infrared object detector

[ant9985]

hi, im trying to build the circuit on the following link:

**broken link removed**

in the circuit diagrams i can see two diodes, and in the parts list it says photodiode, diode, and LED.

id like to know what the LED is used for in the circuit, and which diode is used where, and also, i have all these parts, and nither of the diodes look like they are capable of emitting an IR beam so could somone please help me with this

 

[Nigel Goodwin]

hi, im trying to build the circuit on the following link:

**broken link removed**

in the circuit diagrams i can see two diodes, and in the parts list it says photodiode, diode, and LED.

id like to know what the LED is used for in the circuit, and which diode is used where, and also, i have all these parts, and nither of the diodes look like they are capable of emitting an IR beam so could somone please help me with this

The LED fed from the 555 is an IR LED (fairly obviously), this is the transmitter.

The diode is just a general purpose type, a IN4148 would be fine. The 'photo-diode' is drawn as a photo-transistor on the circuit.

 

[ant9985]

if the phototransistor is actually a photodiode, then how does it recieve the IR beam as there are no entry points for it on the photodiode that i can see, and what am i suppose to do with the phototransistor thats in the parts list if this is case?

 

[audioguru]

Many IR detectors look black so that they block visible light but pass IR. You must look at the datasheet to "see" how to use them.

If you read the entire article you will understand that those kids tried the phototransistor a couple of different ways and ended up with it connected as shown. They never even tried a photodiode and the circuit won't work with one.
If you tried to emit IR with an ordinary diode or a photodiode IR detector then you probably broke them.
Send your broken spare photodiode to those kids.

 

[ant9985]

ok now im confused, nigel says the photo-transistor is actually a photodiode, and your saying they never even used a photodiode and one wouldnt work in this circuit....

 

[bmcculla]

The diode in the 555 circuit is the LED. The transistor in the receiver circuit is a phototransistor. You could replace the phototransistor in the receiver circuit with a Photodiode (pointing up) and the circuit should still work (you might have to resize the resistor above it though). The photodiode isn't used in the given circuit.

Audioguru is saying that if you tried to use a photodiode as an LED then you would damage it.

They never even tried a photodiode and the circuit won't work with one.

Why wouldn't a photodiode work in place of the phototransistor? The photodiode would be in photoconductive mode.

 

[audioguru]

A photodiode won't work in this circuit:
1) The 2K collector resistor's value is much too low for the tiny current from a photodiode. Most IR photodiode circuits use a resistor 50 times this value. Increasing the value of the resistor won't make much difference because of the circuit's next problem.
2) The 100 ohm value of the opamp's input resistor that follows the phototransistor is much too low for a phototransistor with a 2K collector resistor to have much gain. The poor phototransistor has a "dead short" load of (gasp!) only 95 ohms.
No wonder that those kids reported that with a high gain phototransistor their project had a range of a whopping 7 feet!
With a photodiode, if it conducted anything, the project's range might be a whopping 7mm! :twisted:

 

[ant9985]

lol, ok, thnx a lot for the help everyone

 

[ant9985]

also, id like to know, if i connected a 6v sounder to the output, would it work? and how do u connect a sounder to stripboard? i have all the other components connected but im not sure about this one, it has 4 pins.

how do i tell which way round is the +/- way for the diode, LED and phototransistor?

 

[audioguru]

Hi Ant,
What is a 6V sounder that has 4 pins? Is it a buzzer or beeper that draws a fairly high current from a supply of 6VDC?
This project can't drive your sounder without additional circuitry.

I don't know why they called this project a motion detector since it doesn't detect motion. It just simply detects its own IR beam.
This project produces an output whistle of 5KHz when its IR beam is shining on its detector. The whistle stops when the IR beam is broken, is more than 7 or 14 feet away or is not reflected perfectly.

You could add a rectifier circuit to its output and a transistor to drive your sounder. Instead of a sounder you could add a power amplifier and speaker to produce the whistle sound.
Both of these additions would seem to be backwards to what is needed since the sound would be continuous until the IR beam is broken.

For information about which way 'round parts should be connected, lookup their datasheets at www.google.com .

 

[FRIED]

It has a rectifier and comparator to produce a high/low output. It could be improved without much change. Transmitter. Pulse the led at about 10% duty at a higher current. Receiver. Remove 100Ω and replace with short, remove 220pF, reverse diode polarity, swap + and - inputs of comparator.
Transmitter mod. R1 sets max led current. Leds which have 1A pulsed ratings are available.

 

[audioguru]

Hi Fried,
That's a good idea to use a photodiode as a photo voltaic source, and the opamp with massive gain. Then you should remove the 2K bias resistor for it.
I think with a 9 percent duty-cycle it will sound low level and odd.

 

[audioguru]

Radio Shack has (had?) an IR real little motion detector that senses your body's heat with a range of about 10m over about a 90 degree angle. Their shops and many others use them. I recognise its very irritating buzz-buzz square-wave chime.
I nearly finished designing a mod for it to make it sound a pleasant sine-wave fading "ding-dong".

 

[FRIED]

photodiode as a photo voltaic source,

Was not refering to photodiode. Mean the diode on the output of the first opamp, so that it rectifies the positive going pulses, which will be of greater amplitude.
As an afterthought, if the TX is using 12v then several series leds could up the performance some more.

 

[audioguru]

Hi Fried,
OOOOOH! The receiver has a rectifier. I didn't see it so I was mistaken. Now I see that the receiver produces a fairly low current DC output voltage instead of a 5KHz whistle. It might even have enough output current to drive Ant's "6V sounder" if it operates on DC and we find out what its current draw is and what its 4 pins are for. It might be a piezo unit with a built-in oscillator. Maybe the 2nd opamp can be turned around so that the sounder beeps only when the IR beam is broken.

 

[ant9985]

cool thnx a lot for all the help