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I am looking to design a simple circuit that detect objects placed in a spot. So far, the circuit uses simple IR LED and photodiode to detect the object placed between them. The voltage developed in the junction of photodiode is fed to inverting input and Vref to the non inverting input of an opamp. The circuit works as expected, detecting the object placed with some adjustments in the pot.

schematic

simulate this circuit – Schematic created using CircuitLab

My intention is to make an LED blink (not shown in the schematic) continuously (like driving using square waveform or some sort) whenever the object placed in between the IR LED and photodiode is removed. Well so far the op amp gives a steady output which is enough to light up the LED, but not blink.

I have considered the options of using a monostable multivibrator in the next stage using the opamp output as trigger, but unfortunately when the object is removed, the opamp gives high output until the object is placed back, which again will make the multivibrator to stay in a single logic state and again will not be enough to make the LED blink continuously.

I am looking to implement this without the use of any microcontroller. But right now, I am clueless on how to proceed and what to use to achieve the continuous LED blink as output.

Any help or any leads will be very much appreciated.

Frank Donald
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    Relaxation oscillator comes to mind. – jonk Jan 08 '19 at 14:33
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    Your question is not fully clear. You want to modulate the IR light to start with? – winny Jan 08 '19 at 14:40
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    Buy a blinking LED. There are LEDs that have blinking circuitry built in to them. – Elliot Alderson Jan 08 '19 at 15:11
  • @jonk Can you kindly brief a bit please. As far as I knew Relaxation oscillator works without any trigger as well. So how can I make the oscillator generate output only when output from this piece of circuit is high? – Frank Donald Jan 09 '19 at 05:36
  • @winny No I don't want to modulate the IR signal. I want to use the IR LED and photodiode combination to detect the objects placed between them. – Frank Donald Jan 09 '19 at 05:37
  • @FrankDonald It's really simple. These can be done with discrete parts or ICs. For example, with discrete parts you need a resistor and capacitor. As the capacitor charges up, it's voltage reaches a point where a "trigger voltage" occurs. This can be done with an PUJT, for example, or some BJTs arranged correctly. Either way, once this voltage is reached the circuit "fires" long enough to discharge the capacitor "mostly" and the charging starts over again. So when the output of your circuit is high, the circuit will fire over and over and over again until it goes low. – jonk Jan 09 '19 at 05:39
  • @Elliot That's an interesting suggestion. I have done a quick search but most of the blinking circuits is based on Astable multivibrator setup and starts working when power is turned on. But I need the LED to start blink only when the output from opamp gives high output. – Frank Donald Jan 09 '19 at 05:43
  • @FrankDonald See: [full circuit example -- ignore the optodiodes](https://electronics.stackexchange.com/a/364818/38098) and [basic idea, discrete](https://electronics.stackexchange.com/a/344346/38098). That provides some idea about how it might look with BJTs. A PUJT would make this much simpler. But those are rarer to find. If those examples aren't attractive, I can suggest some IC approaches. – jonk Jan 09 '19 at 05:43
  • So where does the square wave modulation come into play? Can you draw a picture/block diagram? – winny Jan 09 '19 at 08:04
  • @jonk I have checked out the answers you have provided, thanks for that. But am still unclear with it. Where should I feed the high signal from Object detection circuit ( given in the question) into the circuit given as example by yourself. Can you explain further with appropriate circuit please if it's not too much to ask? Or Can you brief me about IC approach please ? – Frank Donald Jan 10 '19 at 07:53
  • @winny I can see why we have this confusion. The above circuit given in my question constitutes of IR LED and Photodiode together with Opamp to detect any object and gives high output. So I am looking for a circuit where I can use this Output from Opamp to drive a LED and keep it blinking rather than being lit all the time. – Frank Donald Jan 10 '19 at 08:07
  • @FrankDonald Going to sleep for now. But I'll consider things in the morning and see if there's anything I can contribute further. – jonk Jan 10 '19 at 08:39
  • Oh! That's easy! Blinking LED, relaxation oscillator, 555 or anything similar which has been suggested above would do the trick. – winny Jan 10 '19 at 08:50
  • @Jonk and Winny Thanks for the advice. I got the hint as UJT. Am totally new to UJT, have read some tutorials in Internet and managed to put on this basic schematic design https://imgur.com/sbNQRxi in order make the LED's flash. Can you kindly take a look and advice on this :) It's just that I checked with my local vendor and seems like he don't have UJT's and I want to order them online. Would be great to know whether I am heading in the right direction before buying them. – Frank Donald Jan 13 '19 at 10:22
  • @FrankDonald (1) There's a difference between a UJT and a PUJT (or PUT.) So be careful with terminology there. The 2N6027 (also 2N6028) you selected is a PUJT, which is what I was suggesting. So that's good, in that you didn't select a UJT but a PUJT. Nice catch! But you also selected the wrong symbol for your schematic. You selected the UJT symbol. So there you made a mistake in communication. (2) You probably can't put the LED itself into the PUJT cathode itself. And the problem is still more complicated, having to do with pulse stretching, I think, for the LED to be observable as pulsing. – jonk Jan 13 '19 at 18:55

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