Small Microphone transmitter

[Laserbeamtoast]

Hi, this is my circuit design, and I really need help with it. If you could be so kind as to tell me whats wrong, and how i can fix it, that would be amazing. thank you sooooo much! Oh, and also, the point of this circuit ts to be as small as possible and run off of as small a battery as possible. Thanks sooo much again,
Laserbeamtoast

 

[Gryd3]

Check Here

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[Laserbeamtoast]

it's not a spy tool, its for a mini one way communicator for a present to my little brother, who is now obscessed with the "secret service"...


Geez.

 

[Gryd3]

it's not a spy tool, its for a mini one way communicator for a present to my little brother, who is now obscessed with the "secret service"...


Geez.

To be fair, you did post "Spy Bug" in the title. I could just as easily say 'Geez'.

Have you attempted to build your circuit yet? Do you have a matching circuit for a receiver?

 

[Laserbeamtoast]

ok. i get your point... sorry. Um, I have a circuit, but it really failed, so it doesnt work.

 

[Gryd3]

Failed how? Details help.

Perhaps @Colin Mitchell can give you a couple tips.
In the mean time, give this a quick read http://www.talkingelectronics.com/projects/Spy Circuits/SpyCircuits-1.html

 

[davenn]

hi laserbeamtoast
welcome to the forums

a lot of errors in your circuit ... transistor type, coil and antenna locations to name 3

look particularly closely at the 2nd circuit in the link that gryd3 gave and give that one a go

cheers
Dave

PS I edited your thread title

 

[Arouse1973]

This circuit arangment is not that common and may prove difficult to get working. You could try moving the 47pF to be across the coil. You might also need to reduce the other capacitor to get it to work. Thats what I would try first.
Adam

 

[Colin Mitchell]

Why did you try this circuit ???
I said the circuit did not work:

Before we go to an improved design, here is an unusual circuit using a PNP (BC 557) transistor. Firstly, PNP transistors do not work as well as NPN transistors. I would reverse the 4k7 and electret mic as the voltage between base and 0v rail is very small and the 4k7 is not biasing the transistor - it is not needed! The range will be 50 to 100 metres and the current is about 3mA.
Note: A reader built this circuit. It did not work!!
Try putting the 47p across the coil. The 33p may need to be reduced to 10p.
It is just a BAD design, but a good challenge to see if you can get this type of design to work.

 

[Gryd3]

Why did you try this circuit ???
I said the circuit did not work:

Before we go to an improved design, here is an unusual circuit using a PNP (BC 557) transistor. Firstly, PNP transistors do not work as well as NPN transistors. I would reverse the 4k7 and electret mic as the voltage between base and 0v rail is very small and the 4k7 is not biasing the transistor - it is not needed! The range will be 50 to 100 metres and the current is about 3mA.
Note: A reader built this circuit. It did not work!!
Try putting the 47p across the coil. The 33p may need to be reduced to 10p.
It is just a BAD design, but a good challenge to see if you can get this type of design to work.

Haha, that's why I tagged you and linked your site Colin.
The picture the op used is on your site, and a couple others. I figured having the op read through your page would be a better resource than telling him all the individual points. (That and I am not very good with RF... anything, other than basic theory)

 

[Arouse1973]

Well I got two out of three right, didn't pickup on the microphone issue. I am going to build this and see if I can get it to work. See you around Christmas time
Adam

 

[BobK]

I have tried the circuit (2nd one) from the "Spy Tools" link on a breadboard and it does not oscillate, though it will with higher inductors and capacitors. On LTSPICE it seems to work, so perhaps the problem is with stray capacitance on the breadboard.

Bob

 

[Arouse1973]

I have tried the circuit (2nd one) from the "Spy Tools" link on a breadboard and it does not oscillate, though it will with higher inductors and capacitors. On LTSPICE it seems to work, so perhaps the problem is with stray capacitance on the breadboard.

Bob

The first circuit does not oscillate. I think they may have been relying on the capacitance between the windings off the coil which is why a parallel capacitor had been omitted initially. I have tried it with a capacitor across the coil and with a capacitor from supply to base and a resistor from base to ground I get under damped oscillations but not full oscillation. I think I need a bit more gain. I'll have another try, maybe it needs a choke in the supply to the emitter?
Adam

 

[Arouse1973]

Ok I got it Oscillating with a PNP if anyone's interested. But had to change quite a bit. the values I used for components are the ones I had lying around so might not be optimum.
Adam

Circuit Diagram

Circuit Layout



Pickup Coil



Oscilloscope Output

 

[BobK]

Not quite in the FM band though!

I can get the oscillation up to 1MHz easily. It is when I try to get 100MHz that I have problems.

I wish I could remember the circuit I used many years ago. I seem to remember needing an insanely low resistor (10Ω) as the load.

Interestingly, I saw an ~100Mz very weak signal with the oscillator, but it remained when I removed power. Must be an FM station my tank circuit was picking up. Tank was 5 turns of about 1/4 inch diameter with 33pF across it.

Bob

 

[Arouse1973]

LOL Bob, yeah I guess I am lacking in the Hertz department a bit. I'll see what the max I can get tomorrow and keep you posted.
Adam

 

[Arouse1973]

I got it running at 8.3 MHz but it looked like it was going to give up at any moment.
Adam