Transistor Amplified Capacitive Touch Switch

A touch switch is the simplest variation of a tactile sensor which allows us to use the body's capacitance as a switch allowing us to control operations of various components through touch. A common example of this that most people already have is a smartphone. A smartphone's touchscreen uses many touch switches to give us gesture control over our screens.

Similar to the first prototype of a touch switch invented in the early 1900s using vacuum tubes to amplify signals, this project will make a circuit that allows us to control an LED using the capacitance of our body as a switch. Since then, vacuum tubes have been replaced by a smaller important electronic component, the transistor, so as an improvement of the old vacuum tubes, we will be using transistors to amplify the signal. .

Step 1: Tools and Materials

• 2 pieces of BC337 or BC547
• Breadboard
• 4 pieces of AA battery
• AA holder for 4 batteries in series
• 1 piece of 5mm Red LED
• 100 Ω resistor
• 100K Ω Resistor
• Jumper Cables

Step 2: Circuitry

1. First, connect place both of the transistors, BC337, and the LED onto the breadboard such that the pins are in different places.
2. Label the transistor 1 and 2.
3. Connect the base of the transistor 2 to the emitter of transistor 1.
4. Then connect a 100K resistor to the collector of transistor 1.
5. Connect a 100Ω resistor to the collector of transistor 2, which then connects in series to an LED.
6. Connect the 100Ω resistor to the negative pin of the LED.
7. Connect the positive pin of the LED to the 100K resistor, then to a jumper wire for an open switch, all in parallel.
8. The other side of the switch is connected by a jumper wire on the base of transistor 1.
9. Connect the positive pin of the LED to the positive terminal of the 6V battery holder
10. Lastly, connect the negative terminal of the battery holder to the collector of the transistor to complete the circuit.

[Image Source: MAKE]

Step 3: Demonstration

Power the circuit by inserting all four of the batteries onto the battery holder. To test the circuit, use your finger as the switch and touch both ends of the open circuit jumper wires. This allows the transistor to amplify the signal to the LED, such that when I touch the two wires, the LED lights up. As shown in the picture, any material with some capacitance, like the human body touches the open circuit, acts as a switch due to the transistors.