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help wanted with 555 design: auto resetting IR switch with delay

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dport

New Member
Greetings fellow project enthusiasts,

I've put together an IR switch circuit with a 555 timer whose output is high (non-pulsed) after a 10-60 second delay when the emitter beam is broken. When the beam is detected again, the output is low and the timer automatically resets itself so that when the beam is next broken the output would again be delayed before going high.

The transition time is not critical here. We can assume at least 1 second between beam on/off/on states.

I've attached a design that seems to work. The LED goes on after a suitable delay set by C and R but I'm nervous about shorting THR across the phototransitor. Will this be stable if the beam is unbroken for a long period? Any problems with this design?

Any help is most appreciated!
 

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ericgibbs

Well-Known Member
Most Helpful Member
Greetings fellow project enthusiasts,

I've put together an IR switch circuit with a 555 timer whose output is high (non-pulsed) after a 10-60 second delay when the emitter beam is broken. When the beam is detected again, the output is low and the timer automatically resets itself so that when the beam is next broken the output would again be delayed before going high.

The transition time is not critical here. We can assume at least 1 second between beam on/off/on states.

I've attached a design that seems to work. The LED goes on after a suitable delay set by C and R but I'm nervous about shorting THR across the phototransitor. Will this be stable if the beam is unbroken for a long period? Any problems with this design?

Any help is most appreciated!
hi,
You have the 555 connected as a monostable.

The Threshold isnt really shorted to 0v, its via the 22K.
I cant see why you should have a problem.:)

Note: you must have a resistor in series with the LED.
 
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dport

New Member
thanks!

Yes, a monostable configuration and I did add the 22k as a precaution (although it works fine without it). I did forget to draw in the resistor for the LED but one is there.

Nothing heats up and it seems to continue working. Thanks for the review!

P.S. Designs seem to suggest placing a 0.01u cap between CV and GND. Do you think this is actually needed for my application? I don't care much about minor bounce in the output.
 
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ericgibbs

Well-Known Member
Most Helpful Member
Yes, a monostable configuration and I did add the 22k as a precaution (although it works fine without it). I did forget to draw in the resistor for the LED but one is there.

Nothing heats up and it seems to continue working. Thanks for the review!

P.S. Designs seem to suggest placing a 0.01u cap between CV and GND. Do you think this is actually needed for my application? I don't care much about minor bounce in the output.
Many circuits dont have a Cv cap, but as good practice I fit at least a 10nf.:)

A point to note, if you add additional logic to the circuit, it would be a advisable to add a 100nF/22uF across the +9V supply lines, as the TTL version of the 555 can create spikes on the +V rail.
 
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