I wanted to listen to the morning news while I was preparing for work or whatever in the morning, but I didn’t need the full TV set in the bathroom. All I wanted was to hear the audio so I could catch the weather and a few newsworthy items. So I built a FM transmitter with an RCA plug input so I could plug it into the audio output of my VCR. It was only one or two transistors and a 9V power supply with a wall wart AC adapter.
I used a cheap pocket FM radio in the bathroom, and tuned it in to 90.3 MHz, about the only place on the dial that was not occupied by a FM station. I had it running 24/7 for more than a year, when some neighbor said that someone was looking for me about fixing my cable TV. I told the neighbor that I didn’t have cable TV and ignored it for awhile. Then some engineer for Sprint left a note on my door asking about fixing my cable TV, and left his phone number. I called and talked to him, and told him that I didn’t have cable TV. He said he had traced a radio signal down to my home, and it was interfering with the cellular phones in the 900 MHz band. That really puzzled me because my little FM transmitter was set to 90.3 MHz. Apparently the 9th harmonic of my signal was strong enough to cause interference at that frequency – the cell tower was only a few hundred feet away, close enough to pick it up. So I told him that I would take care of it.
What I did was disconnect the FM transmitter, and I never used it again. Instead I ran a phone wire with two pairs 50 or so feet around the baseboard, and connected it to the TV and a speaker in the bathroom. I could turn it on whenever i was in the bathroom, and the cheap little speaker was good enough to hear what the latest news was. It was completely private – no one else could hear it – and it couldn’t interfere with anything.
‘In my new place, I wanted to do the same thing again. I thought about running wires, but now that I’m not so close to the cellular tower, I decided that I should be able to use a FM wireless microphone. I put a wireless mic that I made next to the TV, put two AA cells in it, and turned it on. I used a cheapFM pocket radio in the bathroom and it worked great. My only problem is that I forgot to turn it off, and it stayed on for hours transmitting the TV. When the TV was off, it was picking up any sound in my living room near the TV. All this time it was running off the two AA cells, which were being wasted while transmitting nothing when things were quiet.
I thought about a solution and considered using the same circuit that I used for my Commercial Killer Junior, three transistors with a multi megohm resistor discharging a 22 uF electrolytic capacitor, and the microamps of current then was amplified to turn on the transmitter. It worked for a few minutes to keep the transmitter on long enough to kill the commercial (see the schematic). But now I need a time interval of much more than a few minutes; probably about thirty minutes minimum. The timing capacitor is going to have to be more than ten times as large, maybe 470 uF. I think the resistors I used were 2.2 Megohms. Now I may have to use even higher values. That means less current for the transistor. I’m thinking that I may be able to use a CMOS FET instead of a transistor so that the base or gate current can be zero.
The CK Jr. doesn’t turn abruptly off; the supply current tapers off until the voltage is nearly zero. This isn’t a problem because the transmitter quits working long before it gets near zero. The one symptom of this is that the transmitter frequency is somewhat voltage sensitive, so it gradually goes off channel. This doesn’t hurt the CK Jr. because you want it to stop blocking the program after the commercial. But it might be a problem with listening to the FM radio.
I built one of these circuits with another transistor, which was used to speed up the turn off as the capacitor reached the end of its discharge.
Other Ideas
I had another idea. I could rectify the audio after it has been amplified, and use it to turn on the transmitter. When there is no audio, the transmitter would turn itself off. This works best if I plug the transmitter directly into the TV. I don’t want the transmitter turning itself on when there is a loud noise near the TV and a microphone picks it up. I might need an override switch so that the transmitter can be turned off at most other times so it will conserve the batteries.
Back to experimenting…