[Laser] Laser TXR questions

[TWOSIG at aol.com]

Leigh

I went to the Oatley web site and downloaded the schematic to AM Laser 
transmitter and receiver.  It may be that I got the wrong circuits, so please check 
over my suggestions to see if they make sense.

Signal strength meter:

I suggest that you measure the audio signal strength rather than the laser 
signal strength.  I did not see a way to add a circuit upstream of the audio 
that would not risk loading the original circuit too much.  The reading will 
fluxuate with the volume of the sent audio, which suggests that you should use a 
CW tone at 100 percent modulation for best results.  See Transmitter circuit 
below.

Adding a switching diode (similar to a 1N914) anode to the junction of C3 and 
VR1.  Then connect a small capacitor ( 0.01 uFd ) from the cathode to ground. 
 This will allow the audio signal to charge the capacitor to a voltage 
proportional to the received audio.  The way you measure must not load down the 
audio voltage going to VR1.  The DC resistance of the voltmeter should be much 
higher than 50K, but you may need to add a resistor in parallel to bleed off the 
charge of the capacitor.  Measure the voltage with an analog meter - the the 
maximum voltage should be about 3 Volts.  

Transmitter Circuit:

The description that I read on the kit said that it did not come with a laser 
diode.  Since you have to put an external laser on the transmitter, I would 
suggest that you build a CW transmitter and switch the laser from one to the 
other with a plug.  Using a common plug / socket would let you build a test 
tool, a "laser simulator" with one or more bright LED and some resistors to match 
the voltage and current required by your laser diode.

By using a completely separate CW driver circuit, you can be sure that the 
laser is turned completely off, so you will get 100 percent modulation.

If you would rather switch your microphone with a tone audio source, you 
should remove the leg of C6 ( 68nFd ) that goes to the junction of R3 and IC2 pin 
3.

I have a 555 circuit that I have used to drive a laser pointer.  It should 
provide audio for the second option.  I will try to send a schematic to you 
separately.  

Hope this helps.  More later


James 
N5GUI


>Leigh Rainbird vk2krr at bigpond.com 
>
>I have an Oatley Electronics AM laser kit.
>
>Is it possible to add some sort of signal strength meter to the RX circuit?
>If so, where would you connect it in at? What type of meter?
>
>What can I add into the TX circuit to make it TX a CW tone of say 600 Hz, so
>that its easier to detect and use for morse code. Could I put a switch in so
>I can choose voice or morse?
>
>Can an input current meter be added to the TX?
>
>Thanks
>
>Leigh VK2KRR
>