Photodiode circuit help

Hello all,

I'm new to this forum, but I started with a search for my topic, but didn't find anything usefull to me.

I want to build a sensor, that detects the presense of a RC controlled car in motion, to determine the speed of the car. There will be 2 sensors with a certain distance, providing a PIC with a wired signal, telling the car has passed - the PIC will then calculate the speed from the time between signals from the sensors and the distance between the sensors.

I have access to IR emitters and photodiodes:

IR emitter 880NM 5mm 100mA
IR detector 880NM 455KHZ

I have no datasheet for the emitters, but I do have one for the detector, you can find it here:

**broken link removed**

I don't know how these things work, but I think that the photodiode detector will conduct current when it's exposed to infrared light with a wavelenght of 880nm. I'm not sure what the frequency of 455KHz means, or which pins are anode and cathode.

I'm thinking of 2 ways to build my setup:

1. A proximity detector type, that detects a nearby object (the RC car) where the emitter and detector are placed side-by-side.

2. A burglar alarm type, that detects the absense of the infrared beam i.e. when the car "breaks" the beam - the emitter will have to be placed 2-3 meters from the detector since the RC car will be passing with 50-60mph and the driver should be able to avoid the circuits. The emitter should then have it's own battery supply.

I like no. 1 the most, since it's the easiest setup in the field where the measurement will take place. But I don't know what the range of this could be - it should be at least 1 meter (again, the drivers skills should be comsidered).

I hope you have lots of suggestions with links, circuit drawings and explanations, since I'm a newbie to IR.


Regards,
Futterama

u have to place your car in between transmeter and reciver
for sensing the car .
u can use 555 for genrating 20K frequency which will do fine for transmiter and u have to convert voltage level for pic programing

RC Car speed trap

It looks like this could be a simple IR emitter and detector beam system, DC to the emitter, point it at the detector and watch for a loss of signal as the beam is broken.
How about setting the emitter and detector side-by-side (as your first idea) and bounce the beam off a mirror on the other side of the track so the beam is broken by a car (your second idea) but all the electronics is at one side of the track ?

The emitter, detector and mirror will probably require sheilding from stray light (a cardboard tube?) to ensure no stray light will cause problems.

Hi,

The idea about the mirror is great, but is it necessary? Shouldn't the beam be reflected by the car itself?

Look at this circuit:
https://www.electronic-circuits-diagrams.com/alarmsimages/19.gif
and the circuit explanation here:
https://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt19.shtml

The author suggests that a nearby object would reflect the beam, but it doesn't say what "nearby" is (10cm or 1 meter?)...

And that brings up another question: Why is there 2 emitter diodes in series in some circuits? Is it to output more "IR power"?

Well, I think I'm beginning to understand what IR is about, I'll try building a circuit and see how it behaves.

More suggestions and explanations are more than welcome


Regards,
Futterama

Edit: Whats with the asterisk * in the URL text?? Is it a hush word?

Speed is defined as the time it takes for one to go from point A to point B.

In your case, each distance is given a unique resistance. The farther away, the greater the resistance.

What you want to do is make or use comparator circuits that will allow only certain resistances pass. You need to assign each resistance to a binary value. Once a desired resistance (or distance) is obtained, a clock needs to start. As soon as another desired resistance (or distance) is reached, the clock is stopped, and the value comes from the clock.

For example, if the distance between point A and point B is 1km, and it took 1 hour to go from A to B, then you can say that the speed is 1km/h.

mstechca: What???

Distance and resistance, what are you talking about? Where do you get the resistance from?

I'll simplify my answer.

An LDR (CDS cell) is a Light-dependent resistor.
Resistance varies with distance.

Now, get yourself a source of bright light (use a lightbulb if you have to), a very dark room, and an ohmmeter. A DMM (digital multimeter) normally has an ohmmeter built in. Connect the LDR to the multimeter. You should see numbers changing. Now move the light closer to the multimeter. The numbers should decrease. Move the light away. The numbers should increase. These numbers represent the resistance of the LDR in different light conditions. I hope you understand now why I say resistance varies with distance.

Futterama said:

mstechca: What???

Distance and resistance, what are you talking about? Where do you get the resistance from?

Click to expand...

Mstecha has imparted more of his unfathomable wisdom.

mstechca: I don't see the connection between my question and your LDR thing, the RC car will not be driven i darkness, so I don't see your point...

I now understand what you meant by resistance and distance, but I just can't see what it's got do to with my question, sorry :roll:

Perhaps I should illustrate my idea:

By the way, perhaps it says that I'm an Electronics Newbie below my username, but that only means that I haven't posted many answers in this forum.

I know about multimeters, oscilloscopes, LDRs, NTCs, PIC microcontrollers, resistors, capacitors, ICs and Ohms law (U=RxI) ect.

Just for your information :wink:

I don't want to spam my own topic, but I played around with the IR emitter and IR detector (photodiode) yesterday, and I came up with the attached circuit.

