Iceman_Chris

Hey,

I need some help with a circuit I designed for a transmitter on my R/C Car. Im not sure with any of the values and I need someone to point out if I need a resistor here, transistor there, etc. Any help at all would be greatly appreciated.

Thanks in advance, Chris

edit: this circuit is probably illegal because of it's frequency but i have my ham radio license and i can change to a different legal frequency if i need to - but for now if you could please tell me the values for the current setup....thanks

Attached Images

transmitter_164.jpg (14.9 KB, 1006 views)

Nigel Goodwin

If you have your amateur licence?, then you should know that the circuit you posted is totally useless!. You should also know what frequencies and modes are available for you to use, under the terms of your licence.

There are specific bands for control purposes, and in particular there's a UHF band where you can buy licence free radio modules, you should use those rather than try to build your own (particularly if the circuit posted is an indication of your radio experience!).

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audioguru

Where's the radio transmitter?
Its antenna is connected to the battery. :roll:

If it had a tuned circuit in Q2's collector then it could probably transmit AM Morse Code at the data rate.

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Uncle $crooge

evandude

Yeah, the way you have Q1 and the DATA_IN signal set up... doesn't look like that's going to work either.

you might be better off placing it between the base of Q2 and ground, as an open-collector pulldown... then when Q1 is on, it turns Q2 off, and when it is off, it allows the oscillator to drive Q2.

However, from what the radio guys are saying... it sounds like you have bigger problems :lol:

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EEgeek.net

panic mode

do you still need licence for 20mHz? :-)

that's only 0.020Hz, i mean i can write letter and seal envelope faster than that...

mstechca

Iceman,

what you have build is a great circuit that can just waste batteries. I don't see how your circuit functions as a transmitter, and if it is a transmitter, then I think you would be jamming random frequencies because you have nothing in your circuit that defines the frequency you want to transmit on.

As for the data portion, I can see your data going nowhere because you do not have a pull-up resistor to the NPN's base.

You need to do a search for an FM transmitter.

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audioguru

Yes there is base pull-up.
When the oscillator's output goes high, it supplies positive collector supply to Q1. When data also goes high, Q1 is an emitter-follower to pull-up the base of Q2.

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Uncle $crooge

Iceman_Chris

Thanks,

I made this while partially awake and I thought I was missing something...(tuned circuit). Anyways, I wasn't planning on going on 20mHz I just put the crystal oscillator in place of where I would design my own or put in a higher (unlicenced band) oscillator. I would be really grateful if you could modify my circuit a little or a lot to be legal and cheap to build.

Thanks again,
Chris

mstechca

Only to the far right NPN. The middle NPN's base doesn't have a pull-up resistor because it depends on whatever data is being injected into the base. Do you see the connection (data to NPN base). If there was a pull up resistor here, then it should be data to +ve through a 100 or so K resistor.

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audioguru

MStechca,
The data signal goes high and low. When it goes high it pulls up the base high too. If the data source wasn't strong enough, then it might not be able to pull a pull-up resistor and the base low enough to turn off the transistor.

I agree that if the data source (and oscillator) is old-fashioned TTL, then its output high voltage wouldn't be very high and a pull-up resistor would help. TTL is strong enough to pull-down a pull-up resistor and the base low enough to turn off the transistor. :lol:

Attached Images

transmitter_pulled-up.jpg (20.2 KB, 857 views)

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Uncle $crooge

JimB

OK now there is a circuit which stands a chance of working, the output will be a (rough) square wave. If the oscillator is running at 20 Mhz, the output to the antenna will have some 20 Mhz fundamental, plus all the odd harmonics.
eg, there will be components at 20, 60, 100, 140, 180, 220Mhz,.......and so on.
You need a good low pass filter to remove all the harmonics.

JimB

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ASKANA

nice hints thanx for everyone's suggestions....

Iceman_Chris

Thanks everyone,

I will most likely change the frequency to a - legal - band and add a low pass filter. But can anyone determine the values for R1 through R3?

Chris

mstechca

ok, I see the pull-ups now.

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-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-

Roff

With DATA IN high and the oscillator low, there is a direct short (limited by device resistances only) through the Q1 collector. Perhaps Q1 could use a series base resistor, or a series collector diode, or...
Also, the Q2 base is left floating when DATA IN goes low - not the fastest way to turn Q2 off, but if the data rate is low enough, it might be OK. I would add a resistor from Q1 emitter to GND.
Trying to get a 2N2222 to switch at 20MHz is problematic if it saturates.
Replacing Q1 with a fast NAND gate (74HCT00), and using a faster transistor for Q2, would be simpler and better.

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Ron