Meteor Counting Using Passive Radar: Receivers

Data interface
Don't rush out and buy an A/D conversion board! All we're dealing with here is an on/off signal that needs to be sampled 100 times per second at the most. This is not leading-edge stuff, so you can interface to the computer through just about any port. I chose the games port as it's electrically pretty basic and happened to be available on my machine, but most observers' setups use a COM port. (See note below.) If you're using a Mac, its serial port would be the obvious choice. For details of pin connections, visit the legendary Pinouts website.

I'd originally used a 741 op amp as a comparator to provide logic pulses suitable to send to the computer, but the circuit below is much easier to set up and is able to cope with a 0.5 - 5v from the receiver's mute line. If the difference between the mute line's on/off voltages is small you may need to juggle the 1M/330K ratio in the voltage divider across the non-inverting input to get reliable switching.

The interface was built on a scrap of veroboard and uses power from the XT supply. It's well worth including the optocoupler on the data output to isolate the receiver from your computer, and vice-versa.

Keeping the computer out of the receiver
I could say I was lucky to find a cheap DEC 486 that was screened in a high-quality metal case, but actually it took several weeks of searching classifieds and making phone calls to find one of that quality at a sensible price.

Was it worth it? Unquestionably! Any computer with a unscreened plastic case will need lots of work to keep its RF emissions inside the box. As it was, even the superbly engineered DEC required a ferrite toroid on the data lead (some scrap figure-8 speaker cable) as it left the case, along with an audio choke (actaully a junked power transformer) and filter capacitors at the receiver end. It also helped a lot to put the receiver on the opposite side of the room, as far away from the computer as possible, and to keep the data lead well away from other leads. It would have been a lot easier and electrically cleaner if I'd used shielded data cable at the outset.

Note: Another advantage of using the games port is that it gives you four input channels at the one port address (201H, bits 4 to 7). This makes it easy to log data from more more than one receiver. My next "creeping feature" is to add a facility that logs the presence of electrical storms in the area, as interference from these can destroy the integrity of any data collected while they're around. A simple AM receiver tuned near 500 kHz (but off a station frequency) and interfaced through a suitably insensitive comparator makes a fine storm detector. (All you need to note are the very large pulses.) The logging software simply tests the detector's bit at the games port address and logs the time in a separate file if there is any storm activity around. This data can be overlaid on a graph of the meteor events to expose data corruption caused by lightning.

Channel

Pin

Pin

Bit

A

2

4

4

B

7

4

5

C

10

4

6

D

14

4

7

When a button is open, the relevant bit is SET (1).
When a button is closed, the relevant bit is CLEAR (0).
+5 volts is available on pins 1, 8, 9 and 15.
0 volts is available on pin 12.