Mode of NE555- Monostable


Monostable mode of operation

Monostable mode of operation of 555 IC can be explained using a simple touch sensitive circuit. Here, timing constant (T) of monostable timer= 1.1*R1*C1. The following circuit has been configured in monostable mode. A monostable circuit produces a single pulse(for a certain duration determined by the Timing Constant(T)=1.1*R1*C1) when triggered at pin 2.

Touch sensor based on monostable mode of 555 Timer

Following circuit shows the working of Touch sensor. It operates in monostable mode. Here we can see resistor 100K and capacitor 100uF determine the monostable timing of the circuit. In place of touch plate, we can use a small piece of wire.Take a wire around 2-5cm long. Longer than 5cm may not work.

Timing(T)= 1.1*47K*100uF = 5.17 seconds

Dark sensor using 555(based on Monostable mode)

This circuit shows how we can convert a simple monostable mode circuit to “dark sensor”.We just need to add an LDR and a resistor in the circuit, then it simply works as dark detector.

When you block light falling on LDR, the circuit switches on the LED for a specific time determined by the values of R1 and C1.

Here, Timing(T)=1.1*47K*100uF = 5.17 seconds

You might have noticed that we can use the same components for making dark sensor as well as touch sensor.

Optimized Dark Sensor

In place of R3-4.7K, if you use a variable resistor rated 100K, you can adjust the sensitivity of the circuit. Besides, use capacitor C1-0.01uF and resistor R1- 68K in place of 100uF and 100K respectively . That circuit would be better than the previous circuit.

Here, Timing(T)=1.1*68K*0.01uF = 0.000748 seconds. So, as soon as you let light fall on LDR, the LED- D1 switches off.

From Schematic to PCB Production, Just Need One Tool - EasyEDA
Free Circuit Design Software:  learn to use Editor quickly and easily Cheapest PCB: 10pcs 2-layers 10cm×10cm only $8.21, 3-day build time, 100% E-test


  1. i used LDR for light work in night only & only senses torch light only….i required to sense sun light…so what i use in place of LDR…reply

  2. this really does work but the time we set is not constant on every attempt…sometimes it glows for very long time..can u kindly tell me why is that so?

  3. hello… can you kindly tell me how to convert this dark detector circuit into light detector circuit… which part and components that i need to change?? tq

    • Just interchange the LDR and the variable resistor that is connected in series and trigger input is connected between interchange the ldr with 100k variable resistor.

  4. here i have interfaced the relay parallel to the led and 220 ohm resistance ……..relay gets activated but do not get turned off after taking my finger off…….plzz help