 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | Sep 22nd, 2008 |
| Location: | Doylestown, Pennsylvania USA |
| Posts: | 130 |
| Status: |
Offline
|
Thanks Joe, interesting information there. My original sweeper built in March did drive a .25W loudspeaker with a 386 (configured x100 gain with the capacitor on Pins 1 and 8 method) and this current circuit might well end up amplified. Smiled as I read the familiar parts list in your box design...different folks building similar designs eh
The SC1088 being at $1 would be reason enough to start with just the chip, but the fact that it comes with a whole circuit around it, earphone jack, earphones, housing and battery compartment is quite amazing.
Here's a link to some audio, as the sweeper scans and resets: http://www.filefreak.com/pfiles/76779/DollarTree.mp3
Currently the reset time is at 8 seconds, but it's easy enough to use the small pot to increase and decrease. One good feature about these SC1088's is the effect of fitting an audio level pot between Pin 9 (bottom right) and +V. The trimming allows full audio scan down to no audio heard as it scans. Not really sure what the point of the trim is lol, but if totally negating terrestrial broadcasts is an aim, then there's its use. Is that like the sweep width on yours ?
I'm having trouble getting a good 99/1 reset though. As you might hear, the duty cycle is about 60/40. I use an LED to indicate sweep and reset taken off Pin 3 of the Astable 555. Tried the usual formula for working it out but for some reason things don't quite match up. The 10uF capacitor isn't new though so I might replace it and see if that improves things with charging/draining.
* pin 9 is a point in the IF signal path and is low frequency FM and DC offset...a pot might change the offset and short IF signal to RF ground (+3volt).( not a good way to control gain)
* the reset timer in my adjustable sweep circuitry applies 4.5volts to the reset pin at adjustable intervals of 3sec to 10sec.
* the scan rate is controlled by a separate timer chip set from 0 to 1sec scan pulse intervals.
* the internal tuning noise squelch is controlled by pin 1 ( the chip automatically mutes the audio during tuning between locked stations.
