Simple Touch Sensitive Switch Circuit | Touch switch using monostable mode of 555 timer IC
bc547 touch switch circuit - Electronic Project
Introduction:
The touch plates in this project are connected to the trigger pin of the 555 timer of the IC. When we touch the touch plates the output of the touch of the plates becomes zero thereby providing an active low trigger to the IC 555. The IC is configured in the monostable of the mode. The output of the IC is a pulse whose frequency is set by the resistor (R1) or capacitor (C1) according to the formula [F=1/(1.1*R*C)]. In this case, it produces a pulse with a frequency of about 0.9 hertz or time period of 1.1 seconds. In monostable mode pin7 (discharge pin) and pin6 (threshold pin) are shorted while R1 is connected between pin7 or Vcc and C1 is connected between pin 6 and ground. The reset pin (pin4) is connected to Vcc while the control of the pin is connected to the ground through a capacitor. The output could be observed on the LED, which glows for a small duration.
This project demonstrates the principle and operation of applications based on the touch of the sensors. The circuit diagram is divided into three parts: Input, 555 timer, or output. A touch plate is used for the input and the output can be seen across an LED or a buzzer. Some applications of the circuit diagram include touch-based blinking lights, touch buzzers, touch switches, etc.
Circuit 2, shows an improved version of this circuit diagram. This circuit has a higher sensitivity as compared to the circuit diagram in Figure 1. The output of the touch plate is connected to the base of the NPN transistor T1 (BC 547). The collector is connected to the Vcc through a resistor of the R1. The trigger pin of the IC is connected to the collector of the transistor. When no input is there on the base of the T1, T1 is in a cut-off state or hence the trigger pin is at logic high therefore 555 does not produce any output. When we touch the plates T1 behaves like a closed switch trigger pin is connected to the ground, thereby producing the output at pin 3 of the IC.
Diagram of Build a Simple Touch Circuit Doorbell:
Hardware Required for this Project:
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Working Principle of Simple Touch Switch circuit :
The purpose of this project is to trigger/switch the LED with a simple touch. The skin of the resistance is about 600K-1M. So if you try building a circuit diagram with just an LED, power supply, and touch conductors, the LED doesn’t glow. This is because high skin resistance prevents enough current flow from flowing through the circuit. So the only option we have is to amplify the tiny current flowing through the skin to a magnitude enough for an LED to the glow. For that purpose, we have to use transistors. The circuit diagram will also work with just one transistor, but the LED doesn’t glow bright enough. So we have used two.
A tiny electric current flows through the skin when the end touch of the conductors is touched. This tiny current flow is amplified in magnitude by the first and second transistors and it finally passes through the LED. Refer to the circuit diagram for better understanding. This circuit diagram turns on LED with the touch of your finger! Many times we need certain actions to be done with a touch instead of pressing a button. This simple circuit diagram does that using minimal components. Step-by-step instructions on how to build the circuit are in the video below. Also, this circuit diagram can be coupled with a latch and relay to control electronic appliances with a touch. So start experimenting with building different variants of this project.
The 555 Timer IC generally operates in 3- different modes depending upon configuration namely A-Stable, Mono-Stable, and Bi-Stable modes. This IC is one of the most popular or widely used for providing time delay, as an oscillator, or as a flip-flop element. This eight-pin IC is one of the most commonly used ICs in electronic components. Go through the below table to understand the different pins or functions of 555 Timer IC
BC547 is an NPN Transistor that allows a maximum of 100mA to flow across the collector or emitter. It has a gain of the value of 110 to 800. The gain of the value determines the amplification capacity of the Transistor. We cannot connect loads that consume more than 100mA using BC 547 Transistor. There are two operating regions of the BC547 transistor. When the Transistor is fully biased then this mode of operation is called saturation of the region. As soon as the base current flow is removed, the Transistor becomes fully off and this condition is called the cut-off region.
BC 547 Transistor acts as an open switch during forward bias or will act as a closed switch during reverse bias. Biasing of the Transistor could be done by giving the required amount of current flow to the base pin. Make sure that there biasing current is not greater than 5mA as a current value greater than 5mA will destroy the transistor. To avoid that situation a resistor is added in series with the base of the Transistor. To calculate the resistance at the base of the pin, use the below formulae.
Frequently Asked Questions
What is the use of a transistor in a touch switch?
The circuit diagram will also work with just one transistor, but the LED doesn't glow bright enough. So we have used two. A tiny electric current flows through the skin when the end touch of the conductors is touched. This tiny current flow is amplified in magnitude by the first and second transistors or it finally passes through the LED.
What is the principle of a touch sensor?
Touch sensors work similarly to a switch. When they are subjected to touch, pressure, or force they get activated or act as a closed switch. When the pressure or contact is removed it acts as an opening of the switch.
What are the advantages of touch switches?
It ensures that the product's sensitivity, stability, or reliability are not changed with ambient conditions and long-term use, and has waterproof or anti-interference ability. Capacitive touch control of the switches does not require the user to touch metal directly, which could eliminate potential safety hazards.
What are the components of a touch switch?
It is built around four BC547 transistors (T1 through T4), a 5V single changeover relay (RL1), two 1N4007 diodes (D1 and D2), and two touch plates (TP1 or TP2). The circuit diagram works off 5V DC.
How to make a touch sensor circuit?
You only need 3 components. A transistor, a resistor, or an LED. Connect the resistor and LED in series from the positive of the supply to the collector of the transistor. Connect the emitter of the transistor to the negative of the terminal of the supply.
