Need timed relay energized

Thread Starter

ajaxoftherockies

Joined Apr 7, 2012
21
Hi. I have a 555 timer that activates when object breaks a laser beam

It works okay so far. I break the beam and the LED on the output comes on. When I ground pin 2 briefly it goes out until I break the beam again

What I want now is for the output to stay on for three minutes and then reset.

So final operation is when the beam is broken a relay is energized for three minutes then releases.

Can someone tell me what else I need?

Thanks!!
 

vu2nan

Joined Sep 11, 2014
129
You need to reconfigure your 555 timer circuit from 'on-delay' mode to 'monoshot' mode.

Regards,

Nandu.
 
Last edited:

vu2nan

Joined Sep 11, 2014
129
Here's a schematic of the 'monoshot' or 'monostable' mode of the 555.

untitled.JPG

In this mode, the output comes on with a trigger pulse and stays on for a time duration determined by the values of 'R' and 'C' (Time in seconds = 1.1 x μF x MΩ).

Regards,

Nandu.
 
Last edited:

AnalogKid

Joined Aug 1, 2013
8,330
Please post your original schematic. This will make it much easier to suggest changes.

Also - what do you want to happen if the beam is broken again in the middle of the 3-minute cycle? Terminate the cycle? Ignore any beam breaks curing the cycle? Something else?

ak
 

Thread Starter

ajaxoftherockies

Joined Apr 7, 2012
21
Please post your original schematic. This will make it much easier to suggest changes.

Also - what do you want to happen if the beam is broken again in the middle of the 3-minute cycle? Terminate the cycle? Ignore any beam breaks curing the cycle? Something else?

ak
Hey AK, thanks for your input! After thinking about your question, I came to the conclusion that this project would be better served with an Arduino Pro Mini ($4 at Micro Center).

For others who may come across this thread, here is the circuit I worked with (per your request) to get my laser trigger beam idea working
1582316212589.png

My answers
1) when beam is tripped, relay should be on for 3 minutes
2) Relay should stay off after the 3 minutes regardless of the beam status
3) I should manually reset the entire operation after discovery that the beam has been broken and relay tripped

The Arduino can do all this with my LDR/R components and a transistor to switch the relay on. I will add a 'reset button' to restart the cycle.

For anyone curious, the project is an electric rat trap. I had a rat in my garage recently (first time for the big guys). There was motion-activated video running, so I recorded the rat tripping a snap-trap. This was one of those giant Victor brand rat traps that would break a finger if you got snapped in it. This rat ran off with the snap-bar on its head and dragged the trap over ten feet before escaping from it and getting away. So, it seems that my new problem isn't solvable with an affordable commercial product.

In my junk pile was a lightweight wood box that is a perfect size/shape. I also found some scrap PCB material to make electrodes on the floor of the box. Another useful find was a mercury lamp transformer that puts out around 400VAC (hoping that is sufficient). I acknowledge that 3 minutes may be a bit long, but I want to make sure the rat is terminated, and then shut off the juice before the fire starts.

If anyone is interested, I can post more about the build and results. So far, I haven't seen the big one on video, and the regular mouse traps have been left untouched since my first post.

Thanks for everybody's responses!!

- a -
 
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