By Steve Roush Arizona, USA
I have been very pleased with Pierre Terrier's meteor detection system. I did encounter one small problem while building his interface circuit - my laptop supplies only +/- 4V through the serial port, so I was forced to use an external +/- 12V power supply. This was accomplished with a 24VDC "wall wart" and a 7912 Voltage Regulator IC.
After I got the system up and running, I began to feel that it was limited by my old PRO-2004 receiver's lack of SSB/CW capability (ie, it could not detect an unmodulated carrier). Instead of buying a new receiver, I decided to monitor the PRO-2004 receiver's Squelch Trigger voltage. This entailed opening up the receiver and soldering a wire to the junction of R150 and R151 - I ran this wire out to a RCA jack which I installed on the receiver's rear panel (the metal panel provided a ground connection).
In AM mode, the Squelch Trigger voltage is typically +1.5V when no signal is present, and it drops to approximately +0.9V when a strong signal is present. The response is affected by the Squelch control on the front panel - for consistent results, I keep the Squelch control set to maximum (ie, fully clockwise).
Pierre's interface circuit is designed to measure frequency, not voltage, so it was necessary to build an additional circuit which would translate the voltage changes into frequency variations. Referring to the schematic, the first part of this new circuit filters out slow (non-meteor) voltage swings and introduces an offset voltage which is required by the LM566 IC. The input at pin 5 of the LM566 should be about 2/3 of the way up between ground and the positive supply voltage. With a +/- 12V supply as shown here, the 10K potentiometer should be adjusted to yield +8V at pin 5.
The LM566 is a Voltage Controlled Oscillator which outputs an audio signal to Pierre's interface. When the receiver's Squelch Trigger voltage changes rapidly because of a meteor, the frequency of the audio signal will vary, and Pierre's program will display a vertical shift of the green trace.
I am afraid that the LM566 is no longer in production, but fortunately, it can still be found on eBay and other online sources.