The red LED is on when the emitter is close enough to the detector. At 5mm distance the voltage at A is around 0.75V - just enough to drive the transistor. At slightly greater distance (2cm) the voltage is lowered to around 0.27V and the red LED is off.

If I disconnect the emitter, then the voltage at A will be 0V.

I guess I need a comparator to sense lower voltages than the 0.75V.

Hi Futterama,
Your circuit uses an extremely low value for the transistor's base resistor. With the 7mA load and a minimum current gain of 100 in the transistor, a 47k resistor would allow it to turn on with about a 5V input.

Your circuit also uses an extremely low value for the photodiode's load resistor. If it is 470k then almost all its current would turn on the transistor.

Then your circuit would be 100 times more sensitive, but much slower if it matters. It will be less than 100 times the range unless you focus the IR beam.

You are pumping a whopping 77mA into your IR emitter. Is it operating within its max continuous current rating? Usually IR emitters are pulsed to remain cool. :lol:

audioguru said:

Hi Futterama,
Your circuit uses an extremely low value for the transistor's base resistor. With the 7mA load and a minimum current gain of 100 in the transistor, a 47k resistor would allow it to turn on with about a 5V input.

Click to expand...

Scrooge, why would you need a base current limiting resistor (the 47k)? The photodiode is a light-dependent current source. Even if that resistor is zero ohms, the photodiode will never generate enough current to zap the transistor. The only reason I can see is to protect the transistor if the photodiode becomes shorted.

Hi Ron,
Good point! :lol:
Without the base resistor then the transistor will receive the full current of the phototransistor, plus since the phototransistor will have a higher reverse voltage aross it, its capacitance will be lower and therefore it will respond quicker.

audioguru said:

Hi Ron,
Good point! :lol:
Without the base resistor then the transistor will receive the full current of the phototransistor, plus since the phototransistor will have a higher reverse voltage aross it, its capacitance will be lower and therefore it will respond quicker.

Click to expand...

Yeah, I was going to comment on the capacitance issue, but I had run a sim that showed (to my surprise) that it didn't make much difference. Now I realize I forgot to include the junction capacitance model in my sim (duh!). I re-ran it with a diode junction capacitance model, and the speed improvement is dramatic when changing the base resistor from 47k to zero. If you reduce the value of the 470k resistor to GND (which draws less current in the absence of the series resistor) to, say, 47k, the speedup is even greater (as you commented on). The sensitivity is reduced somewhat, and our OP may not need the speed improvement anyway.

Hi,

The emitter diode is supposed to handle 100mA.

I'll give your suggestions a try, lower the transistor base resistor to eg. 10ohm and raise the other resistor to eg. 470ohm.

Thanks for your comments

Hi Futterama,
We were talking about making it like this:

RC Car Detector

Two (or more) emitters to get more IR on the target or to get coverage over a wider area without having to aim at the receiver or if you don't know where the receiver might be.

At the risk of offending the electronic purists (again), this looks like a perfect application for some inexpensive lenses to increase the range of reliable detection. I don't know about the emission pattern of your emitter, but the spec sheet for your sensor shows a very broad reception angle - 60 degrees off-axis at 50% sensitivity. Since you have very well defined source and detector locations, you can easily greatly increase the range of reliable detection with a pair of lenses. Notice that companies like Banner and Honeywell use lenses on most of their emitter-detector pairs, even many of those intended for fairly short ranges. Plastic lenses tend to have better IR transmissibility than glass, but I'm not sure it makes much difference at 880 nm. A thin plastic Fresnel lens (from a bookstore or a kids science supply store) at the sensor can give you a greatly increased range.

One reason for using a beam interrupter, rather than looking for the reflection off the car, is that you have much better consistancy of detection in situations where the cars are non-cooperating (do not have retroreflectors mounted on them), if the cars can be at highly variable distance from the emitter-detector (a few cm. to more than a meter), and can be of variable IR reflectivity. Opposed emitter/sensors with lenses at opposite sides of the track will give you the best reliable detection range.

Another way to reduce interference from ambient light (the world is very bright from reflected sunlight at 880 nm.) is to modulate the emitter and have matching narrow band filtering at the sensor. That can also reduce heating of the emitter and allow higher peak emitter currents.

You might be interested in the Opto Diode Corp. OD-50L "Super High Power IR Emitter," with 50 mw, 880 nm. optical power output at 500 ma (much more if pulsed), 500 mw/steradian radiant intensity, 7 degree half-intensity beam angle, $11 each (www.optodiode.com).

awright

Just a thought.

Can one use a laser pointer as the emitter and a photodiode with a red filter, mounted on the bottom of a short tube to reduce ambient light interference.

With a laser pointer, which is dirt cheap at $1 and the beam is well define and totally visible even at long distance in board daylight, the alignment work can be done much easily.

Again, just a thought.

audioguru: I have testet the new circuit with the 470kohm resistor and no base resistor to the transistor.

The circuit is very sensitive to daylight but not really sensitive to the IR diodes (I actually have 3 in series consuming 42mA).

I don't get it...