RC-(remote controlled) Car

[faizanbrohi]

Here is a PCB layout and shcematic for my latest TX-500 500mW FM Transmitter i am building for my RC Remote controlled car using DTMF Signals to control it .
I have not build it but this will hopefully be my 3rd transmitter project . from 40mW and 10mW to 500mW . Well hope this works . I found the schematic on www.electronics-diy.com and then redrawned it and made a PCB . Later i will make the Reciever and the DTMF Encoder and Decoder.

THere is one problem the 4.7uF capacitor is i think a non polar  and the 47uF is also a non polar capacitor but well i can go with a 4.7uf to be non polar but a 47uF , i don't think so or it might be bipolar.

The Red connection on the PCB Layout is for a jumper that was not routed correctly . it might be connected on the bottom side using insulated copper wire.

[audioguru]

Hi Faizan,
Doesn't your country have rules and regulations about radio transmitters like most countries?
Here the broadcast bands are full and a complaint about interference makes the RF cops find the cause of the interference then go after the person responsible. There are certain other frequency bands reserved for RC.

In your circuit, most coils are milli-Henries when they should be micro-Henries or less for 100MHz.
I think the BC549 transistor is too small for the output stage and with a 12V supply it might melt.
The 47uF capacitor is across the power supply so a polarized one is fine.
But using 4.7uF for the input capacitor is rediculous! Calculate its cutoff frequency. It is only 4.3Hz. Use a 0.33uF film capacitor for a cutoff frequency of 60.6Hz, 0.22uF for 90.9Hz or 0.1uF for 200Hz.

You and the web circuit have the LED backwards, and you have its current-limiting resistor R5 in the wrong place.

[faizanbrohi]

Well in my country there are no Radio cops to whom i have interferance . so no problem there. about the R5 resistor yes the other end is backwards ,well i guess i have to redaraw the schematic and PCB with your settings now lets see .

ok i will use a 0.1uf because i have one . for the inductors . well they are not the actual values i have approximatied them , please see the original website and see how they are made. And for The transistors , i think i will go with BF199, MPSH10, BF240 , 2N3926 , MRF237 will do fine as all are RF Transistors. i think bf199 and bf240 are fine RF Transistors and easy to find.

[faizanbrohi]

here is the modified Schematic of the things you posted. And it is also written in the title checked by AUDIO GURU ..  ;D Well i think you should join the www.electronics-diy.com community also , It is in a beta stage so it is growing.

[audioguru]

Hi Faizan,
The original project has its oscillator separate and shielded from its RF amplifier but your layout doesn't.

You will need to add emitter resistors to the output transistors when you change them to RF types, as is explained in the article.

[faizanbrohi]

Today i will go to get ferrite beads and the RF Transistors. Well The emitter resistors i don't know which values to put , do they have to be greater in term of power i mean 1/2w or 1w or 2w maybe or in value and Wattage.
Well i am going to build a prototype and i am going to build three different units first , the Oscillator seperate , the Seperatar/Amplifier seperate , but how do i shield it ???
one more thing i will make the power meter also seperate.

i have another idea of building an oscillator , why not use a opamp with gain bandwidth product of almost 500Mhz , and then use a summer with gain 2 and add or mix the two frequencies . it is more robust i think.

[audioguru]

Well The emitter resistors i don't know which values to put

The article tells you.

do they have to be greater in term of power i mean 1/2w or 1w or 2w maybe

The power supply voltage and current are low, so resistors will not get hot. The emitter resistor will have about 1V max across it and 100mA max in it which is only 0.1W.

Well i am going to build a prototype and i am going to build three different units first , the Oscillator seperate , the Seperatar/Amplifier seperate , but how do i shield it ???

The article shows how they shielded theirs.

i have another idea of building an oscillator , why not use a opamp with gain bandwidth product of almost 500Mhz

I have never seen an opamp with such high gain-bandwidth product. If one is available, how would you modulate it? A single transistor is fine as an RF oscillator.

and then use a summer with gain 2 and add or mix the two frequencies

A modulator is not a summer and is not a mixer. Look at modulators in Google.

[faizanbrohi]

Well First things first here are the pictures of the coils i made (L1 is a tunable coil with ferrite core for tuning) , and you require a lower value of emitter resistors like 100 ohm for the RF Transistors . To shield it you require a metallic shield between the driver and the amplifier. The pcb's will be made shortly and will post them soon.

[faizanbrohi]

pics continued....

[audioguru]

Your coils look good.
Does the wire have enamel insulation on the ferrite-cored coils? It looks like bare wire.

The frequency of this transmitter will drift all over the place. RC cars, planes and boats use a quartz crystal in their transmitter for frequency stability. Some also have a quartz crystal in the receiver.

