Sensors Are Different from PNP Proximity Sensors. NPN Transistors Are on Naturally, That Means the Emit High Signal When No Metal Object Is Detected and Goes OFF or Low, When an Object Is Detected.
NPN Requires More Circuits in Order to Switch ON When an Object Is Detected and Switch OFF When Object Is Not Detected. Usually, An Inverter Circuit Is Used to Make the High Output of The NPN Low when there Is No Object and Also to Make the Low Output of NPN High, when an Object Is Present.
Let’s Consider a Bulb as Load Which Need to Glow When a Metal Object Is Detected and Switch OFF when No Object Is Present.
When No Object Is Detected as NPN Transistors are Naturally On, they Emit a High Signal on the Signal Line, that has a Voltage Just above the Voltage Fed into the Positive Voltage Pin, that Is the Brown Pin.
This High Signal Is Fed into The Input of An Inverter and the Inverter Inverts It into a Low Signal. Thus, With this Low Voltage the Bulb Cannot Be Powered ON.
When a Metal Object Is Detected, a Low Signal Is Emitted on the Signal Line of The Transistor. This Low Signal Is Fed into the Input of An Inverter and Inverter Inverts It into a High Signal. Thus, With this High Voltage the Bulb Is Powered ON.