Super Sencetive IR

[pixelsnpings]

Hey all.

I've been working on an extreamly sencetive IR detector using voltage devision to mark voltage down to the threshold level of a diode aprox - 0.7V.

Heres how its supposed to work...

The Voltage comming from the first voltage devider (R5-R4) is 0.07V
The Voltage comming from the second devider (R1-R2) is 0.63V

The voltage of both deviders therefore is 0.07V + 0.63V = 0.7V.

This should be enough to trigger the diode and turn on the transister.
fliping the VOut line from 0 to 1.

But when I put it to gether it fails to work.

Can someone please look to see if Im missing anything.

I would greatly appreciate any suggestions.

 

[Sceadwian]

Is the output always high, or always low?

 

[pixelsnpings]

Alwase Low. The diode is supposed to only activate the transistor when IR is detected. Even in the faintest of illumination.

 

[pixelsnpings]

Sorry, I have a bread board here and I try exactly what you said and it still failed.

 

[@MR]

in the schematic, if you replace a switch in the position of the phototransistor(with the switch open) you have a 0.63V acros R2. this will not trigger the diode. when the switch is closed, you have a parallel combinations of resistors, try to compute compute the equivalent value of shunt resistors then compute the voltage acros the anode of the diode...

 

[pixelsnpings]

Well thats kind of the point, I only want the diode to trigger when the 0.07V from the first divider is added to the 0.63V of the second. That should trigger the diode no?

Or if Im not understanding, please refrase your point.

 

[@MR]

you can compute for the the value of resistors r4 and r2 that will give a
0.7V drop across the emitter...

 

[@MR]

well the voltage will not be additive because its in parallel connection, its just like a two parallel bateris which wil give you the lesser voltagae. which means, in your circuit the voltage sent to the diode is 0.37V

 

[audioguru]

Hi Pixels,
Your phototransistor is connected backwards but it can't turn on the transistor anyway.

The diode needs about 0.7V and the base of the transistor needs an additional 0.7V so if the phototransistor provides 1.4v then the transistor would just barely begin to turn on.

Why not have the phototransistor directly turn on the transistor like in my schematic #1?
Why not have the phototransistor directly turn on the transistor that has voltage gain like in my schematic #2?

 

[TheVictim]

There is more bad news. Once you get the circuit to work as suggested, it still really isn't going to work. Just about any light is going to trigger your circuit, not just IR. With a good filter on your IR it will block most visible light, but you will need to adjust your circuit so ambient levels of IR from normal light sources in the area won't trigger it.

 

[pixelsnpings]

I had considered TheVictim's point. I thaught I might actually run an IR LED off of an oscillator and have the detector scan only for the specific frequency.

 

[gramo]

It's much more accurate to detect a modulated IR signal, for example, a signal that is pulsed high and low 38,000 times a second (38KHz). This would remove the ambient IR issues your circuit would face, as such a signal would rarely exist (if ever) by natural sources.

Such IR receiver’s already exist that have all the built in circuitry to detect this signal, and even have a logic output voltage (5V or 0V) if a signal is present/not present

I have just recently brought a few PNA4602M-ND ($1.28)from www.digikey.com that do that exact task.

**broken link removed**
~2.5 times larger than in real life​

I use a PWM output on a PIC to create the 38Khz signal, but you could use a 555 timer etc instead and drive an IR emitting diode that operates on the same IR Wavelength (940nm in this case). I used the QED233-ND ($0.38)

 

[Oznog]

I had considered TheVictim's point. I thaught I might actually run an IR LED off of an oscillator and have the detector scan only for the specific frequency.

Exactly.
I believe it is also important to make sure the filtering is done in analog and you should be sure the gain is not so high that the analog signal clips. For example, say you have a very high gain because you use a high resistance on the phototransistor pullup or put an opamp with 100x gain after a DC-blocking capacitor, before the 38KHz filter.

If it clips, the filter won't work. I mean say you've got a DC-blocking cap and all but there's a 38KHz signal along with a much stronger 2 Hz signal from the sun flickering through a window because the wind's blowing through the trees outside. The signal will go up to +5v and back down to 0v at 2Hz and the 38KHz signal will only show up briefly in the middle, that's not readable.

You'd need to make the filter, then your super-sensitive amplifier.

 

[gramo]

The IR Receiver I linked earlier has the following 'built in'

**broken link removed**

But I don't know how 'sensitive' you want to go

 

[@MR]

pixel,

consider audioguru's analysis, you cannot triger the both the diode and the transistor if you want only a triger 0.7V... you have two diode in series, first is the diode in your circuit and the base-emiter junction of the transistor. you need 1.4V to so that both will be triggered...

 

[Nigel Goodwin]

I've been working on an extreamly sencetive IR detector using voltage devision to mark voltage down to the threshold level of a diode aprox - 0.7V.

I think you need to start again from scratch!, assuming you could even get that to work it would be extremely UNSENSITIVE - but you really need to tell us what you're trying to do?.

 

[Ambient]

What is this going to be used for? That will help. There will ALWAYS be IR light around. If you make it too sensitive it will always detect it. Is this for security? I have a working circuit that will detect changes in IR (from 1 to ~80Hz). It will output a signal that can be amplified to saturation if you want it to. It is very sensitive, walking 4 feet past reflects enough IR to set it off. I have not tested range on it yet.

 

[TheVictim]

Modulating an infrared laser and adjusting the diffusion with a focusing lens will GREATLY increase the range. If you used a modulated IR diode array you can get a bit more range, but the laser is going to be your best option.

 

[pixelsnpings]

Thanks

I really appreciate all the pointers.

I've desided to go with an oscillating 40kHz IR signal.

I wrote a simple tuning app to help. All are invited to try it.