Introduction: Touch Sensor Circuit
A Touch sensor is a device that detects physical touch. Touch sensors are making their way into many applications like mobile phones, remote controls, control panels, etc. Present-day touch sensors can replace mechanical buttons and switches.
How does a simple touch sensor work?
The working of a touch sensor is similar to that of a simple switch. When a human touches the wires, the human body acts as a resistor and the circuit is closed ( there is a flow of current). When there is no touch, there will be very high resistance that we could assume R=∞ and that means the circuit is opened, and no current flows.
Human Body Resistance:usually ranges from 1kΩ to 100kΩ depending on some conditions.
Basic Components:
- LED
- NPN-Transistor
- +9v Battery
- Breadboard
- 330Ω Resistor
- 2 Copper wires
Step 1: Let's Figure Out Our Circuit Schematic Diagram
- The Circuit shown is open circuit, but as we explained earlier once a human touches the wires the circuit is closed as the resistance between the wires decreases.
-From approximately ∞ to (100kΩ:1kΩ)
- Current flows from the base of the npn transistor to the Emitter and there will be like an imaginary wire between the collector and the emitter that will close the circuit and makes the LED ON.
*Note that the base current is too small to harm or shock humans but this small current will be amplified by the npn transistor and there will be enough current to turn the LED on.
Let's simulate the circuit in the following step.
Step 2: How Is the Breadboard Connected?
The breadboard has strips of metal underneath the board and connects the holes on the top of the board. The metal strips are laid out. Note that the top and bottom rows of holes are connected horizontally and split in the middle while the remaining holes are connected vertically.
Check the above video for a better understanding, Or skip if you're already familiar with using breadboards.
Step 3:
Connect the circuit as shown in Step 1.
- Connect the Emitter to the Ground.
- Connect the collector to the cathode of the LED
- Connect the 330Ω resistor to the anode of the LED on one side and the other to the +9v battery
- Connect the base of the transistor and the point at which the collector and LED's cathode meet with copper wires.
- Touch the Wires with your fingers and you will see the LED on!
For software simulation, you can replace your touch with a suitable resistor.
