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triggering a 555 timer

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jonnyjames1985

New Member
hi people, new to this site, looks great. im in need of a little help with a circuit im constructing. ive searched everywhere on the internet and cant find what im looking for. ive made a automatic motor control circuit and i want to use a light dependant resistor to trigger a 555 timer. ive tried everything i can think of. i can get it to trigger the 555 timer but stay on, i know the trigger pulse has gotta be shorter than the output. i cant find a way to do this using a ldr. any ideas??? cheers
 

colin55

Well-Known Member
It's very easy to trigger a 555 but a 555 takes 5 - 10 mA. But do you want to use a chip that takes 10uA.
 
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colin55

Well-Known Member
Why do you want a short pulse from an LDR when you said you wanted to trigger and "hold-on" a 555.
 

jonnyjames1985

New Member
sorry didnt mean that. i meant i needed a short trigger for the 555. currently im using an ldr to operate a transistor which triggers the 555 by taking pin 2 negative. but doing this keeps to timer triggered, which i am trying to rectify. im not making much sense i know sorry, electronics fry my brain :confused:
 

MikeMl

Well-Known Member
Most Helpful Member
So each time light falls on the LDR, you want a single, short pulse out of the 555. You do not want another pulse until it goes dark, and then light again? Are you willing to use a 556 or two 555's?
 
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MikeMl

Well-Known Member
Most Helpful Member

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MrDEB

Well-Known Member
new to lt spice ??

where did you find the 555 timers. all I get is a transistor layout of 555.
and what to do to get the wave forms to show up??
I am just starting out with lt spice as well as multisim.
used TINA alot but its limited.
 

Sceadwian

Banned
To get the wave forms you run a simulation, for a few seconds or whatever time frame you need, after that you just click on any spot to get a voltage graph. I think the 555 model mike is using is a behavioral model, he should be able to tell you where he got it from, I used to have it but I forgot where I got it because I don't use 555's really.

Make sure you check out the Yahoo Group LTSpice, there are a lot of useful models in there.
 

MikeMl

Well-Known Member
Most Helpful Member
where did you find the 555 timers. all I get is a transistor layout of 555.
Click on the little AND gate in the Toolbar. The Symbol window opens. Normally you would choose one of the standard symbols, like an NPN. Instead, double click on [misc], which pushes down one level in the heirarchy to some miscellaneous symbols, including the NE555.

and what to do to get the wave forms to show up??
...
You have to choose one of several analysis methods by clicking on the "SIMULATE" menu, and selecting "EDIT SIMULATION COMMAND". I used a time-domain transient analysis. Look at the schematic, and you will see ".Tran 2" on the page. This tells LTSpice to simulate the circuit for 2 sec.

I set up the independent variable V2 (which is not a real voltage; it is just used to control the voltage dependent resistor R2 which is the light sensor. During the 2 sec simulation run, I ramp the "light level" up for the first second, and back down for the second. This creates a voltage which triggers the first 555.

During any simulation, LTSpice will display the voltage at any circuit node, the current through any component, the power dissipated by any component via a powerful waveform viewer.

Anybody that wants to get familiar with LTSpice should sign up for the Yahoo groups LTSpice Group.
 
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