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.
i used LDR for light detection..is work in night only & only senses torch light only….i required to sense sun light…so what i use in place of LDR…reply
Here you can use this Tool to find
the time out
The Timeout Of Monostable Multivibrator Calculator
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?
How touch sensor works?
can i use a PIR sensor instead of an LDR? (Dark sensor using 555(based on Monostable mode))
no
Can u tell me the voltage rating of 100 micro farad capacitor intouch sensor
This circuit can be simulated on:
http://www.cirvirlab.com/simulation/ne555_monostable_multivibrator_online.php
in order to see waveforms and influence of R and C value as well as time position of triggering signal.
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 them.ie interchange the ldr with 100k variable resistor.
i need to let the led lit for 2 seconds
.plz suggest the r1 and c1
i want to construct a bulb by using sensor
What program you are using to draw schematic
Mode of NE555- Monostable | BUILD CIRCUIT
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I want to use 4.8 V light bulb. What do I have to change in the circuit?
Thanks
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