Tag Archives: build your own infrared sensor

Sensing Someone Around

This 3rd article in the series of “Do It Yourself: Electronics”, takes you through the basics of infra red sensing using an IR LED and an IR diode.

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Early morning, Pugs went to the ECE Department to meet Surya, who was creating some experiments for his juniors.

“What are you doing at this time in your department lab?”, exclaimed Pugs to Surya.

“Setting up a robot framework for our ECE juniors, for the robotic showcase planned for today afternoon”, replied Surya

“Interesting. I see that you are also using the IR transmitter and the IR receiver, you asked me to buy. By the name itself, I can make out that one would transmit IR signals, and other would receive the same. But what could we use them for?”

“These can be used for a wide range of applications …”

“To me it seems like another light sensor. Sun also emits IR. So, may be we can detect presence of sun”, interrupted Pugs.

“Why only sun? Even our bodies are good source of IR”, added Surya.

“Okay. So, do we detect sunlight, living beings, etc using the IR receiver? But then what is the IR transmitter for?”

“We can detect, but for that we may need more sophistication to eliminate the unwanted IRs. Anyways, the purpose here is not that. It is more of obstacle detection, and that’s where, we need this IR transmitter, as well.”

“I see. So, how do these IR Tx & Rx pair work?”

“Let me just show you a small working demo of the same.”

With that Surya picked up the various stuff needed, from around the lab, and showed the following:

Pugs went to his room & rebuilt the circuit. But to his surprise, the LED was always on. He tried debugging it, as in his previous light sensing circuit, but in vain. So, in frustration, he called up Surya and said, “Surya, your debugging gyaan is not working out. I tried all your suggested debugging tricks, but not able to figure out the problem in my circuit”.

“What happened? Have you powered up your circuit?”, queried Surya.

“Yes obviously. And that’s what the problem is. In my circuit, the LED is now always powered on”, replied Pugs with a sarcastic voice.

“Where have you put the circuit?”, asked Surya.

“On my table. Where else?”

“Put it below the table, and check out.”

“Why are you joking with me? How does it matter, whether I put it above or below the table?”

“Just try it, na”, emphasized Surya.

So, unwillingly Pugs put his circuit below the table, and viola the LED went off. And then, when he hovered his hand over the circuit, the LED switched on. He couldn’t believe himself. So, he tried the same multiple times. Then, he also put back the circuit on the table to check the problem, again. And yes, it was there. But all fine, when below the table.

“What happened Pugs? Did it work?”, asked Surya, as he didn’t receive any response from Pugs for a while.

“Yaaa. It is working. But how?”

“Now as it is working, I can explain you. Remember, you only told me that sun also emits IR, and your table is near the window. So, the IR receiver was always getting the IR, and hence LED was always on.”

“A ha! That’s cool. So, when I put it below the table – no IR from the sun – and things worked. That’s a great insight, that when we are playing with real world stuff, we need to keep our minds and thoughts open to all possibilities.”

“In fact, there is a simpler way to fix this problem. But I thought, why not have this fun experience.”

“What’s that?”, asked Pugs curiously.

“You may just adjust the pot to appropriately set the threshold value, depending on where you are keeping your circuit.”

“O ya! That’s what we have put the pot for.”

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