Simple beam break sensor

[Archangel]

Is there a way to calculate what resistors and capacitors to use? .

http://www.national.com/ds/LM/LM555.pdf
The formula is in this data sheet near the end, page 10 between figure 13 & figure 14.

[The Master]

Sorry, i dont understand that formula. Its a bit more complicated than i thought.

Ive been reading a few webpages (and re-reading this thread) and i think im starting to understand how it works. I like the 555 idea but if i need to send it in bursts then ill need 2 555s (1 for the 38.5khz frequency and 1 to control the bursts). Other sites also recommend a variable resistor for fine tuning.

I think its best if i do use the PIC chip. I do still have the MCLR pin which can be used as an input only (for a sensor). Hopefully one of the pins on this chip has hardware PWM and hopefully that works how i think it does. That way i can adjust the frequency to suit and have the burst thing without having to make the PCB bigger and use extra components... and i wouldnt have to figure that equation out

Im going to put an order in soon and get some new emitters/PTs and ill see how it goes. No doubt ill be back for more help when it doesnt work :P

[mackrackit]

Any of the PICs output pins can control the 555, look at the trigger pin of the 555. It is basically an on and off pin. The basic stamp folks did it this way before they had a freqout command.

[The Master]

Thats a good idea. Ill add some 555s and the related components to my next order

[Archangel]

A simple beam break sensor, what about a $5.00 US laser module and a simple CdS cell ?
http://www.doctronics.co.uk/555.htm#transducer
easier formula here http://www.doctronics.co.uk/pdf_files/555an.pdf

[The Master]

I dont want people to be able to see the beam but if i cant get IR working then that might be the only option.

Thanks for the links. Ill have a read through them

[The Master]

Im trying to get this 555 circuit working but nothing is happening. Ive been following a few different diagrams on the internet and none of them seem to work

I found this calculator on google. I entered 38.5KHz, 50% duty cycle and 0.001uF (1nF) capacitor. It said to use an 18.7K resistor for RB and 0K for RA. Ive been using a 15K resistor with a 5K variable resistor as another website said. It doesnt really matter if its at the right frequency for now though. the LED just wont light up at all (im using an LED for now so i can see it working).

Ive tried putting the output through a transistor. Ive tested the transistor by itself and it works fine.

My circuit currently looks like the one on that calculator site. Does anyone know what ive done wrong or what to check next?

[Archangel]

I don't want people to be able to see the beam, but if I cant get IR working then that might be the only option.

Thanks for the links. Ill have a read through them

red laser / infrared filter might work . . .