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KeithLuneau

Question regarding the "Honk Kong" wireless camera

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A few years ago, I picked up one of those cheap-o cmos cameras with built in transmitter like the "Hong Kong Camera" project.

Mine was sold as "1.2GHz", has a tuning knob on the receiver, typical junky set. I messed around with it a little bit for a while, and found that the frequency seemed to drift as voltage changed. I've used it all the way up to 12V, even though it was originally set up for a 9V battery. Using a stable voltage (Anyvolt Micro) I was able to get it tuned so that the knob didn't need to be adjusted much in flight, and made a GPP scaled for 1.16GHz as stated in the project here. I did get a handful of flights in using it, thought I wouldn't go so far as to say that it worked well.

I ended up taking the transmitter out of the camera and rewiring it to use an external camera and mic. It's just been sitting around, worthless. Not long ago I picked up a 1.28GHz set which included a receiver like those from Inesun. I decided to see if I could vary the voltage on that little transmitter enough to "tune" it so that the new PLL receiver would pick it up.

With the input voltage for the little transmitter between 3.4V and 5.6V, I can set my receiver to channel 4 and it picks it up, the picture looks great, sound is clean too. Channel 4 is 1.080GHz according to the manual for the receiver. That's a problem, it's not legal in the US. I want to try to tune the transmitter to another legal frequency and see if it'll work.

Even at 12V it never got so much as warm. So my question is this... Does the output RF power vary as the voltage changes? I don't want to go over 12V and burn it up, but at the same time I don't want to stay too low if it's cutting the RF power too much.

I don't ever expect to get a "good" result from this, just playing around with it is all. At most I might stick it on little indoor flyer or something to poke around the yard with when it's dead calm. The transmitter barely weighs a few grams.

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Does the output RF power vary as the voltage changes?

Are you sure there isn't a Vreg built into the cable? Many Hong Kong combo Tx/Camera cubes that run on 9V-12V have one hidden in the cable, so increasing the voltage does not increase RF power.

By the way, increasing voltage on various video transmitters does not always increase RF power. That is because some have a Vreg in them that stabilizes the RF Amp's voltage. Increasing the voltage just makes the Tx tin can hotter, but RF power remains the same. But, some Tx's do not have a Vreg for the RF power amp and so their actual operating voltage will indeed impact RF power a lot.

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No, there's no Vreg in the cable now. There was one, but it was a 5V for the little CMOS camera. The transmitter was fed the full 9V from the battery. There may indeed be one on the transmitter, I'll take a look and see. I know varying the voltage definitely changes the freq. it's transmitting on. Using the pot on the Anyvolt I was able to "tune" the transmitter so the receiver picked it up.

Tomorrow I'll cut the shrink wrap off and take some pictures of the transmitter board. It's super small, no can, just a little board with a whip antenna. I soldered a short lead with a connector so it will plug into my other harnesses in my planes.

If I could use it on 5V it would be great, as small as it is and using such a small amount of power, I bet it would be safe to power it and a small camera off the BEC in a small plane. I'm afraid I'll never get anything close to a legal freq. at 5V though.

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In that case your Hong Kong Tx probably does not have a Vreg on the Power Amp. From what I have seen, those things are so cheaply made that they barely have enough parts to work.

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Here's the close up pictures... There's not a lot there... The coin for size reference is a dime.

Maybe you can make more sense of it than I can. All I see is poor construction and design! :)

Maybe later today I'll do some ground range tests and see what it's doing with such a low supply voltage.

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Edited by KeithLuneau

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It's a bare-bones VCO type transmitter. For reliable/consistent results you should power it with a voltage stabilized source that is set to the rated voltage.

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Ok, that explains why it's so easy to "tune" the output frequency too then...

To quote some info that came up with a search for VCO Transmitters...

A common oscillator implementation is known as a voltage controlled oscillator (VCO) circuit, where an input tuning voltage is applied to an oscillator circuit and the tuning voltage adjusted to set the frequency at which the circuit oscillates.

I guess the final verdict is that I'll never be able to get it to work on a legal HAM frequency within the specified voltages. I did try earlier raising the voltage up to 11.5v and never got it all the way to the next channel on the receiver. I'd have to jump 5 channels to make it to a legal one. :(

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I guess the final verdict is that I'll never be able to get it to work on a legal HAM frequency within the specified voltages.

The low end eBay/Hong-Kong systems are not known for being compatible with "legal HAM frequency" applications. Once the initial fun wears off, it is best to bin it and upgrade to something with a bit more quality. :)

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