What are you going to use for an FM receiver? FM radios have de-emphasis (treble cut) which is the opposite of the treble boost used by FM radio stations. The treble cut will mess-up your DTMF signalling. You could add a pre-emphasis circuit to the transmitter to match the de-emphasis in a receiver.

[faizanbrohi]

i have a FM Radio kit which i have tested and it is built around a samsung am/fm radio IC , it is a full superhet FM Reciever with preamphasis and PLL Tuning , It uses a Gang Capacitor for tuning ,  i would require a frequency filter of some sort for the dtmf frequecies. so when there is no signal the car should not be doing anything. The wire is insulated actually , the color of insulation and the qualtiy of the picture makes it look like bare wire.

The question is how can i apply a quartz crystal here , my fm radio reciever has a ceramic resonater for stability. but how can i apply it in the transmitter section.

hmm but i can also use a varactor diode for stability in the oscillator section , the tunable coil L1 gives good stability also.

Here is a link for the DTMF Remote control. http://wiredworld.tripod.com/tronics/remote_control.html

[audioguru]

Your radio is tuned with a variable capacitor, so it doesn't have a quartz crystal nor a PLL.
It ceramic resonator shapes its IF wideband response curve.

Your transmitter's RF frequency will drift with temperature change of its transistor and parts, with changes in its supply voltage and when anything gets near it. A varactor diode can tune it or modulate it but doesn't help its frequency instability.

There are many circuits on the web with very stable FM transmitters using a quartz crystal and a PLL in a frequency synthesizer circuit. Some are fairly simple. The circuit you have is old, older and oldest.

[faizanbrohi]

Housing it in a mettalic case will reduce the frequency drifts. and the supply voltage is also regulated with a 7805 regulator. but the thing that will have affect is only temprature.

[audioguru]

What kind of battery are you going to use for the transmitter?

[faizanbrohi]

I will be using a 12V battery pack. and also be using MPSH10 VHF/UHF Transistor , and what do you know , it has really low frequency drift.  i will also be making a board layout with ground plane. so well this all will reduce frequency drift , but using a PLL would be really nice to reduce frequency drifts. but PLL Controllers are not so easy to make.

[audioguru]

It is good to hear that your transmitter's RF frequency doesn't drift much.
The MPSH10 is in a small case (low power dissipation) and has a low max voltage limit. I hope it doesn't break.
Won't your transmitter cause interference to FM radios and TVs?
Won't your receiver pickup FM stations?

[faizanbrohi]

well i have completed the Oscillator section and here are the PCb board layouts and schematics .

For your question that i will have crowded bands , the answer is that 105mhz FM Band has no radio station , so it will work for me . and keep in mind this is only a educational project. SO NO LICENCING REQUIRED  ;)

Now i am making the power meter to test the circuit .

[audioguru]

It looks nice, BUT:
Your varactor diodes are forward-biased. They are supposed to be reverse-biased. You should turn them around.

[faizanbrohi]

But in the original schematic it is with this configration , take  a look here .. i am a little confused here  ???

and look at the power meter also ...

really confused , don't know what to do , then just to experiment.  ???

I think the varactor are in the right direction because it is actuallly reverse biased and the voltage from the variable resistor varies at the cathode.

[audioguru]

Your varactor diodes have +9V on their anodes and ground on their cathodes. They are forward biased. Turn them around.

[faizanbrohi]

Hello i am back with some modifications . here is a v1.1 of the oscillator TX-500 . i have added a 1nF capacitor for better stability . the rest is that varactor diodes have changed their posistion. Now i will build the power meter... Asap

[audioguru]

The 1nF capacitor you added reduces modulating frequencies above about 500Hz, and about 5kHz will be down -3dB. Your DTMF decoder might not work properly with the higher tones rolled-off. The varactor diodes look correct now.

[faizanbrohi]

So if i remove it , it will be ok for DTMF and if i remove it . it will work for voice.
now i will post the power meter shortly. one more thing i am putting a 12V voltage regulator for both the VHF power meter and as well as the VHF Amplifer. is it ok.

[audioguru]

An ordinary 12V regulator needs a minimum of about 14.5V on its input to work properly. Your 12V battery pack won't have enough voltage.
Low dropout regulators are made for 5V and if somebody makes one for 12V then it would need a minimum input voltage of about 12.5V.

[faizanbrohi]

WHy not use a step up boost voltage regulator , like the ti's UC2577 , but it has a bulky circuit and high amperage . i think a small step up boost voltage regulator with 0.5 A and 12 V regulation voltage would be ok .
But then again why bother , and just connect the 12 Volt battery pack directly  ;D