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Brad F (B_free)
Junior Member
Username: B_free

Post Number: 5
Registered: 2-2007
Posted on Sunday, February 18, 2007 - 12:50 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

I'm having trouble getting my laser communicator to work. Using the 3 AA batteries I've damaged, like, 4 lasers now. :P (I haven't fried them entirely but the output is significantly dimmer than before I hooked them up to the double-A's.)

My setup doesn't include the bi-color LED yet because I haven't found anything at Radio Shack explicitly labeled "bi-color LED"... maybe I'm being too literal, but I don't want to fry another $10 laser. :-)

I was thinking an alternative to the LED might be hooking up some resistors in parallel to limit the current. Only problem is, I don't know the maximum number of Amperes the Laser can take.

Does anybody know how many Amps the Laser can take before it fries?

Alternatively, does anybody know many Volts/Amps the bi-color LED should use?

(My laser is almost identical to the one in this catalogue. It uses 3 button cell batteries, but it has nothing in it to limit the current/voltage.)

Thank You!!!!! :-)
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Simon Quellen Field (Sfield)
Senior Member
Username: Sfield

Post Number: 1641
Registered: 12-2004
Posted on Sunday, February 18, 2007 - 5:40 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

Resistors won't protect against voltage spikes from the transformer.
That's why we use the shunt diodes (the two diodes back-to-back in
the bicolor LED).

You can use any two diodes in this back to back configuration to make
sure that any voltage spikes are short circuited. You can use two
ordinary LEDs, for example. I specify the bicolor LED (and ship one
in the kit) because there is no chance someone will connect it wrong.
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Blair Calderara (Merseyless)
Junior Member
Username: Merseyless

Post Number: 4
Registered: 9-2007
Posted on Tuesday, September 4, 2007 - 6:49 am:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

but how many amps can the laser take before exploding?
I was sort of hoping that i wouldn't need to get a special box to keep 3 button batteries in.
or does the led somehow keep it all under control.

(Message edited by merseyless on September 4, 2007)
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Gary Patzel (Phantom_menace)
New member
Username: Phantom_menace

Post Number: 1
Registered: 7-2008
Posted on Tuesday, July 8, 2008 - 12:15 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

I recently built the laser communicator complete with 1K/8 Ohm transformer and bi-color LED using a laser pointer which looks identical to the yellow colored pointer in the "Setup and testing" section of "A simple laser communicator" on the SciToys web site.

The LED extended the time for the destruction of this variety of laser by several seconds over the circuit which included the transformer only.

Are there any other simple solutions to this problem which also provide maximum light intensity without the destruction such as Zener diodes in place of the LED, or the additon of a current limiter such as a LM317 IC?
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Simon Quellen Field (Sfield)
Senior Member
Username: Sfield

Post Number: 1904
Registered: 12-2004
Posted on Wednesday, July 9, 2008 - 12:20 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

Perhaps the LED was not connected properly?
It should be across the leads of the transformer. The purpose is
to absorb the energy in sharp voltage spikes from the transformer
and prevent them from getting to the laser.

Check the current your laser normally gets from its normal batteries.
To do this, you will need to connect a milliammeter in series with the
batteries and the laser.

Then temporarily replace the laser with an LED, and measure the current
going to the LED from your transmitter. If the current is higher than
what the laser normally gets, then you will want to add a resistor in
series with the laser to bring the current down to what the laser can
handle. Use Ohm's Law to calculate the value of the resistor to use.

If you use the kit we provide, you will know you are getting a laser,
LED, and transformer that are known to work well together.

If you are using a 6 volt or 9 volt battery, that will fry the laser quickly.
Use only 3 cells (4.5 volts). If you have to use higher voltage, you will
need to add the resistor in series with the laser to bring the current
down to safe levels.
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Gary P (Phantom_menace)
New member
Username: Phantom_menace

Post Number: 2
Registered: 7-2008
Posted on Monday, August 4, 2008 - 12:48 am:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

I believe I will claim the “Twinkie Defense” in the logic of my last post.

I unintentionally used a weak battery in the battery pack I made to provide the 4.5 Volts to test the laser transmitter using the audio transformer and bi-color LED only circuit, and this provided a much weaker intensity light from the laser than the laser pointer produced using only its own battery supply.

When I incautiously added another battery to the battery pack to boost the Voltage to 6 Volts across the transmitter, hoping to boost the light output, the laser soon burned out.

In both cases the bi-color (red/green) LED was constantly glowing orange (Flashing?) with no audio input to the transformer, and I am concerned to know if this is what is meant in the description of the circuit operation, or if this shows a resonant oscillation peculiar to the type of laser pointer I am using.
Are the flashes (red, green, or orange) supposed to be a result of voltage spikes from the audio input?

Also, when I used fresh batteries to make a 4.5 Volt supply for the laser transmitter, the light output still appeared weak, so I decided to change the voltage to 6 Volts with the addition of a current-limiting resistor in series with the laser pointer.

To apply Ohm's Law I measured the current to the pointer from it's original battery supply at approximately 30 mA. However, when I substituted an LED for the laser pointer in the transmitter circuit as suggested, I found that the value for the current measurement varied with the type of LED I used, from 33 mA for a garden variety red LED to 64.7 mA for a high intensity red LED. Both results appeared with the use of three 1.5 V “AA” batteries in series as the transmitter power supply.

If I reason that the transmitter circuit with a 6 Volt battery supply should have 4.5 Volts across the laser pointer at 30 mA and the remaining 1.5 Volts across the current limiting resistor at 30 mA, then the value for the resistor would be 1.5V/30 mA (1.5/.030), or 50 Ohms. However, the actual value I found for the resistor that would allow a current very close to 30 mA was approximately 2 Ohms.

I arrived at that result by substituting a potentiometer in place of the resistor, gradually lowering the resistance until the meter displayed approximately 30 mA and the laser appeared to have a brightness close to what appeared with only it's original button cell batteries. I then replaced the potentiometer with a fixed resistor of 2.2 Ohms. Another laser pointer I tested required a resistor value of approx. 17 ohms.

I am happy to say that the laser transmitter now works very well with very good audio fidelity, and does not seem inclined to burn out despite the remaining mysteries I described here!
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Patrick (Firewire)
Senior Member
Username: Firewire

Post Number: 104
Registered: 12-2007
Posted on Tuesday, August 5, 2008 - 5:59 am:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

Putting the laser on top of a big speaker is enough to make signals that can be received by a simple CdS-piezo earphone-battery receiver...

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