Powering a FM transmitter from a car 12V socket

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Wed Dec 05, 2012 9:53 pm

Circuit diagrams? You don't need any. Simply use a resistor to pass 25mA though an IR LED. Use many LEDs if you need more optical power. Now connect anordinary RF transmitter to the LED via a capacitor of about 1000pf.

Use an attenuator to restrict the power level to the LED to about 10mW or so. With a 50 Ohms 20dB attenuator you can have an input level of 1 Watt and pretend it is the antenna for any QRP transmitter design.

For scouts, use the ARRL handbook for radio amateurs for HF circuits they can build. When the guys get their license then they can change the IR transmitter for a genuine antenna.

Easy :-)



Best regards from Harry - SM0VPO

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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Wed Dec 05, 2012 4:27 am

Admin wrote:
Interesting! Did you know that you can feed RF into an LED that is illuminated with DC and create a radio-frequency spectrum using light? The transmitter is easy and the receiver is just an IR photo-diode connected to an ordinary receiver input.

In the UK I once created a complete HF spectrum for scouts and each team projected/received RF modulated light from the church meeting hall ceiling. I also pumped the real HF spectrum in there to give some real background traffic. Then the kids could build QRP transmitters and puit them on the air.

That is incredible! So, the output from the oscillator stage is fed into an LED?

Do you still have some circuit diagrams avaible? This will be a very interesting project to tackle.

I'm not yet at the point of being able to design my own circuit diagrams - my real work is programming, this is a hobby and sadly I don't get all the time I need to research the theory

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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Wed Dec 05, 2012 4:23 am

Admin wrote:Yes, that is true. I designed several projects for Kits-R-Us, but I never got a single $ for any of the work I did.

The owner, Peter Crowcroft, said he had but little money and asked me if I could design for nothing, then he would add $1 per unit when they sold.

Peter passed away in cancer before he was able to put those words into practice, but his wife did not honour the agreement.

As a result I no-longer support those products.


Well, that is just terrible. I wish I knew this before I bought this last kit - I will stop supporting them as well as a result of this.

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Tue Dec 04, 2012 7:20 pm

Willem wrote:I am addicted to RF circuits, can't seem to find other circuits that interesting, haha

Interesting! Did you know that you can feed RF into an LED that is illuminated with DC and create a radio-frequency spectrum using light? The transmitter is easy and the receiver is just an IR photo-diode connected to an ordinary receiver input.

In the UK I once created a complete HF spectrum for scouts and each team projected/received RF modulated light from the church meeting hall ceiling. I also pumped the real HF spectrum in there to give some real background traffic. Then the kids could build QRP transmitters and puit them on the air.

BR Harry - SM0VPO

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Tue Dec 04, 2012 5:27 pm

Yes, that is true. I designed several projects for Kits-R-Us, but I never got a single $ for any of the work I did.

The owner, Peter Crowcroft, said he had but little money and asked me if I could design for nothing, then he would add $1 per unit when they sold.

Peter passed away in cancer before he was able to put those words into practice, but his wife did not honour the agreement.

As a result I no-longer support those products.

Best regards from Harry - SM0VPO

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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Tue Dec 04, 2012 10:12 am

Will do

To highjack my own thread - I see you also designed the Kitsrus K18V2 Fm transmitter (also with coil etched into the PCB). Just finished building it, a very nice little kit - performs very well

I am addicted to RF circuits, can't seem to find other circuits that interesting, haha

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Tue Dec 04, 2012 10:09 am

That's what I thought :-)

Let us know what happens.

BR Harry - SM0VPO

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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Tue Dec 04, 2012 8:38 am

Great idea, then it would probably be a bit neater as well

Thanks Harry

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Tue Dec 04, 2012 6:43 am

Small idea here, if you get one of these "cheap" 5v USB chargers that you stuff into the cigarette lighter outlet, you can add a 3.9v Zener diode and change them to 9v.

The diode must be fitted in the control connection to the series regulator. Disconnect the lead from ground and connect the zener between the control lead and the ground.

Finally, check the working voltage of any capacitor fitted. Even for 5v, electrolytics should be rated at 10v or 16v, but I have seen Solid Electrolytic Devices (SED's) that have a maximum working voltage of just 5v. I think it is unlikely to find one in these cheap regulators and I have never seen an electrolytic rated at less than 10v, but it costs nothing to check for the possibility of an explosion.

BR Harry - SM0VPO

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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Mon Dec 03, 2012 5:35 am

Thanks Harry

Going to give it a go Smile

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Sat Dec 01, 2012 4:57 pm

If for example you use a 9v regulator, the the input voltage (after the resistor) should be about 11v, so 13 to 11v at about 25mA is about 82 Ohms. The higher the resistor then the better the noise filtering, but with less current.

Without a regulator then 13v to 9v is 4v drop so you need about 150 Ohms. Without a regulator the frequency will vary a loittle as the lights, heater and seat-heater are used.

As regards the regulator, I would use a 10v zener and a BC547 series regulator. You only need 25mA, so use a device that will handle at least 10 times that and the V5 will only tickle it :-)

Almost ANY commercial 9v regulator will work. You can even use a 78L05 with a 3.9v zener in the ground lead ;-)

Regards from Harry
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Re: Powering a FM transmitter from a car 12V socket

Post by Willem on Fri Nov 30, 2012 12:26 pm

Hi Harry

With regards to 9V voltage regulators, would something like an LM2931A work?

What resistor value do you recommend for a 9V circuit (like the V5) and would this be in series with the capacitor between the two power rails?

Thank you

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Re: Powering a FM transmitter from a car 12V socket

Post by Admin on Fri Nov 30, 2012 10:49 am

Easy!

Think about it for a moment, noise is amplitude modulation on the voltage, which causes the circuit parameters to change when the voltage is varied, albeit just a little.

A 9v voltage regulator will literally slice-off all the DC above 9v, so the noise will (almost) dissapear.

If you wanted to filter the noise then simply putting a 1000uf capacitor across the 12v input will work wonders BUT the low impedance of the 12v battery will force the voltage across the capacitor to vary in sympathy with any noise. For this reason you feed the voltage to the capacitor via a resistor. You can use this in conjunction with a voltage regulator.

If you have particularly troublesom HF noise or spikes, then you can supplement the resistor with a DC choke. If you are talking about the V5 FM transmitter then the 230v AC primary winding of a low-power transformer will work fine.

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Powering a FM transmitter from a car 12V socket

Post by Willem on Fri Nov 30, 2012 4:19 am

Hi all,

I want to power a FM transmitter from the 12V socket in my car, but I read that doing this will result in noise. How does one filter this out?

I am fine with 12V DC output, I need to understand what to do with regards to the noise

Thank you in advance

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