On the air for less than $10.00?

Sure, you get what you pay for, but it's pretty rare to have any fun at all for under $10.00 these days, so what the heck?

If you’re interested in trying low power broadcasting without any big commitment, the BH 1417 chip is your friend - a cheap, effective little IC from ROHM. The very good news is that this interesting FM stereo transmitter chip is widely available in a variety of ready-to-use-and-abuse forms from your local discount store or ebay seller.

Sold as a "Car FM Transmitter Modulator for MP3 players" or some similar sounding name, these fully assembled (and often FCC Certified) transmitters can be an easy entry into experimental broadcasting.  A sound source, a windowsill and perhaps a tiny bit of hacking and you're on the air. Most of these low cost, low power transmitters use some form of this schematic, a suggested starting point from the manufacturer.

On the downside, the range will be measured in hundres of yards, not in miles and how long these little set ups will last is anybody's guess. But, we're talking about cheap fun for beginners here, so let's have fun!

What's exciting about these is first of all the low price. As Mad Magazine used to say, "CHEAP!"

Second, the BH 1417 is a fairly stable transmitter that holds frequency well and is easily tuned on newer digital FM radio receivers. In their original, unmolested form, these transmitters are also FCC certified for trouble free use. According to the manufacturer, the BH 1417 has:

  • Pre-emphasis, limiter and low-pass filter circuits
  • Built-in pilot-tone system FM stereo modulator
  • Stable PLL controlled transmitter frequency
  • Data input controlled PLL with 14 FM channels

So, if someone does manage to find my tiny signal, at least they will like what they hear!

You can sometimes find this item sold as the MixSonic FM Transmitter, the DIGIPOWER IP-FM FM Transmitter and the Dynex Portable Wireless FM Transmitter, among many others

I used the Scosche FMT4 Fm Transmitter for my experimentation, and it really worked well! Walmart has Scosche FMT4 Fm Transmitters for about $10.00 (but it has recently been 'red tagged' so get one soon). It's not much bigger than a small of wooden matchbox. The transmitter has a black body with a silver panel detail around the power button. A bright blue LED indicates the transmitter is operating. On the top is a short 5" stereo plug, on the right side is a frequency range switch and on the left are three toggle switches used to choose the frequency.

Don't Buy Any FM Transmitter Until You See Our FM Transmitter

From the manual:

  • Available Distance 3 Meters
  • 14 Available channels
  • Powered by 2 AAA batteries
  • Run Times: In Use 15 hours
  • Standby : 45 days (1 hour per day)
  • FCC ID :RLQAT100

Using the the FCC ID# and the OET search, here are the results. Note photos, schematic and operation page. Nice!

FCC Report on FMT-4, ID #RLQAT100

I've been running one of mine for the last few days and here's what I saw.

Power - I get about three days continuous use out of the 2 AAAs, phenomenally low current drain. A couple of Ds might easily last over a week, and a couple of those big "dry cell batteries" from grade school science class could probably beat a month. Perfect for solar, and perhaps even solar with a couple of rechargeables and with any sun at all could run 24/7.  

Range - Well, the manufacturer says "3 meters" and the certified unit doesn't have an antenna.

According to the FCC filings, "The output of IC2 has the matching network consisting of L5 and C23 that limit the harmonic content and effect the proper coupling of the antenna to the output stage." As is, from a third floor window sill, range is about 90 feet line of sight. It's about 60 feet on the other side of the building, after going through or around 20 feet of building.

Hacking - this part of the schematic shows the RF at pins 10 and 11, with 10 being the RF ground. Since the XMTR has no antenna, I could probably get a magnifying glass and get a lead on the RF out at L5. However, a short lead attached to the ground side would also form half an antenna. If you remember the Tiger Tail sold in PopComm magazine, that's an Instant station - FMT-4 + NOAA Weather Radioexample.

I tucked a scrap section of wire about 24 inches long between the (-) post on one of the batteries and the terminal spring in the battery case, effectively connecting to "ground." The range went up a bit to over 100' but the main benefit was a more solid signal all the way around. FCC note - this trick would void certification but the range isn't exceeding what I would expect from a compliant unit. It is also reversible, in that I can remove the wire :-)

More hacking - It will be fun to take one of these and put a bit more voltage to the system, it likely has an internal regulator and more voltage doesn't mean more range unless it's a spark gap transmitter. However, as this unit was designed to be battery powered it is possible the unit is more geared to work OK with not very much juice. It would be interesting to see what making 4, 5, or 6 volts available might do. Adding the other side of the antenna will be interesting, too.

After a few days I changed to a clearer frequency (88.9) and adjusted the wire scrap to be 1/4 wavelength (~33")

Oops! Don't do that if you want to stay compliant - range went way over the limit - around 500 feet (1000' diameter circle), tuned with an EOM Pioneer FM receiver in a Toyota with a winshield antenna. And the content was intelligible (but not pleasant!) for another 300'

A half wave dipole (33" wire on the RF out side as well) could probably approach 1/4 mile if it was elevated a bit, so I'll probably not do that

Lot's of experimental fun for $10.00

Random thoughts on Solar Power

If my unit used a set of batteries in 3 days (or 72 hours), and if a AAA battery has a capacity of about 1000 milliamp hours, and If the batteries are in series the current drains twice as fast so the voltage can double, then the unit uses 1000mAh/72h, the "h"s cancel, and unit used 13.88 mA for each hour of operation by this calculation, say 14mA/h to make it easier.

So, I want a system that can deliver 14 mA per hour and can store enough current to power the unit when there is no sun. I'm in the Pacific Northwest, so I'd plan 19 hours of darkness capacity :-) 14 mA per hour X 19 hours = about 270 mAh of storage. A rechargeable NiCad AAA size battery is about 1.2 Volts and 300 mAh, so a battery pack of 3 in series would deliver 3.6 volts (which is what two brand new AAAs deliver at 1.8V each)

So, what output solar cell? If the battery pack should hold 270 mAh, and If the sun is only available 5 hours a day, Then the panel needs to deliver (270/5)+14mA per hour in sunlight So a perhaps 3.6 volt, 100 mA/h panel This is pretty simple stuff, so I might go forward without a charge controller - just see if the batteries can take it. An example of what looks like a fairly inexpensive and simple low power controller, available for around $10.00 at digikey, is here:
MAXIM MAX712/713 NiMH/NiCd Battery Fast-Charge Controllers

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