Unlike mechanical switches, a touch switch has no moving parts, making it more durable and reliable. The MMBF170, a small-signal N-channel MOSFET, is ideal for this project due to its high input impedance and ability to function effectively in low-power applications.
Let’s dive into the details and build our own touch-sensitive switch!
Project Overview
This touch switch circuit will activate when you place your finger on a metal touchpad. The MOSFET will detect the small voltage change caused by your touch and use it to control an output device, such as an LED. The circuit is simple, requiring only a handful of components.What You’ll Need
● MMBF170 MOSFET – The heart of the circuit, responsible for detecting touch.● 1MΩ resistor – Helps set the sensitivity of the circuit.
● 10kΩ resistor – Provides a pull-down function to stabilize the circuit.
● LED – The output indicator.
● Metal touchpad – This could be a small coin, a piece of aluminum foil, or a custom metal plate.
● Battery (3V–9V) – Powers the circuit.
● Breadboard & jumper wires – For easy assembly and testing.
How It Works
When you touch the metal pad, your body acts as a conductor, transferring a small voltage to the gate of the MMBF170. This turns the MOSFET on, allowing current to flow and activating the LED. Once you remove your finger, the MOSFET turns off, and the LED goes out.Step-by-Step Assembly
Step 1: Preparing the Components
Gather all the components and ensure you have a clean workspace. If you’re using a breadboard, make sure it’s in good condition to avoid poor connections.Step 2: Placing the MOSFET on the Breadboard
Identify the three pins of the MMBF170 MOSFET:● Gate (G) – The control input.
● Drain (D) – The output that connects to the load.
● Source (S) – The ground connection.
Place the MOSFET in the middle of the breadboard with enough space to connect the other components.
Step 3: Connecting the Touchpad
● Attach one side of the 1MΩ resistor to the gate of the MOSFET.● Connect the other side of the resistor to the metal touchpad (this will act as our sensor).
Step 4: Adding a Pull-Down Resistor
To ensure the circuit resets properly, connect a 10kΩ resistor between the gate and source of the MOSFET. This prevents accidental activation due to stray signals.Step 5: Connecting the Output (LED)
● Connect the drain of the MOSFET to the negative leg of the LED.● Connect the positive leg of the LED to the positive terminal of the battery.
● Connect the source of the MOSFET to the negative terminal of the battery (ground).
Step 6: Powering the Circuit
Attach the battery to the circuit. Double-check all connections to ensure nothing is shorted.Testing the Touch Switch
- Touch the metal plate. The LED should light up, indicating that the MOSFET has switched on.
- Remove your finger, and the LED should turn off.
- If the circuit doesn’t work, check your connections, particularly the gate resistor and power supply.
Project Enhancements
Once your basic touch switch is working, consider these improvements:● Use a relay: Control higher voltage devices like lamps or fans.
● Add a delay circuit: Keep the LED on for a few seconds even after removing your finger.
● Increase sensitivity: Experiment with different resistor values to adjust the response.
Conclusion
This simple MOSFET-based touch switch is a great introduction to using the MMBF170 in real-world applications. It demonstrates how small electrical signals can be used to control larger loads without mechanical switches.With some modifications, you can integrate this circuit into home automation projects, interactive art installations, or even wearable electronics. Try it out, experiment with different setups, and have fun exploring the world of electronics!
US 13793 
GB 11618 
CA 9609 
AU 8941 
IE 5194 
BR 2379 
NZ 1933 
SG 1152 
