Popular

# ldr engineering

LDR Engineering

LDRÂ (Light dependent resistor) Engineering is a technique of making several electronics circuits following theÂ logic of a simple light andÂ dark sensorÂ using transistors, 555 IC andÂ 741Â IC. Out of several optical sensors, light dependent resistor can beÂ used for making light/dark sensors. A dark sensor switches on transistor orÂ LEDÂ wheneverÂ the incident light intensity is decreased, whereasÂ a light sensor works in opposite way. In LDRÂ Engineering, we first understand the working principle of any circuit using LDR, and then we replace the Â LDRÂ with a fixed resistor or some other electronic component that makes the simple automatic light/dark sensor work as a completely different device. Making automatic light/dark sensor circuits using different methods can make you observe several electronics components. On the other hand, modifying those automatic light/dark sensor circuits to some functionally different circuits like touch switch,water level indicator,Â clap switch, etc. can make you understand about those circuits. In this article, we will see how we can use the concept of a dark sensor to make a touch and aÂ clap switch.

Light dependent resistor:

Light dependent resistor(LDR), also called as Cadmium Sulphide (CdS) cell or photoresistorÂ is a resistor whose value depends upon the incident light intensity. In darkness, theÂ resistanceofÂ LDRÂ becomes high and in light, the resistance gets low. The resistance variation phenomenon of LDRÂ has been takenÂ as a base for making circuits ofÂ LDRÂ Engineering.

Circuit 1: Dark sensor usingÂ 555

The following circuit works as a dark sensor. If the light falling on LDRÂ is blocked, it switches on the LED D1. It has been configuredÂ in monostableÂ mode of 555 timer which means that the circuit switches on the LED D1Â for a specific time. The switching time is determined by theÂ combination of resistance R1Â and capacitor C1.

In this circuit, time (T)Â = 1.1xR1xC1 = 1.1x47x103x100x10-6= 5.17sec.

Circuit 2:Â Touch sensorÂ using 555

We can make the previous dark sensor circuit work as a touch sensor by making small changes in the schematic. We just need to remove the LDR, put a small wire and connect it to the coupling capacitor. Note that the electrode wire connected to capacitor C2Â should beÂ around 2-5cmÂ long. As we touch on the wire, the circuit triggers the LED for a specific time as determined by the resistor R1Â and capacitor C1. Â You can connect the wire to a small metallic touch plate using an alligator clip.

Circuit 3:Â Dark sensor using two transistors

Here, we have another dark sensor that works with two transistors. When light falls on LDR, its low resistance drives transistor Q1-BC547Â into conduction. This keeps transistor Q2 cut-off due to low baseÂ bias. The LED D1Â does not get power as long as ambient light falls on LDR. When the resistance ofÂ LDR becomes high in darkness, transistor Q1 stops conducting and transistor Q2 starts conducting to turn on the LED.

Circuit 4: Clap switch

In order to make a clap switch, we have to combine the previous touch sensor and the transistorizedÂ dark sensor circuits. We also need to connect a condenser microphone in place of LDRÂ and the collector of Q2 has to beÂ connected to the terminal of capacitor C2. Whenever there is loud sound produced near the condenser mic, pin 2 of 555 gets triggered and it switches on the LED D1. We can adjust the sensitivity ofÂ the circuit using a variable resistor in place of fixed resistor R5.

Circuit 5: Light sensor from dark sensor

If we follow the schematic of figure 6, we can convert the two transistor dark sensor to single transistor light sensor by removing R2, D1Â and Q2. The circuit works as a light sensor which switches off the LED wheneverÂ light falling on LDRÂ is blocked.

Circuit 6: Water level indicator
WARNING: NEVER TRY THIS CIRCUIT WITH AC POWER. USE 6V(4 AA size) OR 9V-PP3 Â battery FOR THIS EXPERIMENT.

Now, the light sensor can beÂ easily converted to a simple water level indicator. We just need to remove the LDR, and dip the wires into water. When both wires are dipped, the circuit switches on the LED D1. We can use copper electrodes for better performance, but for normal experiment, we donâ€™t need any electrode. VR1-470KÂ is for adjusting the sensitivity of the circuit.

>>>>

The experiments given in this article have been testedÂ and verified. We can make several other circuits using the idea of dark/light sensor. You can explore more LDR Engineering based circuits on buildcircuit.com.

## 7 thoughts on “ldr engineering”

1. ayo okunlola says:

i love dis site wish i culd get d company and come and learn more about electronics

2. aleksandar says:

i would like to make acircuit with ldr and 68ch12 micro, the function of ldr will be to change from logic 1 to logic 0 and via versa, depending on the light lux flow, which will trigger the micro and will display different numbers, using 7 segment. any ideas?

3. prinil das says:

iam searching a circuit for a line follower

4. Rajalakshmi says:

any other topics related to ldr or some basic electronics plz suggest me

5. i would like to make a circuit with ldr and 68ch12 micro, the function of ldr will be to change from logic 1 to logic 0 and via versa, depending on the light lux flow, which will trigger the micro and will display different numbers, using 7 segment. any ideas? – See more at: http://www.buildcircuit.com/ldr-engineer/#sthash.8IIrBDNw.dpuf

6. krystel says:

please do something with a 12 volts input, we’re going to connect it to 12 volts output of a solar panel

7. ankur says:

what is dmosfet