 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | Mar 3rd, 2008 |
| Location: | Muskogee, Oklahoma USA |
| Posts: | 349 |
| Status: |
Offline
|
What can you expect for such a small sum ?
Well, headphones included, for a start. Sweepable FM band radio. A flashlight ! with its own button on the unit. A good reception. Takes just 2xAAA batteries :D
Seemed a great candidate for making a sweeper out of.
I bought 3 or 4 and walked out of the shop with a smile!
As things were, as with any sweeping radio, it would sweep upward til it found a good strength station and then stop sweeping. Pressing the sweep button again made it go a little further and stop again when it got to another station.
Decided to take it apart and see what was what.
The chip inside is a Silan SC1088 - datasheet here: http://www.alldatasheet.com/datasheet-pdf/pdf/86814/SILAN/SC1088.html
Seems it's FM and AM capable...even better.
Quickly realising the 'Mute' input on Pin 1 was readily negated, that was the first thing I did. It required running Pin 1, via a resistor to the +3V. A bit of experimentation with a 1k variable pot resulted in a situation where the sweep can be silent of received station signal or up to full signal reception as it sweeps...a handy enough feature.
Next stage was to get that sweep to reset back to the start.
There is a 'Reset' button on the unit, but of course we wish to do this automatically at the end of the sweep.
Instant thought was a triggered system, something that would click in after a timed interval. Our friend the 555 timer seems very well suited. However, these devices usually start to work correctly at 4.5V, bit of a dilemma. Several other chips datasheets were looked at, anything with Scmitt triggers especially, but all started at +5V or were chips I didn't have. Next, I thought about CMOS versions of the 555. I had a TL555CP - excellent! these start to run at 2V and can be supplied with a lot more voltage if required.
So, for 6hours today I experimented with the whole setup. First, the flash rate was too quick and didn't quite relate to online calculation guides of Duty time etc, but eventually I got it to work on an approximately 8 seconds rate, the time it takes the unit to scan through the whole FM band
Integration of the pulse of reset to the button that normally resets took up most time. Eventually, it was realised that there was an issue with the radio circuit picking up interference from the 555 when the 555 was supposed to be idle and causing a cancelling of radio reception at best. A diode on the 555 output fixed that !
Now, the radio scans around at 8 seconds time spans AND the audio can be heard as it does so :D
Quite the set up for something measuring about 1.5" by 1" and 2XAAA battery powered.
Audio will follow, but here's a pic of the prototype circuit.
