Ok , call me thick!!! Im trying to build a circuit that uses a IR beam break detector, Im trying to get my head round the IR's and how they work. I set up a little start circuit to try to understand this as below 12v------204 ohm resistor---------IR Transmittor--------0v | | | | |----IR Reciver---------| I put a voltmeter from the negative lead of the IR reciver to the earth to see what voltage come out when the beam is on or broken, so I could make sure it operates a transistor. But its zero volts. I thought IR's work on like a internal resistance that changes when the beam is on of off. Where am I going wrong guys and girls??? Thanks Russ Hmm, that picture out come out right, but basically the reciever is connected between the positive and negative of the transmitter, in parallel
The receiver is possibly an NPN opto-transistor, if so you need a resistor between collector and +Vs or you'll burn it out. Do you have part numbers?
Or in plainer English, it will take 30v max, and the max current allowed through it is 20mA. You can ignore the 300mV or so voltage drop across the "on" transistor, and just calculate the load resistor as SupplyV/.015 (for 15mA), so 820R for 12v, 330R for 5v, between +V and collector pin. You could add a led in series as well to see when it works.
errr, bit lost'ish So the 204 ohm resistor I used was to small and has probably blew the IR components??? The supply adaptor Im using is a 12V 0.8 amp supply?? The 30 V, do you mean I can just pump 12v stright through the IR Reciever
You've got two totally separate components. They are not the same. Don't get them mixed. One is the transmitter which is an IR LED that needs a series resistor to limit the forward current to around 50mA (so 200R is OK on 12v supply). The other is the receiver, an IR photo-transistor that needs a series resistor to limit collector current to around 15mA (so 820R is OK on 12v.) If they've got a code printed on them, find the complete datasheet, might give some basic circuits.
I see, I was treating them as the same, thats where was going wrong. So with the transmiter, you need to limit the voltage to it, that same as an led, and so you use the equation of 12-1.8 / 0.05 (12v supply, 1.8v led, 50mA led) With the reciever, you need to limit the current, and so use 12 / 0.015 (12v supply, 15mA IR reciever) Is that correct????
oh, and one more thing, Im using a 12v adaptor, but when I measured the voltage acrossd it, it was pumping out 16v ish, is that ok, do I take it as 12 or 16
Cos it's a transistor, not a led. Voltage drop Vce(sat) is around 0.3v so use it if you're picky. Your unregulated adaptor supply will give more than 12v at loads below whatever current rating it is, regulated wall-warts are safer.
Hi, Ive set up the circuit on a bredboard to test it using an LED like you suggested. How ever, the LED come on, but nothing happens when I break the beam, so Im presumming the beam system aint working. Do you have any ideas what could be wrong, or any ways I could cheak the IR's are working and not broken
Ok, I think I have fianlly got this circuit working, however, its not quite how I want it. Ive used a transistor, in a similar set up to this.....IR Reciever Circuit But Ive used a 1K variable resistor. First, The beam only seems to break, or have any effect on the led, when I cover the whole of the IR reciever with my fingers!!! Yet if I just pass something between the middle or cover the tranmitter, nothing happens. How come??? Second, When the beam breaks (by coverig the whole of the reciever) the LED only dim's slightly, that all that happens, How do i get it so that output of the transistor (operated by the IR reciever) is either on or off, not dimming!!! (I.E, I think its to give a digital out put instead of an analoge!!! Think thats right!!!) Thanks all Russ
ust to make things a little easier to read (and to keep it on one page for for longer) try to edit your previous post if is the last one on the page. As for your problem, the reciever might be for a wide angle, see if you can find one for a narrow angle, or a IRLED with a narrow angle, just make sure the wavelength matches the old one. as for the dimming problem, not sure there, but it might be that you aren't cvoering it enough, what happens if you cut the power to the transmitter, does the led turn all the way off, or just dim?
The receiver is wide-angle (70° ), looks like it's picking up lots of stray infra-red from your lights, etc. If it doesn't have a built-in filter you may need one, also could try mounting the receiver down a tube so it can only see straight out. On the dimming, the circuit you link is emitter-follower, so it will. For on-off you'll need a transistor switch and a sensitivity setting, or a comparator design. Good news is, both bits seem to be working.
Got you, is the 2n3904 transistor Im using not a switching transistor then, how would I set up a switching circuit, and what value variable resistor should I use for sensitivity any advice or help
