Laser Tripwire Project

[AbsoluteZedo]

Ok I have a new problem, I want the laser to run off a battery and it needs to do this continually without eating up battery power, which means I need to somehow pulse the laser so that it will draw less power from the batteries....I was looking into 555 timers and then I realized that if I used a 555 timer the IC will actually require a lot of voltage to operate to begin with.

 

[qumefox]

a 5mw red should have minimal current draw.. It should be good for several hours even running off a couple of AA's.. How long of a battery life were you looking to get?

 

[lasersbee]

Ok I have a new problem, I want the laser to run off a battery and it needs to do this continually without eating up battery power, which means I need to somehow pulse the laser so that it will draw less power from the batteries....I was looking into 555 timers and then I realized that if I used a 555 timer the IC will actually require a lot of voltage to operate to begin with.

A quick check on a standard 555 Data Sheet shows a supply
current of 3mA at 5Vdc supply and 10ma at 15Vdc...
That is minimal...

http://www.national.com/ds/LM/LM555.pdf

If on the other had you use a Cmos version of the 555 timer..
the data Sheet shows a supply current of 100uA at 5Vdc and
150uA at 12Vdc.....

http://www.national.com/ds/LM/LMC555.pdf

That is almost nothing compared to your Laser...
Your 5mW Laser will draw at least 20-50mA in CW mode...


Jerry

 

[Cyparagon]

Maybe a 635, but all the 650s I have draw 10-20mA.

 

[lasersbee]

The dollar store red Lasers that I have draw between 20 and 50mA
on 5V... (I had just tested them to be sure)
The cheap Sure 5volt red Laser modules draw ~24mA...
Either way that is low enough to modulate and work off
batteries...


Jerry

 

[AbsoluteZedo]

I did a LOT of math and running of 10 - 50mA will only last like 20-40 hours, which is too substantial for a real design. Anyway, I figured out how to pulse and am investigating monostable multivibrators/missing-pulse detectors, I think I got this figured out. And laser bee is definitely right, the 555 uses almost no energy, especially the low power version, so the energy i would save would substantially increase if I modulated the beam (I did a lot of datasheet research and my initial assumptions on the 555 were wrong). (I look at a lot of battery datasheets and estimated the life)

 

[qumefox]

20-40 hours isn't enough? Ok, now I have to ask. What exactly are you going to be doing with this thing? heh

Also if size isn't a huge concern, you can always go with two battery packs paralleled to double your capacity.

And if it's outdoors, there are options like solar recharging, etc.

 

[chipdouglas]

this is going to be mounted in a stationary postition, why would it have to be battery operated. even if you pick up and move it to another door or window you will pretty much always have a power outlet near by. It sounds like reliability are a factor. You wont get reliability with a battery.

michael.

 

[lasersbee]

I did a LOT of math and running of 10 - 50mA will only last like 20-40 hours, which is too substantial for a real design. Anyway, I figured out how to pulse and am investigating monostable multivibrators/missing-pulse detectors, I think I got this figured out. And laser bee is definitely right, the 555 uses almost no energy, especially the low power version, so the energy i would save would substantially increase if I modulated the beam (I did a lot of datasheet research and my initial assumptions on the 555 were wrong). (I look at a lot of battery datasheets and estimated the life)

There are plenty of Missing pulse detectors using a 555 out there...
I've used them before quite reliably...

Here are two... (I've used the Transistor input circuit)

Jerry

 

[random person]

Shinning a laser at a LED works pretty darn well for speed. Alot better than a photosensitive resistor. In addition, the signal is decently large to begin with. Tried it out tonight. I used the cheapie 3usd laser from hightech dealz. surprisingly it was able to PWM rather decently with the driver attached. CH1 is the output voltage of the LED being used as a sensor.

The main issue I was having that was the whole idea behind the PWM-ing the diode was to save current draw. I noticed as I increased the duty cycle of my pwm circuit that it would draw more current. =/ Will have to look into this more. It could be the internal vreg of the laser driver... not sure. Still a work in progress

 

[lasersbee]

I did a LOT of math and running of 10 - 50mA will only last like 20-40 hours, which is too substantial for a real design. Anyway, I figured out how to pulse and am investigating monostable multivibrators/missing-pulse detectors, I think I got this figured out. And laser bee is definitely right, the 555 uses almost no energy, especially the low power version, so the energy i would save would substantially increase if I modulated the beam (I did a lot of datasheet research and my initial assumptions on the 555 were wrong). (I look at a lot of battery datasheets and estimated the life)

Since you are only using this as a trip wire... if you set the duty
cycle to something like 1% ON and 99% OFF (or less)... a Photo Diode
could still easily see the pulses and the current would really be low.
You could also set the pulse rate at 100 to 300ms if detecting humans
tripping the beam...


Jerry

 

[random person]

Since you are only using this as a trip wire... if you set the duty
cycle to something like 1% ON and 99% OFF (or less)... a Photo Diode
could still easily see the pulses and the current would really be low.
You could also set the pulse rate at 100 to 300ms if detecting humans
tripping the beam...


Jerry

I'm actually finding it rather hard to get a low power PWM circuit that has a net current save. Using a low power 555 and a low power lm339. I think the trick is to use a BJT for starters when driving. Even though a bjt requires current through the base, the current amount needed to keep the bjt in the saturation region when driving a 5mw laser is in the micro amps. As opposed to a mosfet where they have gate capacitance. Even the 555 timer takes a decent amount of power... 1mW for a low power version: "Low power" TLC555 http://www.datasheetcatalog.org/datasheet2/0/01ok8odyj3ol0u3ijalwe020llky.pdf has a quiescent power of 1mW.

Has anyone here tried building a low power PWM circuit and then driving it? Anyone mind reporting the min amount of current that they were able to do it at?

 

[lasersbee]

I just slapped together a quick circuit...

1) I used a CMOS 555
2) I used a N-Channel 500mA FET
3) I used a SURE red Laser module that tested at 3mW
4) I used a regulated 5VDC supply

a) the 555 circuit alone draws 0.6mA
b) the SURE 3mW Laser alone draws 11.6mA
c) the 555 circuit gives a ~1ms pulse every ~100ms
d) the total current draw with the Laser pulsing is 1.0mA


Jerry

 

[random person]

Nice Laserbee!

Found out my main issue was using the driver that came with the LED from high tech dealz. Removed the driver and put a 100ohm resistor in series with the LD. Pwmed it at 37khz 4% duty cycle. Got about a 600uA total current draw. Signal was much more weak on the receiving side but it worked.

Also switched my fet to a IRF510

Now time to work on buffering that receiving signal and then put it through a missing pulse detector.